by dr a. laden burg translathi) from tiir second …

L e c t u r e s o n t h e H i s t o r y o f t r i e

D e v e l o p m e n t o f C h e m i s t r y s i n c e

t h e T i m e o f L a v o i s i e r

B Y D R A. L A D E N B U R GIMMU'I MNOU «•!•• <HKM!!"itHV IN THfe flNI VKltMtTY 'flf HIIKNhAU

TRANSLATHI) FROM TIIR SECOND GHRMAN RDITION

BY LEONARD POBBJN, VuA).UKfTVUKfl US ('HKMirAL TlftiOltV AND Him'MtV, ANU AK'itHTANT IN t.'IlKM IHTIlV

IN' THK t'NlVKHHITV (»!•' KI>!NlK'H<Jlt

tuhhtuni'. ami itirfcitititfi In/ the Aklhtor

REVI8ED EDITION

I'UIH-ISHKI) KOK TniC ALK.VUiiC CLUB

JAMKS TMIN, 55 Snirnr MKUICJK

SIMPKIN, MARSHALL, HAMILTON. KRNT & CO., LTD.

3 0 5

5A-0-9

A U T H O R ' S P R E F A C E

I N placing these lectures before a wider section of the

pub l i c , I consider it essential to indicate the point of view

iVom which they have been prepared. I regard them as an

u t t e m p t to follow the development of our ideas of to-day

f r o m those that were formerly current. Hence I have only

g o n e back as far as Lavoisier ; because our science assumed

ii new aspect in his hands, and because it may be held that,

u s regards development, we are still passing through the

e p o c h inaugurated by him.

It has been my wish to arrange the matter of the

lec tures in such a way that the student may be enabled to

o b t a i n a survey of this portion of the history of chemistry

w i t h little trouble, and at the same time so that it may

. s e r v e as a guide for those who may desire to engage their

a t t en t ion more particularly with special investigations in

t h i s department. On this account I have expressed myself

a s concisely as possible, whilst, on the other hand, I have

suppl ied moderately complete references to the original

l i t e ra tu re in connection with the subjects treated of. A

vi PKEFACK

twofold result appears to me to be attained in this way,

inasmuch as the reader is placed in the position of being

able to form an opinion forthwith regarding the value of

the narrative, and to correct errors and omissions, while

the labour of subsequent investigators is lightened. While

I could scarcely consider it possible to give an absolutely

accurate representation of the period in question, with its

great wealth of discovery, still it has been my aim to furnish

a useful contribution towards the history of the chief

chemical facts and theories.

It is almost unnecessary to say that the book has no

pretentions to completeness. I only felt justified in taking

notice of those investigations and ideas which have exer-

cised an influence upon the further development of chemistry,

whereas I have at most merely referred to other investigations

which, in my opinion, will still exert such influence. An

objective treatment of the subject appeared to demand that

it should be handled in this way.

I have not hesitated to carry the history of the develop-

ment of chemistry down to the present day, although the

difficulty of the task has been greatly increased by my

doing so. It is certainly in this part in particular that

many corrections will still be necessary before the end in

view is attained. How different the latest phases of our

science will appear to subsequent investigators I And yet

the opinion of a contemporary is not without value also,

when it is moderate and free from prejudices or special

PREFACE vii

leanings. This is exactly what I have striven to attain. If

I have not always been successful in doing so—if here and

there I may have underestimated the merits of some and

unduly asserted those of others—this has been unintentional.

If I have been severe in my judgment, I have at least been

free from any personal feeling, and it has always been the

matter alone that I have attacked. Should I have ap-

proached in some cases too closely to the limits of historical

accuracy, or should I not have succeeded in representing

fairly the claims of every one, I am fully prepared to rectify

my error as soon as I am convinced of it.

If my colleagues are interested in the subject, and assist

me with their knowledge and advice, it may soon be possible,

perhaps, to obtain an objective picture of the chemical

theories of the last hundred years. I desire that this book

may be regarded as an attempt in that direction, and that

it may be judged indulgently.A. LADENBURG.

KIEL, December 1886.

A U T H O R ' S P R E F A C E T O T H E

E N G L I S H E D I T I O N

THIRTY years after the appearance of the first edition of

this book, an English translation of it is now being pre-

pared. I regard this as a favourable indication of the per-

manent value of the book, since it is evident that the

standpoint then adopted is intelligible at the present day

and is still unsuperseded. Moreover, it may be concluded

that the exposition of the subject is not marred by national

prejudices.

In order to keep pace with the constant progress of the

science, two new lectures have been added to the original

fourteen. One of these appeared fourteen years ago, upon

the publication of the second German edition of the book;

whilst the other—the sixteenth—is here published for the

first time.

The English edition is a faithful translation, and so far

as I am able to judge, it is written in a good style. For

these features my best thanks are due to the translator.

I venture to express the hope that the book will find

friends amongst the English-speaking peoples, and that it

may contribute to stimulate interest in the history of our

science.A. LADENBURG.

GRASSKNDAT.E, SOUTHBOURNE-ON-SEA,September 1899.

T R A N S L A T O R ' S N O T E

THK translator wishes to express his sincere thanks to

Professor Ladenburg for the very cordial manner in which

he agreed to the preparation of this translation of his well-

known history, as well as for his kindness in specially

writing an additional lecture for the English edition,

thereby bringing the latter up to date ; also for the great

care he bestowed upon the revision of the proof-sheets. He

further wishes to thank a number of friends to whom he is

indebted for advice and assistance upon a variety of points.

T A B L E O F C O N T E N T S

LECTURE IPACK

fntroduction—The Phlogiston Theory in its First and in its LaterAcceptations—Chemical Knowledge of the Phlogistians—Fallof the System . . . . . • . i

LECTURE ITRevolution of the Views regarding Combustion—Priesiley—Scheele

—Lavoisier—Indestructibility of Matter . . 15

LECTURE IIIChemical Nomenclature—Tables of Affinity—Berthollet's Views—

Controversy regarding Constant Composition . . 31

LECTURE IVRichter's Investigations—Dal ton's Atomic Theory—Gay-Lussac's

Law of Volumes — Avogadro's Hypothesis — Wollaston'sEquivalents . » • • - . - 47

LECTURE VDavy's Electro-Chemical Theory—Discovery of the Alkali Metals—

Discussion regarding their Constitution—Does HydrochloricAcid contain Oxygen ?—Hydrogen Theory of Acids . . 67

LECTURE VIBerzelius and his Chemical System—Dulong and Petit's Law—

Isomorphism—Prout's Hypothesis—Dumas1 Vapour DensityDeterminations—Gmelin and his School . • • <Q5

xiii

xiv CONTENTS

LECTURE VIIPAGE

Organic Chemistry at the commencement of its development—Attempts to determine the Elementary Constituents of OrganicCompounds—Isomerism and Polymerism—Views "regardingConstitution—Radical Theory . . . . . 10S

LECTURE VIIIFurther Development of the Radical Theory—Views concerning

Alcohol and its Derivatives—Phenomena of Substitution—Dumas' Rule — The Nucleus Theory—The Equivalent ofNitrogen . . . . . . . . 130

LECTURE IXGraham's Investigation of Phosphoric Acid—Liebig's Theory of

Poly basic Acids, and his Views with respect to Acids in general—Adoption of the Davy-Dulong Hypothesis—Discovery ofTrichloracetic Acid—Attack upon the Electro-Chemical Theory—Replies of Berzelius—Copulas , . . . 1 5 0

LECTURE XInfluence of the School of Grnelin—Theory of Residues—Coupled

Compounds—Gerhardt's Determination of Equivalents—Dis-tinction of Atom, Molecule, and Equivalent by Laurent—NewCharacteristics of Polybasic Acids—Molecules of the Elements 173

LECTURE XIReasons for the Assumption of the Divisibility of Elementary Mole-

cules—Fixing of the Molecular Weights, by Williamson, byMeans of Chemical Reactions—Theory of the Formation ofEther—Fusion of the Radical Theory with Dumas' Types—Substituted Ammonias — Polyatomic Radicals — Gerhardt'sTheory of Types and System of Classification . . . 1 9 7

LECTURE XIIMixed Types—Relation between Kolbe's Views and the Copulse of

Berzelius—Radicals containing Metals—Conjugated Radicals—Kolbe and Frankland and the Views regarding Types—Poly-basicity as an Evidence for the Accuracy of the new AtomicWeights—Discovery of the Polyatomic Alcohols and Ammonias 220

CONTENTS xv

LECTURE XIIIPAOR

Ideas regarding the Types—Elucidation of the Nature of theRadicals by the Valency of the Elements—Quadrivalence ofCarbon—Specific Volume—Constitutional Foimulce — Separa-tion of the Ideas of Atomicity and Basicity—Isornerism amongstAlcohols and Acids —Physical Isomerism—Unsaturated Sub-stances . . . . . . . . 249

LECTORK XIVTheory of the Aromatic Compounds—Determination of Position of

Substituted Atoms or Groups—Quinones—Artificial Dyes—Ring Compounds—Constitution of the Alkaloids—Syntheses—Condensation Processes . . . . . . 2 7 2

LECTURE XVThe Fundamental Conceptions of Chemistry—Phenomena of Dis-

sociation—Abnormal Vapour Densities—Constant or VariableValency—The Doctrine of Valency in Ino:ganic Chemistry—The Periodic Law—Later Development of the Doctrine ofAffinity—Spectrum Analysis—Synthesis of Minerals—Con-tinuity of Matter in the Liquid and Gaseous States—Liquefac-tion of the so-called Permanent Gases—Thermo-Chemistry—Electro-Chemistry — Photo-Chernistry — Molecular Physics —Morphotropy . . . . . . . 297

LECTURE XVI

The Doctrine of Phases—Van der Waals's Equation—Theory ofSolution —Electrolytic Dissociation — Electro-Chemistry—At-tainment of High Temperatures—Low Temperatures—The NewElements in the Atmosphere—The Chemistry of Nitrogen-Transition Temperature — Stereo-Chemistry — Racemism —Syntheses in the Sugar and Uric Acid Groups—Iodoso-Com-p'mnds—Terpenes and Perfumes—-New Nomenclature . 334

INDEX OF AUTHORS' NAMES . . . 360

INDEX OF SUBJECTS . . . . . . . 366

NOTES RESPECTING THE REFERENCES TOJOURNALS, ETC.

IT is not anticipated that the abbreviations employed for the

titles of journals, etc., will, as a rule, present any difficulty.

The following unfamiliar abbreviations may be explained :-—

A. C. R. * Alembic Club Reprints.

E . (following a reference to a foreign book) =• English

Translation. (This contraction is only employed in the

cases of a few well-known translations.)

In cases where journals have been issued in two or more

series, references to volumes belonging to the second or

any subsequent series have the series indicated by prefixed

numerals enclosed within square brackets.

H I S T O R Y O F C H E M I S T R Y .

L E C T U R E I.

INTRODUCTION—THE PHLOGISTON THEORY IN ITS FIRST AND IN ITSLATER ACCEPTATIONS—CHEMICAL KNOWLEDGE OF THE PHLO-GISTIANS—FALL OF THE SYSTEM.

THE value of historical narratives is undisputed. Thisvalue no doubt varies with the subject matter which

is dealt with ; but the history of human actions and of humanknowledge always forms one of the most interesting inquiries.If we are adherents of the Darwinian theory, and grant tothis theory a warrantable latitude, the importance of aretrospect of bygone centuries is thereby enhanced. Weare then obliged to recognise a steady progress of develop-ment ; history is no longer a mere enumeration of isolatedfacts in chronological order, as these succeeded one anotherfortuitously, but it embraces the development of the humanmind and of human civilisation. It shows us the results of theinfluence which the most varied causes have exercised uponthe most different natures, and may perhaps at some timeenable us to discover the laws which regulate these results.From this point of view it cannot be denied that thedevelopment of the present condition out of any formerone becomes of increased importance ; and hence theinterest which the thinking public has taken in Buckle's" History of Civilisation J) is easily understood.

I do not, however, go so far as to assert that this actualstandpoint is necessary in order to lend due importance to therepresentation of the past. The facts cannot be overlooked,

2 HISTORY OF CHEMISTRY [LKCT. I.

that knowledge itself affords a certain satisfaction to thehuman intellect, and that every one eventually seeks todraw lessons for the present from the destinies of nations informer times. The most pronounced opponents of Darwin,for example, must admit that a connection exists betweenthe predominating character and the fate of a nation, andeven they will attribute the success or the non-success ofgreat undertakings to material causes and circumstances.

Assuming as a basis, then, the standpoint mentionedabove, it may be asserted that a historical account of anyscience possesses an interest extending beyond that particularscience. In a comparative study of the history of all intel-lectual sciences, certain general tendencies of speculationmay perhaps be recognised which were predominant atparticular times, and owed their existence to real, definitecircumstances. In this respect the history of philosophy, inparticular, is of importance for early times ; while for moderntimes the historical exposition of the natural sciences, in myopinion, possesses just as great, and probably even greater,importance. The subject matter treated of in the presentwork may hence find an application some day : it may beregarded as one of the many preparatory studies which willbe required if the question of writing a history of thedevelopment of the human intellect should ever arise.

If we limit our view, however, and inquire as to theinterest which the historical representation of a science pos-sesses for that science ; or, what concerns ourselves still moreclosely, if we merely consider the advantage which accruesfrom it for the study itself, or for the student, the points ofview which then become paramount are entirely different.

A retrospect of the past, especially in the exact sciences,alone affords a proper comprehension of what is acceptedto-day. It is only when we are acquainted with the theorieswhich preceded those accepted at present, that the lattercan be fully understood ; because there is almost always anintimate connection between them. It might appear in ourscience (where any final result is arrived at by the test of

•LECT. i.] HISTORY OF CHEMISTRY 3

experiment) that the earlier views, which give expression to alimited number of facts only, must not merely be supplantedby the later theories, which deal with a larger class of pheno-mena, but that they must lose their importance altogether.For the most part, however, this is not-the case. On thecontrary, a certain connection between successive hypothesescan very frequently be observed. When the general develop-ment is followed up, the effects of the earlier ideas can berecognised in the later views, and it is in this way that thelatter first come to be properly understood. The abandon-ment of a theory is not always accompanied by a revolution.Such, indeed, is scarcely conceivable in the higher stages ofthe development of science ; and even when new modes ofexplanation are proposed, traces of former opinions may stillbe recognised in the direction which these take.

But quite apart from this real advantage of the study ofhistory, which thus, in my opinion, leads to a clearer under-standing of our present position, yet another advantage maybe adduced which is perhaps of still greater value to thestudent: namely, the accurate estimation of the value oftheories. An examination of the past shows us the muta-bility of opinions ; it enables us to recognise how hypotheses,apparently the most securely established, must in course oftime be abandoned. It leads us to the conviction that welive in a state of continuous transition ; that our ideas of to-day are merely the precursors of others ; and that even theycannot, for any length of time, satisfy the requirements ofscience. We learn from any historical exposition that ournatural laws are not incontrovertible truths or revelations,but that they can be regarded as the expression, for the timebeing, of a certain series of facts, which are thereby summa-rised and, as we say, explained in the most practical way forus. We recognise that these laws do not originate suddenlyin the head of a single individual, like Minerva in the headof Jupiter. It is only slowly that the fundamental ideaswhich underlie them mature, and that the requisite facts areascertained by the labours of many ; until, at last, the law

I H S T O K Y 1 ) 1 "

c o m m o n t o t h e m i l l i - a n n n n * * i 4 ' ', • ••, * . * ^ » i

s e v e r a l p e r s o n * " i m t t l t a m »MJ I 1 , . I ' l i t ' l c ) . '» \ ' * < f !

h i h t o r j n o r f . t i t h i n «ui f! i s??*!*» >. ^ L V . U I , %« : * % f*» «%!

p r o d u c e s p c r n i i i M i i * t»!i«*f * I>\ •' *: J« • , ^ \ \ » ^ '. f ^

o r i g i n a l «lf*v<*loj)me»«t »•• ! h i ; ; J ! i . < i ' i •)

O n f l u o f h e i h a n d ^ * a ! «« «<*?i » f '* . i . ^ " b f

t h e a c t u a l t e t ( 3 i n » n ^ » J J* ' i ^ t »«^ ; . . ; ,s }. l : , ' ^i

i n t e l l e c t u a l ^ i g i i i l i i « i i i * i « AM » i ; i i v ! i a < ^ u > ¥ t I H * * * * {

i s o l a t e t i cil»wf*i\ *tti» »JI * h\ i < n ( . i i i ' f f l< s* *?li * * ! l i s

o h s i T v . t t i M n , t h a n i n f l u ^ I M I I ^ I ^ ' ^ I ? * H * I » j * ' r

W h e n t h e p * » m f *•! v i t * w w h i « Ii 1 ?• | \ u < l ,i < r . i i . /» ' i

. s u b j e c t i s t h t i s t t M i i r < k if , it \ u < l *fT i n , 4 ^ I ? ^ »? ? ! . +< t <3

m y u t t c n t i u n p t i i u i j M i l y t " » t h > <«) t • 4n>t^*u\ !»A< i ; ? in i

o f t h o ^ c e x p i ^ i m c f U a H n v t ' ^ i ^ ^ M ' i M ^ w J u * h h i # » - ' ^ * w , J

t o t h e e » t a b l i * t h m r t i t ntr f l u : n v i f f l i f ^ w nt & nut n w w f |

T h e e a r l y h i M o r y o f o u r ( c i t > t t r r ! I » I . t x H I t r * ' 4 t n j i

e x c e l l e n t l y . Mid i n c k l a i l f ! »v I f f i f » i i s i * i K * * J » J » ; *\u 1 h>t

r e a s c H i I c o i i f i n t m y n t * l f lc» l l i r j m i ^ ' J i , , * ' i n * ' "

I ^ i v o i h i e r ; t l u i t i s , t o f l u ; j»« n o r ! o f ! i / / | i i n '}>>\ru

t o t h a t o f q u a n t i t a t i v e t u v f «ti|4-t}i*< i< .• I ui j * J^MJ '!

h o w e v e r * t o g i v e a * h m t t l e ^ f t p t j *?i ! *j*< w » v % w

p r e v a i l e i l i n c h e m i s t r y j i r i u r I n L n ? *%;'-i

* I ' h e i n f l u e n c e o f lift* < J u ' e l * U J M I 4 i i 4D«I l i ? r t f | * n i |

t h e i r r c a w a k e n i n g a f t f T u v c r ^ U n i m i r f I } ' $i # " 4 ^ * J ^ # V M K

k n o w n t h a t i t i i w l nut i i t j u i t n if » t i«-- * t r r % * i i '

i n f l u e n c e i n M ' i e n e * ? a\ 'f*» 1 1 i * r !• nt vU m* r t * * 1 h » » }•» 4 i

w a t c i , e a r t h , fir«% i i i c l air, u h * h , H I I n j * * i l » * •.» .rin

r c p r e s t m t u t t v i t o f i l n * f « i i i i l a t ^ i i M 1 p r ^ j t r f ,r< , U P > ^ , ' * t » *

a n d c o l t ! » : i i i . i | i i s f t % f « 4 i i i i l t » i r < t M h n h g u if i i i i j ^ i ^ r r * .

i i i g t i t e l i t r e a m o u n t I lit? d r i t t r n ! ' ' , ««i*l i*. • « i t K» , 4 11*^*4

a s a m a t e s i . i l **fiif t t a i i u * , A \ w t ' l w l l l»Mnji I I I M S I . I ! b»h< ^ 1

r h t c h e m i c a l t h i ( n i a a > h a v e u f r r e j u r t i » I I H T j i i f ' n M ^ v t i

j«4ifc *tfi« l l r pu* iii4fi«i t»K ll*r t^fl l irf fr in f i l l . ! ' * * f if»«

LBCT. r.] HISTORY OF CHEMISTRY 5

combustion, and the phlogiston theory becomes more compre-hensible when we minutely study the views of the Greeks andthe Romans. Amongst these peoples, combustion is alreadylooked upon as consisting in the separation of the materialof fire; and Pliny regarded the easy inflammability of sulphuras a proof of its being largely composed of a fire material.2

At a later date sulphur was itself assumed to be the firematerial; and from that view the hypothesis that all metalscontained sulphur, unquestionably arose.

These few words concerning the chemical theories of theancients appear to me sufficient in order to understand Becherand his follower, Stahl. Both of these based their views uponthose of the Greek and of the Roman philosophers ; in thesame way that we find so many imitators of Greek art at thesame period, that is, in the seventeenth century.

A difference may, it is true, be pointed out betweenthem; namely, that only the latter intentionally and know-ingly followed in the footsteps of the ancients, whilst theformer declared themselves to be their opponents. ThusBecher says : " A good peripatetic is a bad chemist." Hereplaces the four elements of Empedocles by three others :the verifiable, the inflammable, and the mercurial earths.3

It is not my business here to inquire whether it wasBecher or Stahl who thought and wrought most with respectto the phlogiston theory. Still I will not omit to drawattention to the great modesty of Stahl, who wished thathis own services should be attributed to his teacher andfriend Becher: "Bechciiana sunt quae profero^^ Suchexamples are rare.

The adherents of the phlogiston theory regard combus-tion as consisting in a decomposition : u only compoundsubstances can burn" ; these all contain a common principlewhich Becher calls terra pinguis and Stahl calls Phlogiston.During the combustion this principle escapes and the otherconstituent of the substance remains behind.

2 Kopp, Geschichte. 3, 102. u Ibid. 1, 179. 4 Ibid. 1, 188.

6 HISTORY OF CHEMISTRY [LECT. I.

This theory was applied to all combustible substances.Thus, according to Stahl's views, sulphur consists of sul-phuric acid and phlogiston ; a metal, of its metallic calx (ofits oxide we should say) and phlogiston. According to Stahl,sulphur was not identical with phlogiston, but, as with Pliny,it was rich in the principle of inflammability, which he did notknow in a separate state. Soot appeared to be the substancerichest in phlogiston ; in fact as almost pure phlogiston. Itwas in consequence of this that the conversion of the metalliccalx into the metal by heating it with soot succeeded so well;for the soot handed over its phlogiston to the metallic calx, sothat a metal was produced again. In his experimentutn novumStahl tries to prove that the phlogiston in soot and in sulphuris identical. He shows here, how a sulphate can be convertedby means of charcoal into liver of sulphur, from which sulphuris precipitated by the action of an acid. From the reductionof the metallic calces by means of soot, Stahl further infers theidentity of the phlogiston of the metals with the inflammableprinciple in soot and in sulphur; and thus he arrives at aproof that there exists only one such principle, which he callssimply Phlogiston (from <£Aoy«rro9, combustible).

The phlogiston theory was, for a century, the basis of allchemical considerations; nevertheless we shall find thatduring this time the conception of phlogiston did not alwaysretain its first signification, and that the whole mode ofregarding it was altered in consequence.

We can understand Stahl and his immediate followersquite well if we assume a loss of oxygen in every place wherethey speak of the taking up of phlogiston, and vice versa; aphlogisticated substance is, with us, a substance free from orpoor in oxygen. It might perhaps be said, in short, thatphlogiston is negative oxygen.

Stahl borrowed from the ancients the view that combus-tion is accompanied by destruction, or decomposition. Thishe retained, although, even in his time, facts were well knownwhich proved an increase of weight on combustion. EvenGeber, an alchemist of the eighth century, appears to have

LECT. i.] HISTORY OF CHEMISTRY 7

observed this in the cases of tin and of lead ; and the chemicalliterature up to Stahl's time furnishes several statements ofthe same kind. Highly interesting, for example, are theobservations of Jean Rey, of Mayow, and of Hooke, as wellas the conclusions which they drew from them. I shall enterinto these in the next lecture.

Can we avoid being astonished when we read that Becherand Stahl knew of these experiments and still defended theirviews; that they regarded the increase of weight merely asan incidental, unimportant phenomenon ; and that either theauthority of the ancients, or the phenomenon of combustionitself, which with them was so intimately associated with theidea of destruction, was a sufficient ground for neglecting factswhich must otherwise have overthrown their edifice ? It ismore particularly noteworthy, however, that Boyle—one ofthe most considerable thinkers of the seventeenth century, apredecessor of Stahl, calling himself a follower of the Baconianschool; who was aware, from his own experiments, of theincrease of weight on combustion ; who knew that the airwas necessary for this, and who had made the observationthat during combustion a part of the air is absorbed—couldnot make up his mind whether sulphuric acid was a con-stituent of sulphur, or on the other hand whether sulphurwas contained in sulphuric acid.5

Amongst the successors of Stahl we find, it is true, somewho direct their attention more fully to this increase of weight.Lemery, for example, towards the end of the seventeenth cen-tury, states his views about it at length.0 At the same timehis belief in the existence of phlogiston remains unshaken,although combustion now becomes a sort of double phe-nomenon. It remains a decomposition ; that is, the burningsubstance separates from its phlogiston, but simultaneouslyit unites with a ponderable fire material. Lemery obtainedhis ponderable fire material from the same source as thatfrom which Becher had taken his terra pinguis and Stahl his

0 Kopp, Geschichte. I, 166. 6 Ibid. 3, 123.


phlogiston. It was a new application of the element fire.This double principle—the combustible principle on onehand, the ponderable fire material on the other—satisfactorilyand completely explained the phenomena of combustion tothe chemists at the close of the seventeenth century. Wefind these views first shaken by Newton, for whom fire is nota special substance. He suggests that every strongly heatedand glowing substance burns; that red-hot iron or woodmay be called fire; and that those substances which emitmuch smoke burn with a flame.

The assumption of this ponderable fire material was firstrecognised as really fallacious in consequence of a highly in-teresting experiment by Boerhaave, who weighed masses ofmetal both cold and red-hot and found their weights to beidentical in both cases.7 The explanation of the increase ofweight next brings about differences of views amongst thechemists of the eighteenth century. Some seek to regard it,as Stahl had done, as an unimportant phenomenon whicrmiaybe neglected; others, on the contrary, and amongst themBoerhaave, assume a union with certain (saline) portions of theair, and in this way seek to take account, at the same time, ofthe necessary presence of air during the combustion and of theincrease in weight. According to others again the air merelyserves to take up the separated phlogiston, which, in theirview, cannot escape from one substance if there is not anotherpresent with which it can unite. In the middle of theeighteenth century we also find the notion that phlogistonpossesses negative weight, or absolute levity. It seems quitenatural to the upholders of this hypothesis that the weightincreases on the separation of phlogiston. Others still, whohave difficulty with the conception of absolute levity, regardphlogiston as lighter than air. This view is upheld, forexample, by Guyton de Morveau,8 whose explanation of theincrease of weight is based upon the Archimedean principle,and does not altogether tell in favour of the clearness of imagi-

7 Kopp, Geschichte. 3, 127. 8 Ibid. 3, 149.


nation of this noted chemist. He says in effect:9 If webring two lead balls of approximately equal weight into equili-brium under water, on a balance, and then attach a piece ofcork (an object lighter than water) to one of the balls, this ballwill ascend; it becomes, therefore, apparently lighter althoughclearly we have increased the weight. A similar thing holdsin combustion ; in this case we weigh in air; the metal—thecompound of the metallic calx with phlogiston—appears to belighter than the calx because the phlogiston, just like thecork,is specifically lighter than the medium in which we weigh.—Itake it for granted that the reader perceives the fallacy in thismode of regarding the matter; and in this respect he is inadvance of the celebrated Macquer who could not with-hold his admiration for this explanation.—Even Boyle hadalready observed that the metallic calces were specificallylighter than the metals, but Guy ton does not take this intoconsideration !

As the reader will have observed, I have not hesitated tocall attention to the contradictions of the phlogiston theory,and to its weakness with respect to any reasonably tenableexplanation of the increase of weight during combustion. Inspite, however, of those hazy conceptions, which constitutedthe basis of the chemical opinions of the period, there weremen amongst the phlogistians who have scarcely been excelledin the fertility of their discoveries by any of the chemists ofthe present day. In this connection may I venture to make ageneral statement ? Am I not justified in asserting that falla-cious theories are not always obstructive to the developmentof science, and in supporting the view that it is better to pos-sess definite theoretical bases, even if they do not explain allthe facts, than to represent these facts themselves as the soletriumphs of science ? Facts certainly play a great part in thefoundation and in the overthrow of a theory ; indeed theyalone should have any influence in such matters; and if wenow turn our attention to the decline of the phlogiston theory,

9 Kopp, Geschichte. 3, 149.

10 HISTORY OF CHEMISTRY ("LECT. I.

we must consider, in a general way at least, the chemicalknowledge and labours of the phlogistians.

Their chemistry consisted especially of a rather incompleteknowledge of the chemical and physical properties of a seriesof substances which occur in nature. They had learned howto prepare other substances from these, and their endeavourswere directed to the discovery and recognition of new sub-stances. Hence we find an already remarkable developmentof qualitative analysis, which we owe chiefly to Bergman,whilst quantitative methods were almost wholly unknown.Naturally, the theoretical bases did not permit of any valuebeing attached to chemical proportions by weight.

In order to give a general idea of the substances known atthat time, I shall mention some of them. Sulphur, charcoal*gold, silver, copper, iron, tin, and lead were certainly knownto the most ancient peoples; the discovery of mercury belongsto the Greek period; that of antimony, bismuth, and zinc tothe middle ages ; that of arsenic, phosphorus, cobalt, nickel,platinum, etc., to the period of phlogiston. Scheele, who wasthe mostfertilediscoverer amongst the phlogistians, discoveredmanganese and chlorine. The metallic calces, or oxides^ aswe should say, were looked upon as different by all t h echemists of the period ; yet Macquer thought this differencemight be referred to the more or less incomplete expulsionof the phlogiston, and he therefore assumed the existence ofthe same earthy constituent in all the metals.10 Amongstthose earths which were not classed with the metallic calces,they knew lime, alumina, and magnesia. Scheele discoveredbaryta. They divided the alkalies into the caustic and themild (carbonated), the latter being regarded as substanceswhich might pass into the former by taking up fire mater ia lPotashes were in use from the earliest times ; the Arabiansprobably made known the preparation of caustic potash frompotashes and l ime; nitre was also known, and served forthe manufacture of gunpowder. Soda or potash had been

10 Kopp, Geschichte. 3,143.


already employed by the Egyptians in the manufacture ofglass,11 but Stahl was the first to discover that common saltcontained an alkali differing from potash.

Amongst the acids known at that period, I mentionhydrochloric, nitric, sulphuric, and acetic. We are indebtedto the Arabian alchemists for the introduction of the use ofaqua rcgia. Scheele considerably increased the number ofthe organic acids. He discovered hydrocyanic, malic, uric,lactic, citric, oxalic, and gallic acids. The discovery of hydro-fluoric acid also stands to his credit. So we see what a largenumber of salts the phlogiston period had at its commandin consequence of these discoveries. I do not enlarge uponthis, but turn to the knowledge respecting the gases, whichare all the more interesting because they led to the downfallof the phlogiston theory.

All gases were for a long time regarded as identical withair, and this in turn was considered to be an element. VanHelmont, in the middle of the seventeenth century, was thefirst who assumed the existence of different gases. Nearlyanother hundred years passed after this assumption beforethe recognition of a gas which was certainly different fromair ; the difficulties of the manipulation make this easily com-prehensible. We are especially indebted to Black, Cavendish,and Priestley for surmounting these difficulties. The firstexamined carbonic anhydride, or the so-called fixed, air, andcorrected the views as to mild and caustic alkalies. Hisinvestigation12 is one of the most important of the phlogistonperiod. In it (as was done by Lavoisier at a later date) wefind the relations by weight brought forward as the most im-portant consideration in the arguments. Cavendish studiedthe properties of hydrogen, whilst Priestley discoveredoxygen, nitric oxide, and carbonic oxide, as well as sulphur-ous and hydrochloric acid gases, ammonia, and silicon fluoride.

11 Kopp, Geschichte. 4, 27.12 Experiments upon Magnesia Alba, Quicklime, and some other Alcaline

Substances. See Alembic Club Reprints, No. 1.

12 HISfOkY OF CHEMISTRY [I^CT. I.

I shall deal more fully in the next lecture with the dis-covery-of oxygen and the theoretical revolution, connectedwith this discovery. I shall now say something with respectto Cavendish's investigation of hydrogen, and shall referparticularly to the modification in the prevailing phlogistontheory, which Cavendish and some other chemists introducedas a result of this investigation.

Cavendish prepared his hydrogen from iron, tin, or zincby dissolving any of these in hydrochloric acid ; he studiedits physical properties, called it inflammable air, and provedthat it was quite distinct from common air. Basing hisopinion on its mode of preparation, he regarded it, in thesame way that Lemery had already done,13 as identical withphlogiston. Priestley and Kirwan further developed thisview, the former basing his opinion upon his own observationthat the metallic calces were reducible by means of hydrogen.11

The phlogiston theory in this new form is really basedupon the following views: When a metal is treated with adiluted acid it decomposes into free phlogiston (hydrogen),and a metallic calx which dissolves in the acid. If the acidis concentrated (nitric acid or sulphuric acid) the phlogistonunites with the acid and a phlogisticated nitric or sulphuricacid is produced (nitrous or sulphurous acid). The explana-tion of the reduction of the calces by means of hydrogenwas very simple : what occurred was merely the taking upof and the combination with phlogiston, whereby the metalwas regenerated.

These ideas, in which we must recognise a touch ofgenius, were pretty generally adopted by the phlogistians ofthe period. They furnished the last glimpse of sunshineaccorded to the theory. The same person who discoveredthe facts which rendered their advancement possible, soonafterwards furnished the experiments which brought aboutthe downfall of the theory. . . . . . . . .

The phlogiston theory, in the sense understood by Caven-

13 Kopp, Geschichte. 3, 152. * Ibid. x, 242.

LECT. r.] HISTORY OF CHEMISTRY 13

dish and Kirwan, was, however, easily disposed of. It ex-plained the conversion of the metals into their calces by meansof acids—a matter which had begun to present difficulties tothe older phlogiston theory, but it no longer took cognisanceof the real phenomena of combustion. In the calcination ofa metal, where did the phlogiston (the hydrogen) go to ?A previous assertion made by Scheele15 that during the com-bustion of sulphur in air, the air takes up phlogiston andunites with it, whereby its volume is diminished, was easilyrefuted now when the properties of phlogiston (i.e., of hydro-gen) were known ; and the phlogiston theory thus became,iri its new form, no longer applicable to that class of pheno-mena which it was advanced in the first instance to explain.

The facts which contributed to the fall of this theorymultiplied from year to year. In 1774, Bay en found thatmercuric oxide was converted into mercury on heating.Whence came the phlogiston which was required to bringabout this change ? Bayen perceived the importance ofhis discovery, and regarded mercuric oxide as different fromthe metallic calces proper. He found at the same time thatthe loss of weight in the reduction of the mercuric oxidewas equal to the weight of the air obtained. How littleattention was, in general, bestowed upon a fact so important,is proved by the views of Macquer who assumed that theremust be a loss of weight in connection with the oxidationand the subsequent reduction of a metal. When Lavoisiercame forward at a still later date, in opposition to thephlogiston theory, Macquer stated that the news thatimportant facts had been discovered, adverse to the phlo-giston theory, caused him some concern, but that he wasquite composed again when he ascertained that it was merelya question of relations by weight.I6

Others, however, thought differently ; and Tillet, afterhaving again confirmed the increase of weight during theformation of litharge from metallic lead, drew attention, in

15 Kopp, Geschichte. 3, 201. m Dumas, Lemons. 133,


a report to the French Academy in 1762, to the circumstancethat the explanation of this remarkable fact had not yet beengiven ; but that it was to be hoped that the immediate futurewould furnish some elucidation of the matter.1T

On the discovery of the composition of water the phlo-giston theory was, in my opinion, no longer tenable, andhad of necessity to be abandoned, since another theory,which was in conformity with all the facts, was ready tohand.

The fact that defenders of Stahl's views were still to befound from ten to fifteen years later proves, however, howdifficult it is to eradicate prevailing opinions. It shows ushow conservative we are by nature, and should make us useevery endeavour to shatter our own faith in authority.

17 Kopp, Geschichte. 3, 129-130.

L E C T U R E II.

REVOLUTION OF TPIE VIEWS REGARDING COMBUSTION—PRIESTLEY—SCHEELE—LAVOISIER—INDESTRUCTIBILITY OF MATTER.

A STRUGGLE of great importance for chemistry was carriedon between the years 1774 a n ^ 1794- This struggle wasconcerned with the removal of the fetters laid by the Greekphilosophers upon the thinkers of that period, and with theconsistent upholding of the Baconian philosophy. It had todo with the recognition of the experimental method (or themethod of observation under definite conditions) as the basisof all theoretical conclusions and speculations, and with theclearing away of those prejudices which had been createdin the minds of the period by the method followed forcenturies—that, namely, of giving the foremost place tospeculation, and of adapting observed facts, as well as mightbe, to the established system.

These twenty years are not only rendered conspicuousby a series of brilliant experimental investigations, but theypossess also a universal importance in chemistry because theyled to the establishment and recognition of a principle whichconstitutes the basis of all our chemical experiments ; andwhich is to so great an extent involved in our generalscientific considerations, that deviations from it seem incon-ceivable to us. This is the principle of the Indestructibilityof Matter. It is only with the greatest effort, and from anextremely objective standpoint, that we are in a position tounderstand scientific treatises in which this basis is wanting.

Although innumerable experiments are in agreementwith this principle, yet we must be doubly careful in theadoption of any such law, seeing that it constitutes the basis

16 HISTORY OF CHEMISTRY [LECT. II.

of all our scientific views. Even here, we must not commitourselves to any blind belief, neither must we regard thislaw as absolutely exact; and, however difficult we may findit to build up a scientific edifice without it, still we mustnever forget that, just like all other laws, it is merely theexpression of facts which we have observed ; that there areerrors connected with all our observations ; and that onthis account the possibility is not excluded that succeedingcenturies may reject the law itself.

Meanwhile, however, we must regard this law as the great-est achievement of chemistry, and as one of the firmestsupports of all natural science; while from the period of itsgeneral acceptation we date the commencement of a new erain chemistry ; that is, of the chemistry of to-day. It will thusbe understood why I desire to direct most especial attention tothe period at which this law was stated and was put to the test;and why I enter upon a detailed account of Lavoisier's experi-ments, from which the accuracy of the principle was deduced.

Many hold the view that the reorganising effect which ourscience experienced, is to be attributed to the discovery ofoxygen, which fell—not altogether fortuitously—within theperiod mentioned. This is not the case, however; and thehistory of chemistry itself furnishes proof of the fact, inas-much as Priestley and Scheele were the discoverers of oxygen,while Lavoisier was the reformer of chemistry. I cannotresist the temptation to point out how the endeavour wasmade to bolster up phlogiston, and how Priestley and Scheelemade every conceivable effort to bring the astonishing pro-perties of oxygen into harmony with the existence of phlo-giston (which had never yet been demonstrated).

Priestley discovered oxygen in 1771. He isolated andexamined it, and the priority of the discovery is his. Hepublished a detailed account of it in 1775.1 Scheele's in-vestigation appeared two years later, but it has been shown

1 Priestley, Experiments and Observations on different kinds of Air,Vol. 2, London (1775), 29 ; Alembic Club Reprints, No. 7, 5.

LECT. ii.] HISTORY OF CHEMISTRY 17

that his experiments were independent of and nearly simul-taneous with those of Priestley.2 Both chemists employedalmost similar methods for its preparation. They obtained itfrom mercuric oxide, pyrolusite,minium, nitre, etc. Lavoisieralso wrote a treatise on oxygen, but Priestley states that hehad previously informed Lavoisier of his discovery, althoughthe latter makes no mention of this.3 It is to be deplored,but unfortunately it seems to be established, that Lavoisierrepeatedly tried to appropriate to himself the merits of others.I do not enter further into this matter here, because I regardit as inessential in the history of the development of chemistry.A man's own period is concerned with his personal qualities,and history with his works. Lavoisier paid with his lifeboth for faults which he committed and for faults which hedid not commit. His own time judged him. Posterity mayregard him with admiration and indulgence.

The different views which were held with respect tooxygen by its discoverers are of interest to us.

Priestley, the worshipper of chance, who asserts that hisgreatest discoveries are due to the latter only, and for whomevery new experiment is a source of new surprises,4 describesin detail how he discovered oxygen and studied its properties.He recognises that combustion proceeds better in this gas thanin any other, and assumes, further, that atmospheric air owesits property of supporting combustion and respiration, to thepresence in it of the gas which he has discovered. He finds,moreover, that it is absorbed by nitric oxide, whence he derivesa method of determining the quantities of oxygen in mixedgases. What does he conclude from all this, however; howdoes he explain these phenomena ? According to him, whena substance burns its phlogiston must be able to separate from

2 Nordenskjold (see Carl Wilhelm Scheele, Nachgelassene Briefe undAufzeichnungen, Stockholm (1892), xxi.) even endeavours to prove that thepriority belongs to Scheele rather than to Priestley, but with this I do notagree. 8 Priestley, The Doctrine of Phlogiston established and that of theComposition of Water refuted. Northumberland (1800), 88. 4 Priestley,Experiments, etc., 2, 29, 39, 42, etc.; A.C.R. 7, 5, 12, 15, etc.

B

18 HISTORY OF CHEMISTRY | > C T . it.

it.5 But in order that this may take place the phlogiston mustfind another substance with which to unite. Combustion ispossible in air; therefore air can take up phlogiston, but thatonly to a certain extent, for after some time it becomes incap-able of supporting combustion any longer. It is then saturatedwith phlogiston. Substances burn in the oxygen gas discoveredby Priestley better than they do in air: it is dcphhgisticated air(a name which Priestley proposes for the new substance) or airdeprived of phlogiston, and it is thus better fitted for taking upphlogiston than ordinary air is. The nitrogen, on the otherhand, which remains behind after the oxygen of the air hasbeen absorbed (and with regard to which Priestley is awarethat it supports neither combustion nor respiration) is air satu-rated with phlogiston, or,phlogisticcited air. With Priestleythe existence of oxygen was no argument against the assump-tion of phlogiston, which he defended till the end of his life.Thus we find him early in the nineteenth century,when the majority of chemists had given up the phlogistontheory, addressing letters to the French Academy fromAmerica (whither he had withdrawn, chiefly on account of hispolitical opinions), in which he requests refutation of his views.0

This was not difficult to give, and although it was refused himby the learned French Society, I must not omit to point outwhat is fallacious in his mode of regarding the matter.

" When a substance burns in air, the latter becomes phlo-gisticated "—if we burn phosphorus, we obtain phosphoricacid (or phosphorous acid), while nitrogen, the phlogisticatedair, remains behind. But if we burn a candle or coal, we obtaina mixture (consisting of nitrogen and carbonic anhydride), partof which can be absorbed by means of alkali, thus exhibiting aphlogisticated air, possessed of properties different from thoseof the preceding one. If we burn phosphorus in dephlogisti-cated air, nothing at all remains behind—the phlogisticatedair vanishes. The contradictions to which Priestley's systemleads, become manifest when it is applied to the facts known

5Kopp, Geschichte. I, 242. 8 Dumas, LeQons. 11$. Priestley, TheDoctrine, etc. x. and xii.


even at that period. Priestley failed to recognise this, becausehis general chemical knowledge was small;7 because heplaced no value upon the results obtained by others ; andbecause he uniformly defended, with the most dogged per-sistence, the ideas which he once adopted.

What, on the other hand, were Scheele's theoretical views;how did he regard oxygen ? Scheele—the ideal of a pure ex-perimental chemist, the discoverer of numberless substances, aman who carried out the most difficult investigations with themost slen der resources, who possessed in the highest degree thefaculty of observation, so that an error can scarcely be found inany of his very numerous researches ; Scheele, who does not,as happens to-day even with the best and most capable obser-vers, overlook half of the points, but grasps the phenomenain their entirety and examines them one by one, and for whomevery new experiment forms a mine of great discoveries—whatintellectual progress did he introduce into our science ?

I must, unfortunately, reply that this was very small. Hisgeneral views are so confused that I only enter with reluct-ance upon the task of giving an outline of them.

Scheele laid down his views chiefly in a small work on aAirand Fire.n The principal difficulty in giving an account of hisopinions is due to the fact that phlogiston, the basis of them, isan unknown substance, to which he can assign every possibleproperty; so that sometimes he endeavours to identify it withan element which is known to us, while at others he seeks toplace it side by side with the medium which the physicists* callether. In consequence, it often seems as if Scheele adoptedthe hypothesis of Cavendish and Kirwan, and by phlogistonunderstood hydrogen,8 and yet, on the other hand, this is notin accord with many of his other views. With him phlogistonis,generally speaking, asubtlesubstance weighing but little5andconcerning which he assumes that it is capable of penetratingthe walls of his vessels. He regards oxygen as a compound ofwater with a hypothetical saline substance,9 in which compound

7Kopp, Geschichte. I, 239. «Ibid. I, 262. 9 Ibid. 1, 261.

20 HISTORY OF CHEMISTRY [LECT. n.

there is, according to him, very little phlogiston. During com-bustion, the phlogiston of the combustible substance escapes,along with this saline substance from the oxygen, in the formofheatand light; the other constituent of the combustiblesub-stance—the metallic calx, for in stance—remains behind, unitedto the water of the oxygen. With Scheele, hydrogen is almostpure phlogiston, which, however, contains a small quantity ofthat hypothetical substance (matter of heat) which is also pre-sent in oxygen. When hydrogen is burned with oxygen, thewater of the latter separates, and the matter of heat from theoxygen unites with the hydrogen—the compound of phlogistonwith little of the matter of heat—and produces heat and light.Thus it was only necessary to add some of that hypotheticalsubstance to hydrogen in order to convert it into heat or light.

Scheele's views are at variance with all the relations byweight, about which he troubled himself very little. Inaccordance with his views, the metallic calx, for instance,should weigh less than the metal plus the oxygen consumed;since the phlogiston of the former escapes, in combinationwith the matter of heat from the latter, as heat and light.The assumption of a ponderable matter of heat, which plays agreat part in his arguments, was at variance with the earlierexperiments of Boerhaave (compare p. 8), so that Scheele, inhis theoretical views, came nearer to those who would retainStahl's doctrine at any cost, than to those who desired an ex-planation of the observed facts, free from preconceived ideas.I leave these, however, and I do so all the more willingly,because I now wish to turn to the ideas and observations ofLavoisier, which are accessible and comprehensible to everyone, and constitute the basis of the chemistry of to-day.

It is not requisite that I should enumerate and describeall the researches of this accomplished investigator ; thiswould exceed the claims which any one could make, in sucha historical sketch as I intend to give. On the other hand,the importance of the philosopher with whom I have now todeal, requires that he should be treated of apart from hiscontemporaries.

I K ] HISTORY OF CHEMISTRY 21

e e i i aracteristic which distinguishes his researches fromof most of the other chemists of his time, is the sys-ic consideration of the quantitative relations, which con-ss, In his hands, a decisive criterion with respect to>mena. Before Lavoisier's time—and I recall this inten-ly Rey,10 and after him, Hooke11 and Mayow,12 hadd t r i e i r attention to the increase in weight on combustion.r i e w s which they stated, come very close to the correctna t i on of the process—Mayow approaching nearest tou t l i . In Mayow's opinion the real substance which> combust ion is the "nitro-aerial spirit'1 which is presenta i r and unites with the metal during calcination. (Theo f this substance is intended to recall its occurrencen n i t r e and in the air.) For any process of combustiona r e requisite, according to him, not only inflammableles (which he designates "sulphureous particles"), butl e presence of this nitro-aerial spirit, the taking up ofexpla ins the increase in weight.13 The establishment

phlogis ton theory, which occurred at this period, and thea n c e that it met with, show to how small an extent thesew e r e then understood, or, indeed, definitely established,•t withstanding this, the priority in the mode of explain-3 p rocess of combustion cannot be claimed for Lavoisier.i s a m e time, the latter did not obtain his views from the3ts above mentioned, whose works were not widely dis-t t e d , and were disregarded. But what places Lavoisiera n y of them is the fact that he not only stated, as they>ne, an idea which could be employed to explain somem e n a , bat that, with the balance in his hand, he alsoa t e d , by means of a series of brilliant investigations,iversal i ty of the principle of the conservation of matter.rs p roved that he possessed not only a speculative head,a t t ie was also a scientific thinker and worker, whoh i s views by means of intelligently conceived experi-a n d , from these, further created new ideas.

sinbic Club Reprints, No. ir. " Ibid. No. 5. 12 Ibid. No. 17.n Kopp, Geschichte. 3, 135.


It cannot be asserted, at least 1 have not been able to readit out of his works, that Lavoisier stated the principle of theindestructibility of matter as an axiom. But he recognisedthe truth of the law ; for why, otherwise, should he have hadconstructed for one of his first investigations, " On the Con-version of Water into Earth," a balance which surpassed inaccuracy everything that was known at the time in suchinstruments ? He recognises the truth of the principle, but hedoes not state it. Proof is to be sought in experiments, andnot in words, and so he holds back until a suitable oppor-tunity; just as he held back the attack upon the phlogistontheory until he saw that the moment had arrived when, witha single blow, he could overthrow the house of cards, heldtogether as it was by decaying preconceived notions only.Accordingly we find his ideas about this fundamental doctrineexpressed in his works only here and there, where it is neces-sary for him to furnish, at once, grounds for an opinion, theexperiments in support of which are not yet completed. Aninstance is furnished, for example, in his first treatise on thecomposition of water (which substance he finds to consist ofhydrogen and oxygen), where he would like to prove that theweight of the water produced by the union of hydrogen andoxygen is equal to the sum of the weights of the two gasesemployed—a point which he had not, at that time, establishedby experiment. He there states that this necessarily follows,since the whole is equal to its parts,14 and nothing but wateris produced by the combustion. At that time the priority ofthis discovery was disputed, and had, without injustice, beenascribed to Cavendish and not to Lavoisier. As it appearsfrom a letter of Blagden's15 and also from a letter from Laplaceto De Luc,10 Lavoisier was acquainted with Cavendish's in-vestigation prior to his own experiments, and hastened topublish his results. It is in this way that we obtain knowledgeof a fundamental doctrine which had long been clear to him,

H Lavoisier, Oeuvres. 2, 339* 15 Crell's Annalen. 1786, 1, 58,18 Jfbpp, Beitnige.3, 271,

LECT. n.] HISTORY OF CHEMISTRY 23

but which was only at once adopted by a. very few chemists.At a later dateLavoisier expresses himself still more distinctly,stating that the substances employed and the products ob-tained can be brought into an algebraical equation ; and thatif any term is unknown it can be calculated.17" This is thefirst idea of those equations which we now employ daily.

Still we do not foresee the course of the development ofthis great thinker's ideas, if we follow him, in a general wayat least, from his first experiments onwards. It may be said,however, that the development of his views is that of thechemistry of his period.

One of Lavoisier's earliest investigations deals with thesupposed conversion of water into earth.18 He points out theinaccuracy of this supposition, which was generally held at thetime. It is interesting to follow him in his experiments. Heseals up a quantity of water in a glass vessel, which was knownat that time as a pelican and is so arranged that a glass tubewhich is fused on to its neck above, leads the condensed waterback again into the body of the vessel. He weighs this empty,and then with water in it, after closing the single opening bymeans of a glass stopper. He then distils the water for ahundred days. The formation of earth appears after a month,but he proceeds with the distillation until the quantity formedseems to be sufficient. He now weighs the apparatus again,and finds it to be just as heavy as before; whence he concludesthat no fire material has penetrated i t ; for otherwise, he con-siders that the weight must have been increased. He nextopens it, weighs the water along with the earth, and finds itsweight increased, but that of the glass diminished. This leadshim to the assumption that the glass has been attacked by thewater, and that the formation of earth is not a conversion buta decomposition. His conclusions accord exactly with hisexperiments; yet he does not permit himself to be blindlyled by them. Thus he finds the increase in the weight ofthe water to be a few grains more than the decrease in the

V Dymas, Lemons. 157. m {.avoisier, Oetivres. 2, r.

H HISTORY OF CHEMISTRY [LECT. II.

weight of the glass. Another person might perhaps have con-cluded from this that there had been a production of matter;Lavoisier, however, explains it as an error of experiment.Although this is a bold view for the period, yet it showsthat he°is guided by profound ideas, and understands how tocriticise his own experiments. All later investigations haveconfirmed the accuracy of his explanation.

Scheele 19 on his part, was occupied about the same timewith similar investigations, and arrived at the same results ;but the mode in which the Swedish chemist conducts theexperiment is very different. He analyses the earth andfinds that it consists of the same substances as the glass inwhich the water was heated.

A later paper of Lavoisier's treats of the increase in weighton combustion. As early as 1772, he hands in to the FrenchAcademy a sealed paper, in which he shows that the productsof the combustion of phosphorus and sulphur are heavier thanthe substances burned. This he attributes to an absorption ofair (of air, because oxygen had not then been discovered).20

In an investigation on the calcination of tin,21 he causes thisoperation to take place in closed vessels; these he weighsbefore and after, without observing any difference ; whencehe concludes that no fire material is taken up. He shows,further, that the metal has increased in weight by just asmuch as the air has lost.

Oxygen is discovered shortly afterwards, whereupon La-voisier repeats the experiments of Priestley and of Scheele ;but his conclusions are totally different from those of the twoother chemists. He is already prepared for this discovery,and, with him, it becomes the foundation of a new theory.Oxygen he at once recognises as that part of the air whichunites with the combustible substance during its combustion ;and he calls it u air erninemment respirable" In the samepaper he shows that fixed air is a compound of carbon withthis air, and that the latter is also contained in nitre.22

19 Dumas, Lemons. 129. *> Lavoisier, Qeuvres. 2, 103. 21 Ibi<J,2, 105. 22 Ibid. z} 128,

LECT. IL] HISTORY OF CHEMISTRY 25

Some time afterwards, in 1777, he advances a completetheory of combustion.23 He says :—

1. Heat and light are disengaged during every combustion.2. Substances burn only in pure air.3. This is used up in the combustion, and the increase in

weight of the substance burned is equal to the loss in weightof the air.

4. The combustible substance, by its combination withpure air, is usually converted into an acid; the metals, onthe other hand, are converted into metallic calces.

Tn a paper on the composition of nitric acid, Lavoisiertries to prove, in the case of this acid also, the last of theabove statements, which is of importance later when wecome to deal with theories of acids.24 He there shows thatthis acid contains oxygen, while he is not aware that it containsnitrogen. The latter fact was discovered a few years afterwardsby Cavendish, on passing electric sparks through mixtures ofoxygen and nitrogen,25 whereby nitric acid was obtained.

Lavoisier at this time groups together the facts that car-bonic acid (carbonic anhydride) consists of carbon and oxygen,sulphuric acid (sulphuric anhydride) of sulphur and oxygen,phosphoric acid (phosphoric anhydride) of phosphorus andoxygen, and nitric acid of " air nitreux " (nitrous air, i.e.nitric oxide) and oxygen. He further shows that an acid isobtained by treating sugar with nitric acid (that is, bysupplying oxygen), and from this he concludes that Priest-ley's dephlogisticated air must contain the acidifying principle{Principe \acidifiant—principe oxygine)^ From jthis timeonwards, he regards all acids as consisting of a basis, orradical, and of this firincipe oxygine. His u air pur" on theother hand, besides this acidifying principle, contains alsothe " rnatiere de chaleur " (matter of heat).

I t is certainly remarkable to find even Lavoisier speakingof a fire material, which he afterwards designates " cahrique,"and of which I shall explain the signification.

2:J Lavoisier, Oeuvres. 2, 226. ** Ibid. 2, 129. 28 Kopp,Geschicjite. £, 231, 26 Layojsfer, Oejjvres. 2, 249.

2 6 HISTORY OF CHEMISTRY [LECT. H.

The "matiere die feu " is not possessed of weight. Lavoisiershows this by burning phosphorus in closed vessels, wherebyheat is liberated but no loss of weight takes place.27 Further,he causes water to freeze in closed vessels, and here, likewise,he finds no change of weight. Since he is aware, from his ownexperiments, that heat is disengaged in the process, he con-siders that he is justified in assuming that heat has no weight.A better notion of what Lavoisier calls "matieredufeu" willbe obtained by my stating his views on the constitution ofmatter, which I take from his " Reflexions sur le Phlogis-tique." 2S According to him, matter consists of small particleswhich do not touch one another, since, otherwise, a diminu-tion of volume by lowering of temperature could not beexplained:29 the matter of heat exists between these particles.The hotter a substance is the more of the matter of heatdoes it contain. In the investigations into the specific heatsof various substances, carried out along with Laplace, Lavoi-sier further proved that, for a like increase in temperature,substances do not take up like quantities of the matter ofheat.30 Into the consideration of these experiments, as wellas of those on the heat of combustion,311 do not enter here.Lavoisier knows that, by the addition of heat, ice is first con-verted into water, and the water then into steam. Hencegases contain most of the matter of heat. This is what weshould understand when he says that his a air pur" consistsof the acidifying principle and the matter of heat. Duringcombustion the former unites with the combustible substance,and the matter of heat is liberated. It produces heat and light.

The following statement is very characteristic of Lavoi-sier's standpoint :—32 " Heat is the energy which results fromthe imperceptible movements of the molecules of a substance ;it is the sum of the products of the mass of each moleculeinto the square of its velocity." Here we find him in com-

27 Lavoisier, Oeuvres. 2, 618. >28 Ibid. 2, 623. ^ Compare alsoLavoisier, Traite de Chimie. 1, etc. -10 Lavqisier, Qetivres. 2,* Jbic}. 3, 31$ and 724. ™ Tfrd, 3, 286,


plete agreement with the fundamental doctrines of themechanical theory of heat. His views with respect to theheat disengaged during combustion, although not quiteaccurate, are also of great importance. He considers83 thatwhen a solid substance (phosphorus) burns in a gas (oxygen),and the product of the combustion is solid (phosphoricanhydride), the disengagement of heat is due to the con-densation which the gas has undergone, in order that itmay become solid. If the product is gaseous (carbonicanhydride), he attributes the disengagement of heat to thealteration of the specific heat. He advances the general viewthat the heat of combustion must be greatest when two gasesunite to form a solid substance. How correctly he understoodthe application of these fundamental ideas, is shown by hismode of explaining tfte lowering of temperature produced bydissolving salts in water. Lavoisier assumes, as we do, thatit is the change of state of aggregation which occasions theabsorption of heat.34 He shows, further, that the evolutionof heat which occurs on mixing sulphuric acid with water, isaccompanied by a decrease in volume, and that both maximacoincide ; so that theory and experiment agree.

I must not, however, enter too deeply into these matters,which belong, partly at least, to physics ; and, therefore, Ireturn to his purely chemical investigations.

Lavoisier adheres to Boyle's definition of an element,85

which we retain to-day. With him, an element is anysubstance which cannot be further decomposed.8(J Whatthe significance of this definition is, and of what importancethis idea of an element has become for the whole of naturalscience, has been specially pointed out by Helmholtz.87

The metals were first regarded as elements by Lavoisier.88

In a long paper he disputes the prevailing view, which as-sumed the existence of phlogiston in the metals. These inter-

33 Lavoisier, Oeuvres. 2, 647. ** Ibid. 2, 654. •'» Kopp,GescMchte. 2, 275. m Methode de Nomenclature Chimique. ParisC1787), 17- :i7 Tageblatt der Naturforscherver mmlung in Innsbruck,J869, 37, 38 Lavoisier, Oeuvres. 2, 623.


esting disquisitions, which contain the annihilation of thepreceding system, only appear towards the end of his shortand brilliant scientific career. At first there was no explana-tion forthcoming for a series of phenomena which were inagreement with Kirwan's phlogiston theory. I refer to thebehaviour of the metals towards acids ; to the hydrogenliberated ; and to the reductions carried out by Priestley bymeans of hydrogen. It is only after the composition of waterhas been ascertained by Cavendish, Watt, and Lavoisier,39

that Laplace arrives at the idea (as Lavoisier relates40) thaton dissolving metals by means of acids, water is decomposed—that the hydrogen is, therefore, evolved from the water,while the oxygen of the latter unites with the metal to forman oxide- The«phenomena of reduction now become clearalso. The hydrogen unites with the oxygen of the metallicoxide, forming water, and the metal remains behind. Lavoi-sier tries to prove all these points by a series of excellentexperiments. His investigations upon the decomposition ofwater, are, especially, of extreme interest.41 He passes watervapour over weighed iron turnings, heated to redness, andcollects the hydrogen in an eudiometer. In this case also,he weighs everything—the water, the gain of the iron, andthe hydrogen. In this way, he succeeds in finding out thequantitative composition of water ; and the latter, alongwith the quantitative composition of carbonic anhydride,which he determines somewhat later,42 constitute the start-ing point for his researches on organic analysis.43

I wish to say at least a few words with regard to theseresearches ; since, even although the numbers obtained arenot very exact, the methods are so important that I cannotpass them by unmentioned.

Lavoisier places a weighed piece of charcoal in a dish,under a bell-jar containing a measured volume of oxygen, and

39 With respect to the part played by each in this most importantdiscovery, compare Kopp, Die Entdeckung der Zusammensetzung desWassers. Beitrage. 3, 235. ^ Lavoisier? Oeuvres. 2, 342, 41 Ibid. 2, 360,42 Ibid. 2 403. tt IbicJ. 2, j86f-


standing over mercury. Besides the charcoal, the dish con-tains a trace of phosphorus and tinder. By means of a bent,red-hot, iron wire, he ignites the phosphorus, which communi-cates the combustion to the tinder, and thus to the charcoal.After the combustion of the latter has ceased, he takes out thedish and weighs it, and so finds the quantity of the charcoalburned. He measures the volume of gas in the bell-jar,absorbs the carbonic anhydride by means of potash, andmeasures again. In this way he obtains the volumes of thecarbonic anhydride produced and of the oxygen employedin the combustion; and hence, all the data necessary tocalculate the composition of carbonic anhydride.

This he makes use of in carrying out the analysis of organicsubstances, such as spirit of wine, oil, and wax. He had satis-fied himself, at an earlier date, that water and carbonic anhy-dride are aloneproduced by the combustion of these substances,whence he had quite correctly concluded that they containcarbon, hydrogen, and oxygen only. In the estimation of theirquantitative composition, Lavoisier employs an apparatussimilar to that indicated above. For example, he places a spiritlamp, which he weighs before and after, under the bell-jar, andburns some of the spirit; he also determines the quantities ofcarbonic anhydride produced and of oxygen employed, andfrom these data he is able to calculate the composition of thespirit.

With this I shall close the consideration of Lavoisier'schemical investigations. A superficial estimate of his merits isall that I have been able to give. It is only by a minute studyof his works that any complete idea of his significance, and anyproper understanding of how much our science owes to his greatintellect, can be obtained. There are, however, certain direc-tions of his activity that I have not even mentioned, as, forexample, the researches on respiration, about which I still wishto say a few words. Priestley already knew that oxygen wasnecessary for respiration.44 Lavoisier shows how it is used upin the lungs, in the formation of carbonic anhydride and of

44 Compare p. 17.

30 HISTORY OF CHEMISTRY [LECT. nv

water, and how this process, which he properly classes as oneof combustion, furnishes to man the heat necessary for hisexistence.45 He demonstrates that the expired carbonic anhy-dride derives its carbon from the blood itself; that in the pro-cess of respiration we thus, to a certain extent, burn ourselves,and would consume ourselves if we did not replace, by meansof our food, that which we have burned. Since he next finds,by special experiments, that in strained activity the breathingis hastened, and that the consumption of carbon is therebyincreased, he arrives at the conclusion that the poor man,compelled as he is to work, consumes more carbon than theindolent rich man; but that the latter, by an unfortunatecircumstance in the division of worldly possessions, can satisfyhis smaller requirements—and that too by means of betterfood—much more readily than the poor workman can. Hetherefore calls upon society to remedy this evil by means ofits institutions, to improve the lot of the poorer classes, andin this way to smooth away, as far as possible, those inequali-ties which apparently are established in nature. He closesthis ingenious treatise with the words :—46

" I t is not indispensable, in order to deserve well ofhumanity and to pay his tribute to his country, that a manshould be called to those public and pompous functions whichco-operate in the organisation and regeneration of empires.The physicist, in the quiet of his laboratory and of his study,can also exercise patriotic functions; he can hope to diminishby his labours the many ills which afflict the human species,and to increase human pleasures and prosperity. And if heshould only contribute, by the new methods which he mayhave shown, to the lengthening of the mean age of man bya few years, or even by a few days, he also may aspire tothe glorious title of benefactor of humanity."

His own time rewarded him badly for his endeavours.—Four years later, in 1794, he was guillotined by order of theRevolutionary Committee.

45 Lavoisier, Oeuvres. 2, 331. 40 Ibid. 2, 703.

L E C T U R E III.

CHEMICAL NOMENCLATURE — TABLES OF AFFINITY — BERTHOLLET'SVIEWS—CONTROVERSY REGARDING CONSTANT COMPOSITION.

IT will readily be understood why a new era dates fromLavoisier, and why the latter is designated the reformer ofchemistry, if we consider what were the theoretical views heldbefore his time, and what they were at the period of his death.

Lavoisier lived to have the satisfaction of seeing his viewsgenerally recognised, in France at least; they graduallygained ground also in England and in Germany, where hisworks were translated, so that it may be justly said thatphlogiston, at the beginning of the nineteenth century, haddisappeared from scientific works.

Lavoisier not only overthrew the old theory, but it is hischief merit that he introduced a new one in its place, and itis perhaps advisable to state here the most important headsof his theory.

1. In all chemical reactions it is the kind of matter alonethat is changed, whilst its quantity remains constant; conse-quently, the substances employed and the products obtainedmay be represented by an algebraic equation in which, ifthere is any unknown term, this may be calculated.

2. In the process of combustion the burning substanceunites with oxygen, whereby an acid is usually produced.In the combustion of the metals, metallic calces are produced.

3. All acids contain oxygen, united, as he expresses it, witha basis or radical which, in inorganic substances, is usually anelement, but in organic substances is composed of carbon andhydrogen, and frequently contains also nitrogen or phos-phorus.

If we contrast these three statements with the views ofthe phlogistians, i.e., with the theories which prevailed prior

31

32 HISTORY OF CHEMISTRY [LECT. HI.

to Lavoisier, we shall appreciate the reformation introducedby him into chemical science. The direction of chemicalthought was entirely changed, and the facts hitherto ascer-tained appeared in a new light. It was necessary, in a sense,to translate them in order to understand them, and as it wasrecognised that a new language was required for their propercomprehension, the need of a system of chemical nomencla-ture made itself apparent.

I pass over all the attempts which had been made prior tothis period to secure a uniform mode of expression, as thesedid not lead to any results worth mentioning, and as theyoccurred during a period to which I can only slightly refer.I wish, however, to mention that Bergman repeatedly ap-proached the French chemists with a view to securinguniformity in the naming of substances. In 1782 Guytonde Morveau, probably stimulated by Bergman's suggestions,travelled to Paris, and laid before the Academy there a pro-posed system of chemical nomenclature. This system con-tained much that was new and good ; but it could not securethe approval of the principal chemists of the period, as itassumed the existence of phlogiston, which even at that timewas vigorously disputed by Lavoisier. The latter afterwardssucceeded in convincing Guyton of the accuracy of his new-views. Guyton agreed to reconstruct his system, and in 1787,in-conjunction with Lavoisier, Berthollet, and Fourcroy, hepublished the Nomenclature Chimique. As this system em-bodies the principles and constitutes the basis of the chemicalnomenclature now employed, I cannot pass it by withoutmention, and I shall therefore state at least its more import-ant features. In doing so, it will frequently be necessary toemploy French words. For practically all of these there areexact English equivalents.

Substances are all divided intoielements and compounds.Amongst the former there are included all those substanceswhich could not then be further decomposed, and these areclassed under five headings. Of these headings the first em-braces those bodies which are of very common occurrence, and

LR(:T, ,„.] HISTORY O F CHEMISTRY 33

whose behaviour seems to indicate that they are not decom-posable. Examples of these are:—-I, Heat (Cahrique); 2,Light ; 3, Oxygen ; 4, Hydrogen ; 5, Nitrogen {Azote). Thesecond class contains the acidifiable bases, such as sulphur,phosphorus, carbon, etc. The third embraces the metals ; thefourth the earths ; and the fifth the alkalies, which, as is wellknown, had not at that time been decomposed. The namesof the substances belonging to the second, third, and fourthclasses are for the most part unchanged ; the alkalies arecalled potash, soda, and ammonia.1 For all these substances,which, with the exception of ammonia, were regarded aselements, the authors observed the principle of designatingeach by a single word.

The radicals constitute an appendix to the elements.These are substances which they regard as decomposable,but which exhibit certain resemblances to elementary bodies.

Next come the binary substances, consisting, as they do,of two elements. The acids occur in this class. According tothe theory of Lavoisier, the acids all contain oxygen. Theirnames are in each case composed of two words, of which thefirst is common to them all and indicates their acid character(acide)} while the second is a specific name Indicating theelement or radical occurring in each. Thus we have acidessulfimque} carbonique, pkosphortquey nitrique, etc. Two acidscontaining the same element o r radical are distinguished bythe different termination of t h e specific name ; that contain-ing the smaller proportion of oxygen receiving the termina-tion eux, whereby such, names as acides suljfureux^ nitreux^etc.,. are obtained.2 Hydrochloric acid h called acide muria*tique, and the existence of oxygen in it is assumed ; whileoxygen is supposed to be present in still greater quantity inchlorine—the acide muriaiique ^xigene?

The names of the binary substances of the second group,i.e., of the basic compounds containing oxygen, arc formedin a manner exactly similar* JFor these the general designa-

1 Nomenclature Chimi<|ue. 67. *! Ibid. 8$»S6. :I Ibid. 87.C


tion oxides is introduced, and to this word the specific nameis appended in the genitive ; for example, oxide de zinc)

oxide de plomb, etc.The remaining binary compounds are distinguished as

sulphur, phosphorus, carbon, etc., compounds, and receivethe class names: sulftires) phosphures, carbures, etc.

Compounds of the metals with one another are calledalliages (alloys), the expression amalgames being retained,however, for mercury alloys.

Amongst the ternary compounds, the salts alone need bementioned. They obtain their class namesfrom the acids fromwhich they are derived, and are called accordingly : sulfates,nitrates, phosphates. The termination ate becomes ite whenthe salts are derived from the acid poorer in oxygen instead offrom that richer in oxygen. The name of the base is appended ;for example, sulfate de zmc) de baryte, etc. When the salthas an acid reaction, the word acidule is employed ,* on theother hand they call basic salts sursature de soude, etc.4

Relatively few double salts were known at that time. Thedesignation introduced for these was not very convenient; forexample, tartar emetic was called u tartrite de potasse tenantd^antimoine " 5 (tartrate of potash containing antimony).

This general review may suffice. Berzelius, as is wellknown, considerably extended these beginnings of a rationalnomenclature, and I shall refer to some of his improvementsand expansions when considering his period.

On comparing the science of to-day with what I stated inthe preceding lecture regarding Lavoisier's views, it will bepossible to judge of the extent to which the latter have beenretained. Lavoisier's theories required modification on severalpoints ; but on others his ideas were attacked without result,since it has been necessary to return to them again. ThusLavoisier's theory of acids is now abandoned by the majorityof chemists. The introduction of the new views only took

4 Nomenclature Chimique. 93 and 97. 6 Ibid. 52; on p. 235 it Iscalled "tartrite de potasse antimonie" (antimoniated tartrate of potash).

LPXT. in.] HISTORY OF CHEMISTRY 35

place, however, long after his death, and therefore I postponethe consideration of this matter to a later lecture. We mustoccupy ourselves here with another attack upon Lavoisier,which was eventually decided in his favour, and is of im-portance on this account, that through it a strict separationof mixtures from compounds was first brought about.

This attack had to do with the question whether chemicalcombinations are possible in all proportions, or whether sub-stances can combine in certain fixed proportions only. Thelatter view, as is evidenced by many of his investigations,was assumed by Lavoisier ; and indeed it seems to have beenaccepted as self-evident by all the chemists of his time, withouthaving been proved. But a book appeared in 1803 whichattracted the greatest attention in the scientific world, both onaccount of its contents and of the form in which these were setforth ; and in this book, amongst other things, the constancyof chemical proportions was denied, on the ground both oftheoretical speculations and of experimental investigations.

The work to which I refer is Berthollet's Stotique Chimique,and if I am to render intelligible the importance of the attackwhich it contains, I must give at least a slight sketch ofBerthollet's extremely interesting general theoretical ideas. Iextract these from the work just mentioned, and from somescattered essays by its author upon the same subject.0

Berthollet'sbook will always be of importance in chemistry,chiefly because the fundamental doctrines to which the authorsubordinates all chemical reactions, are the principles of me-chanics and of physics ; and these must necessarily possess avalue in chemistry. And even although many of RertholletJsconclusions do not harmonise with experiment, and have longsince been disproved, still this does not damage the basis ofhis conceptions.

The work as a whole is chiefly directed against the falseview which had been adopted with regard to the affinities ofsubstances; and against the misuse which was made at the

8 Ann. Chim. 36, 302 ; 37, 151 and 221 ; 38, 113 ; 39, %


timeof so-called tables of affinity. The latter were tables whichpurported to express the strength of the affinity of substances,and they had been drawn up by a large number of chemists.The earliest originated with Geoffroy,7 and dated from the year1718. It consisted of various tables in which the other sub-stances were so arranged with respect to a particular one, thateach preceding substance always decomposed the compoundwhich the next succeeding one formed with this particularsubstance. Thus, for example, his table for acids in generalran : fixed alkali; volatile alkali; earths ; metals. The con-struction of such tables was one of the chief occupations ofchemists in the middle of the eighteenth century. With themthey associated the erroneous view that the affinity of one sub-stance towards another is invariable; andit was only by degreesthat chemists were convinced that this was an error. In 1773Beaume pointed out that the affinities were different at ordin-ary and at very high temperatures (in the wet and dry ways),and that for each substance it would thus be necessary to con-struct two tables which should express its behaviour towardsall other substances under these two different sets of condi-tions.8 Bergman undertook this task,9 and it is truly aston-ishing what enormous pains he took in carrying it out. Foreach substance he constructed two tables, in which he com-pared its behaviour towards fifty-eight others, and these latterwere so arranged that each preceding substance decomposedthe compound which the next succeeding one formed withthe particular substance concerned. From these tables it was,seemingly, possible to foretell the results of all reactions;hence they were held in great esteem. When a new sub-stance was discovered, such a table of affinity was at onceconstructed for it, and even Lavoisier respected this usage onthe occasion of his investigation of oxygen, although hepointed out at the time that a similar table would really berequired for every degree of temperature.10

7 Kopp, Geschichte. 2, 296. 8 Ibid. 2, 299. s Ibid. 2, 301.10 Lavoisier, Oeuvres. 2, 546,

LECT. in.] HISTORY OF CHEMISTRY 37

Berthollet, however, is the first to point out the error towhich chemists had committed themselves in drawing upthese tables. He destroys their significance by advancingthe doctrine that the effective action of a substance is relatedto its mass.

Berthollet illustrates, especially by reference to salts, thelaws in accordance with which chemical compounds areformed. He assumes that the same chemical effect is alwaysconnected with the neutralisation of any given quantity of aparticular base (or acid). He represents this effect as theproduct of the affinity A) and the saturating capacity «S(thatis, for example, the quantity of acid necessary to neutralisea unit of weight of alkali). This gives—

AS^ ConstantA Constant

A ~ — s -that is, the affinities of two acids are inversely proportionalto their saturating capacities,11 or just the reverse of whatBergman had regarded as correct.12

But, according to Berthollet, the quantity Q of any sub-stance present exercises, quite generally, an influence uponthe chemical action, which he considers to be proportional tothe product of Q into the affinity of the substance. He callsthis product the chemical mass.13 In the case of acids, thechemical mass may also be stated as proportional to the pro-duct of the saturating capacity S into Q, the quantity present,as Berthollet likewise points out.14

The effects produced by affinity do not, however, dependexclusively upon the chemical mass : they are varied besidesby the state of condensation of the substance concerned, andare subject, therefore, to the physical conditions of the experi-ment (to the pressure, temperature, etc.). As regards the stateof condensation of matter, it is, according to Berthollet, a con-

11 Berthollet, Statique Chiroique. I, 71 ; E. I, 45. vi Kopp, Ges-chichte. 2, 314. u Stat. Chim. I, 72 ; E. I, 45. 14 Ibid, x, 16 ; E,I, XXIV.

38 HISTORY OF CHEMISTRY [LKCT. m.

sequence of the two opposed forces, cohesion and elasticity.The predominance of the former brings about the solid state,and that of the latter, the gaseous state ; while in liquids abalance exists between the two. If all acids were in the samestate of condensation, then that one should be regarded asthe strongest, of which the smallest quantity is necessary tosaturate a given weight of a base ; or, as we should now say,of which the equivalent is smallest.

Berthollet applies these principles especially to simple andto double decompositions. According to him, when we addan acid to a dissolved salt, a partition of the base takes placebetween the two acids in the ratio of their affinities, that is, inthe ratio of their masse chimique}5 Both salts and both acidsare thus present in the solution ; yet this only holds whenboth salts possess approximately the same solubility, becausea balance is then established which is dependent not onlyupon the strengths of the acids, but also, and in particular,upon the quantities of each present. He further draws atten-tion to the fact that conviction as to the accuracy of thisview cannot be obtained by evaporating the solution andpermitting the salts to crystallise out, because as soon as thequantity of water is no longer sufficient for complete solution,the phenomena which take place depend chiefly upon theforces of cohesion and crystallisation, that is, upon the dif-ferent solubilities of the substances.10

Thus, upon crystallising out after mixing a solution ofpotassium nitrate with sulphuric acid, the more sparinglysoluble salt potassium sulphate is alone obtained ; whereas,according to Berthollet, potassium nitrate and potassiumsulphate are both present in the solution.

When one salt is much more soluble than the other, it isthe less soluble one that is principally formed ; and if one isquite insoluble, there is no partition, but complete decomposi-tion. In this way Berthollet explains, for example, the com-plete precipitation of barium nitrate by means of sulphuric

15 Stat. Chim. I, 75 ; E. I, 49. 16 Ibid. I, 82 ; E. I, 54.

. in.] HISTORY OF CHEMISTRY 39

acid. The barium sulphate, in consequence of its insolu-bility, is removed from the reaction ; a constantly progres-sing partition taking place until the whole of the bariumsulphate is precipitated.17

A similar thing occurs in the cases of volatile acids orbases. In this case also a partition takes place, in the ratio ofthe masse chimique; but as one product (carbonic anhydride,for example) escapes, the decomposition proceeds to the end.18

Nevertheless, it is only in cases of extreme preponderanceof cohesion (insolubility) or of elasticity (volatility) thatcomplete decompositions are observed. Cases of partialdecomposition are far more frequent. Thus, according toBerthollet, calcium salts cannot be completely precipitatedby means of oxalic acid.10

His views regarding double decomposition are similar.Four salts are, in general, produced in these cases, and theformation of two only is confined to cases where the cohe-sion, or solubility, is totally different.

This furnishes the explanation of the so-called reversiblereactions. Amongst these reactions there are, in the firstplace, the various crystallisations which may be obtained atdifferent temperatures from the same mixture of salts, ifthese salts possess solubilities which vary greatly from oneanother with changes in temperature. Berthollet adducesseveral examples of this kind,20 and I shall mention one ofthem. If a solution contains soda, magnesia, sulphuric acid,and hydrochloric acid, Glauber's salt crystallises out.from itat a very low temperature (o° C), whereas sodium chlorideis obtained on evaporation. Hence, at o°, magnesium suhphate and sodium chloride must change into sodium sulphateand magnesium chloride, whilst at high temperatures thereverse takes place.

In the same way Berthollet is also able to explain the phe-

17 Stat. Chim. I, 78 ; E. I, 51. ]a Ann. Ghim. 36, 314. U) StatChim. I, 78-79; E. 1, $1-52. m IW'3« li 101-102 and 129-130; E. 1,71-72 and 395-396.

40 HISTORY OF CHEMISTRY [LECT. m.

nomena which are dominated, according to Bergman'sviews, by the affinities " in the wet and dry ways." Thus,for example, dissolved silicates are decomposed by almostall acids, whereas, on the other hand, silicic acid drivesmost acids out of their salts at a red heat.

But Berthollet goes still further. Cohesion determines notmerely the nature of the compound formed, but also the pro-portions in which combination takes place. His conceptionof a chemical compound is not associated with the constancyof proportions which had been assumed prior to his time.On the contrary, there exist, in his view, chemical compoundswith all possible proportions21; and it is only special reasons,such, for example, as considerable condensation on combina-tion (that is, change in the cohesion of the constituents),which occasion constant proportions. Thus hydrogen onlycombines with oxygen in a definite proportion because water,the product of the combustion, is liquid, and the contractionwhich takes place presents too great an obstacle to the pro-duction of other compounds.22 But if, on combination, thereis no change in cohesion, or only a slight change, then com-pounds are formed with variable proportions. As examplesof these he mentions alloys, glasses, and solutions. He saysthat in these cases the limits are determined solely by thequantities required for mutual saturation, but that, betweenthese limits, the most varying proportions occur.28

It will be observed that Berthollet thus classes solutionsand alloys amongst compounds, and it will now be understoodhow he was able to distinguish amongst them some with vary-ing proportions. But it is much more remarkable that Berthol-let also assumes variable proportions amongst the oxides. Inone of his essays upon the laws of affinity,24 in which he speaksof metallic precipitations, he assumes, in accordance with hisprinciples, that the two metals distribute themselves over theoxygen : there are thus formed, according to him, oxides con-

31 Stat. Chim. I, 373 ; E. I, 282. ** Ibid. I, 367 ; E. I, 276. ^ IbidX. 373-374 5 E- *i 282-283. ** Ann. Chim. 37, 221.

LECT. in.] HISTORY OF CHEMISTRY 41

tain ing different quantities of oxygen. He afterwards developshis views upon this point still more clearly.25 He says : u Ihave to show, then, that the proportions of oxygen in theoxides depend upon the same conditions as those in othercompounds ; that these proportions may vary progressivelyfrom the limit at which combination becomes possible up tothat at which it attains the highest degree.'T The limits them-selves h e regards as determined by circumstances of cohesion.—In the same way he believes in the existence of salts with vary-ing quantities of base. If the base is precipitated, by means ofan alkali, from a salt with an insoluble base, he supposes that acertain quantity of acid is precipitated along with the base, andthat this quantity is variable.26 In short, compounds with con-stant proportions are, according to Berthollet, exceptions, andthe proportions in which substances combine are, as a rule,dependent upon the conditions of the experiment.

If we summarise Berthollet ?s views once more, we maysay that affinity appeared to him to be a force identical withgravity,27 whose phenomena are more varied only because itsets the molecules themselves into motion, and its effects aredependent upon the size and shape of the particles. In apply-ing these physical principles to chemical reactions, he arrivesat the conception of chemical mass, which he defines as theproduct of affinity and quantity present. The chemical effectsdepend upon the magnitude of this chemical mass, and uponthe cohesion of the substance, that is, upon its solubility andits greater or less volatility. This then leads him further totwo general conclusions :—

1. Tables of affinity are useless, since in them affinity isassumed to be constant and independent ofphysicalconditions.

2. There are compounds with varying and progressivelyincreasing proportions of their constituents.

The first of these conclusions is adopted generally, and wefind that the tables of affinity disappear soon after the appear-

'^ Stat. Chim. 2, 370; E. 2, 316. 2(i Ibid. 1, 86 ; E. I, 58. * Ibid.1, T ; E. I, vii.


ance of Berthollet's Statique Chimique. The second, on theother hand, meets with vigorous contradiction ; Proust, afellow-countryman of Berthollet's, especially opposing theviews there stated. Thus there arises that renowned contro-versy between these savants which is remarkable not onlyfor the talent which the opponents exhibit, but also for theextreme politeness which is observed on both sides.

Berthollet was at that time held in high esteem by thescientific world. The sagacity which he had manifested in ahigh degree in the working out of his book was justly admired.It will be understood, then, that it was no small undertaking toattack views which he had stated with much confidence, andhad endeavoured to prove by experiments. At the same timeI may state here that the experimental part, especially, of theStatique Chimique leaves much to be desired. When Bertholletasserts that in the oxidation of the metals oxygen compoundsare formed with widely varying proportions, the reason of thisis that he analysed the crude product directly, and did notfirst try to satisfy himself that he was not dealing (as wasgenerally the case) with a mixture. If, in addition to this,the backward condition in which quantitative analysis stoodat that time is taken into consideration, it will then be under-stood how Berthollet arrived at these erroneous results.

Proust, on the contrary, proceeded very cautiously. Heendeavoured to find proofs of the purity of his substances, andbestowed the greatest care upon the determination of the con-stituents. He thus succeeded in discovering the hydroxides,which had hitherto been entirely overlooked28 and were re-garded as oxides containing a special proportion of oxygen.We are indebted to Proust for investigations of the most ofthe metals, which he usually published under the title Faitspour servir a Vhistoire, etc.29 Further, he wrote detailedtreatises upon the sulphur and oxygen compounds,30 in which

>i8 Ann. Chim. 32, 41 ; Journ. de Phys. 59, 347. y9 Journ. de Phys.51, *73; 52> 409; 55, 32S, 457 5 Ann. Chim. 32, 26; 38, 146 ; 60, 260,etc. m Journ. de Phys. 59, 321 ; Sulphur compounds, ibid. 53, 89 ; 54,89 ; 59. 265-

LECT. in.J HISTORY OF CHEMISTRY 43

he shows that many metals form only a single oxide ; thatmany, however, form two ; and that in those cases where threeoxides exist, the intermediate one can be regarded as a com-pound of the other two.31 In the same way, he endeavours todetect the error in Berthollet?s view regarding the existence ofsulphur compounds with variable proportions of sulphur. Inall these investigations he emphasises the distinction betweenmixtures and compounds. He says that the latter are charac-terised by quite definite proportions, which hold for com-pounds occurring in nature as well as for those obtained inthe laboratory, and that this pondus naturae lies just as littlewithin the discretion of the chemist as does the law ofaffinity, which governs all combinations.32

But Berthollet also responds by means of facts. Heexamines the carbonates of the-alkali metals,33 and finds thatwhen the base is saturated by carbonic anhydride underpressure, crystals are obtained which differ in composi-tion from the carbonates previously known. He shows thatthese give up carbonic anhydride on dissolving and heating,and yield salts differing again in composition. He disputesthe fact asserted by Proust,34 that by leading a trace ofcarbonic anhydride into an alkaline solution only a few mole-cules are saturated, while the others remain uncombined.According to Berthollet, a solution of this kind evolves car-bonic anhydride on the addition of a drop of hydrochloricacid, and consequently contains a souse arbonate^ that is, heconsiders that the trace of carbonic anhydride present is dis-tributed over the whole quantity of the base.

Rendered cautious by Proust's rejoinders and excellent re-searches, Berthollet now no longer assumes allpossible propor-tions between oxygen and the metals as actually occurring.He limits these to a few ; yet, in his examination of the oxidesof lead, he asserts that he has isolated four different stages ofoxidation which are attained by heating the metal in air.80

All the same he has thereby moved a step nearer to Proust.31 Journ. de Phys. 59, 260. ** Ann. Chim. 32, 31. **8 Journ, dc Phys.

64, 168. ** Ibid. 59, 329. «s Ibid. 64. 181. m Ibid. 6x, 352.

U H I S T O R Y O F C H E M I S T R Y [LECT. m.

Still the struggle is not on this account at an end. Even yetBerth ollet will not recognise the difference between mixturesand compounds which had been advanced by Proust. Forboth of these conceptions he demands sharp definitions.37

Now, as a matter of fact, Proust cannot furnish these defi-nitions ; still he shows how mixtures can be distinguished fromcompounds in special cases, and in doing so, he succeeds in dis-proving a great many of Berthollet's statements. Obviously Icannot follow this matter in all its details, and I shall, therefore,only show by a single example Proust's mode of adducing hisproofs. Berthollet had previously asserted that, by treatingmercury with nitric acid, a series of oxides was obtained, inwhich the proportion of oxygen increased steadily from a defi-nite minimum.88 Further, he had observed that these oxides,on treatment with hydrochloric acid, were converted into twochlorides, and had assumed that it was the insolubility of mei>curous chloride which caused the oxides to betake themselvesfrom the stage of oxidation at which they stood to the two endstations.39 Proust considers that too much intelligence is, onthis assumption, attributed to the oxides. He shows that inthe dry way also, only two chlorides are formed, and thatthese correspond to the only two oxygen compounds of mer-cury into which Berthollet's mixtures can be separated.

Thus this controversy, which began in 1801, continues until1807, but, about this time, the interest that the scientific worldhad at first taken in the two opponents diminishes consider-ably. The authority of Bertbollet had made it possible that,in consequence of his attack, a doctrine which had previouslybeen regarded as accurate a priori, should appear doubtful tomany. But the researches of Proust on the one hand, andthose of Klaproth and Vauquelin on the other, had restoredconfidence in that doctrine. Berthollet's rejoinders began tolose their effects. He was forced to admit the existence, inever widening classes of substances, of compounds with con-

37 Journ. de Phys. 60, 347. 38 Ann. Chim, 38, 119. n Proust, Journ.de Phys. 59, 335-

LFXT. in.] HISTORY OF CHEMISTRY 45

stant proportions ; which, as a matter of fact, he had neverwholly denied. In 1809 he still regards compounds withvariable proportions as possible,40 but in this opinion he standsisolated. Too much support has now come to the opposing side.Richter's investigations, carried out from 1791 to 1800, had atlength become known ; Dalton had formulated his atomictheory, which was irreconcilable with Berthollet's view andwas already beginning to constitute the basis of chemical con-siderations ; Gay- Lussac's classical work on the proportions byvolume in which gases combine was concluded; and Berzeliushad published his first important papers. Thus the contro-versy ends, apparently with the complete defeat of Berthollet.

I have treated this subject at length, because I consider itvery important. We have here to do with a general doctrinewhich constitutes one of the foundations of our theoreticalconsiderations, and settles the distinction between mixturesand compounds. It is for the latter alone that our chemicallaws hold, since mixtures are not subject to them. In anyparticular case, therefore, it is very important to know whichclass of substances is being dealt with. What, then, areour means of forming an opinion ?

It is to be found stated in text-books, that compoundspossess a homogeneous character, whereas mixtures can veryfrequently be mechanically separated into their ingredients. Itis further stated there that in compounds the properties of theconstituents have disappeared, whilst they are present side byside in mixtures. Finally the constancy of proportions is thenadduced as characteristic, and I wish to direct attention tothis whole matter. There are cases in which mixtures are,in their whole behaviour, no longer distinguishable from com-pounds. We then have recourse to analysis to solve the ques-tion. We prepare the substance in various ways, and observewhether it always possesses the same composition. We thusinvert the doctrine discussed by Berthollet and Proust. Theformer regarded compounds with variable proportions as pos-

40 M^m. d'Arcueil. 2 470.

46 HISTORY OF CHEMTSTRY [LECT. in.

sible, whilst the latter assumed that substances combine in afew definite proportions only. We call a substance a com-pound when it contains its constituents in invariable pro-portions.

I do not know whether the difference between the twoconceptions has been made clear. In order to appreciatethe full bearing of the question, a person must himself haverequired to decide as to whether he had a mixture or a com-pound in his hands. Even yet we are without a definitionwhich shall suffice for every case ; that is, without such adefinition as Berthollet repeatedly demanded from Proust.It is true that we have certain means of judging with respectto chemical compounds, as, for instance, capacity for crystal-lising, and invariable melting point in the case of solids, andconstant boiling point in the case of liquids. Yet these arefrequently insufficient. I need only recall the phenomenaof isomorphism, when we must admit that mixtures also cancrystallise. I mention the solutions of hydrochloric acid,hydriodic acid, etc., in Avater, regarding which Roscoe hasproved that they are only mixtures (solutions), and we mustadmit that these likewise can possess a constant boilingpoint. In short, this distinction forms one of the most dif-ficult and most important problems, and in point of fact it isoften insufficiently attended to. In the study of chemicalpapers opportunity is often afforded of observing how errorshave arisen through neglect of this very matter. How oftenhave formulae been advanced for substances and theoreticalconclusions based upon their existence, before their com-pound character has been conclusively settled ! The pur-pose of the foregoing remarks is to serve as a warning againstany such error, and I therefore hope to be excused forhaving, for a short time, quitted my proper theme.

L E C T U R E IV.

RICHTER'S INVESTIGATIONS— I) ALTON'S ATOMIC Tin-on V •• <'*-\Y-LUSSAC'S LAW OF VOI.I;MI«S AVOC,,U*H< >'X l\Yi*mti-:>v> --WOLLASTON*S R(]UIVAl.V.NTS.

I SHALL now proceed to describe the development of theatomic theory, up to the second decade of the nineteenthcentury, in so far as it possesses .scientific interest* If »beyond my intention, in doing sot to enter into the hypo-theses of the Greek and Roman philosophers regardingthe constitution of matter. That Leucippus and Demo-critus regarded matter as composed of ultimate particles,and that Lucretius afterwards expounded these view* «itlength, merely shows to us, what, we have l^ug known,that there were men amongst the C# reeks a»«l KUJIMIIHwho might in every respect be placed henide our thiukcm ofto-day. These philosophers made use of tlie deduct i ve irtct it* >dof reasoning ; they started from general principle*, althott^htheir conclusions from them were not alwayn in accord withobservations. The latter were of relatively ftniali inifif *rinucvt

especially as at that time cxpcrinientationt or the art uf in-vestigation understated cmtdkknn^ wan practically unkiuiwii*For this reason, Dcmocritus cannot be placed iff frouc of Kaitf,who, starting from the opposite view-- -from the dynamic:hypothesis—constructed the uiiiversein perhapnJIIH! ;iH|«^tc;da fashion. The expenditure of talent and sagiiciiy wliicli wt*observe oo the part of the supporter* of the two vicw^ wmin vain ; the observations by nteau» of which such tjiienttofticould be solved, were entirely wanting.

The scientific development of the atomic theory dependedsimply upon the discovery of a series of fact9 which were con-nected together by it, and found in it a »imple cxplanattrm,It is my^business now to mention theft* experiment*, attnJ in

48 HISTORY OF CHEMISTRY [LECT. IV.

give an account of the chemical researches which renderedthe assumption of the atomic theory necessary, and which werebrought to a conclusion by means of the theory.

In the preceding lecture I dealt with one of these regu-larities ; that is, with the law of definite proportions. Thisalone^ however, would not have sufficed for that theoreticalconception, and another law was also necessary, namely, thelaw of multiple proportions. The latter was propounded byDalton in 1804, that is to say, prior to the close of the con-troversy regarding the constancy of the proportions by weightin which substances combine. T have intentionally departedfrom the chronological sequence of the facts, in order to securetheir logical arrangement. The law of multiple proportionshas no meaning, so long as the law of constant proportions isnot proved. It includes the latter within it, and can only standalong with it. We may well be surprised that it should havebeen propounded at the very time when doubts were enter-tained as to the accuracy of the law of constant proportions.The explanation probably lies in the fact that Berthollet andProust lived in France, whereas Dalton made his discovery inEngland, and withheld the publication of his investigationsuntil 1808 ; whilst prior to that date the scientific world onlyobtained a short statement of the results of Dalton's experi-ments from Thomson's System of Chemistry, in which theinvestigations are mentioned. Dalton's theory, which verysoon gained favour, undoubtedly exercised a decisive influenceon the views of the chemists of the period with respect toconstant proportions ; and it is partly to be attributed to thelabours of Richter, of Dalton, and of Wollaston, that Berthollet,if he did not exactly withdraw his previous assertions, at leastdid not further endeavour to bring about their acceptance.

It may be that Dalton, as his biographer, Smith, asserts,1

had no knowledge, or only a very incomplete knowledge,of the work of Richter, which might otherwise have contri-buted materially to the establishment of the atomic theory.

1 Memoir of John*Dalton and History of the Atomic Theory, 214,

LECT. iv.] HISTORY OF CHEMISTRY 49

He may have arrived quite independently at those ideas whichexercised the greatest influence upon the subsequent develop-ment of chemistry, but we must here take notice of all the factswhich are of importance in this connection, and we must notoverlook Dalton's predecessors. Almost simultaneously withthe conception of the atom, that of the equivalent was also de-veloped. The recognition of the latter contributed to procureadmission and general favour for the atomic theory, and itis therefore advisable, in my opinion, to deal with both ideasside by side as they arose chronologically. In doing so, itwill be observed that Dalton's atom was introduced indepen-dently of the equivalent, but that Wollaston, in particular,endeavoured to replace the atom by the equivalent. Thisafterwards led to the identification of the two ideas, a factwhich had a detrimental effect upon the science.

The first experiments which could have led to the establish-ment of equivalent quantities, were carried out by Bergman inthe second half of the eighteenth century.2 Bergman observedthat neutral solutions of metals were precipitated by othermetals, without the production of an acid reaction and withoutthe evolution of gas. As an adherent of the phlogiston theory,he explains the observations quite correctly in accordancewith the principles of the theory. He assumes that the pre-cipitated metal has taken up just as much phlogiston as theprecipitating metal has parted with ; and in consequence ofthis view, he perceives a means of determining the quantitiesof phlogiston contained in different metals. The quantities ofthe dissolved and of the precipitated metals respectively, muststand to each other inversely as the supposed quantities ofphlogiston contained in equal weights of these metals.

Lavoisier, who repeats and extends Bergman's experimentsa few years afterwards,8 recognises that they must show, interms of his theory, the quantities of oxygen which combinewith equal weights of the metals. Where Bergman had spoken

3 Bergman, Physical and Chemical Essays. Translated by E. Cullen,M.D.7 2 O784), 349 etseq. 3 Lavoisier, Oeuvres. 2, 528.

D


of the taking up of phlogiston, Lavoisier only needs to assumethe giving up of oxygen, and vice versa: the inverse ratio of thequantities of the precipitated metal A and of the dissolved oneB gives Lavoisier therelations of the quantities of oxygen com-bining with equal weights of A and B ; or otherwise, and moreclearly expressed, the quantities of the precipitated and of thedissolved metal respectively, which the experiment furnishesdirectly, have the property of being able to combine with thesame quantity of oxygen. The generalisation, in the latterform, was not emphasised, however, either by Bergman or byLavoisier, otherwise it would probably have led to the notionof equivalence. This result did not follow, and even the ex-periments of both these chemists were but little heeded. Itdid not fare much better with the investigations of Richter,which were carried out between 1791 and 1802, and were sup-ported by far more observations. Richter was the first to statethe law of neutrality, and to deduce accurate conclusions fromit.4 This merit was formerly attributed erroneously to Wenzel,who arrived, however, at exactly the opposite result. Theerror which passed into many of the older text-books appearsto have been caused by Berzelius,5 and was pointed out bySmith,6 and others.7 On the other hand, the merit of havingfirst stated a fundamental proposition of the doctrine ofaffinity belongs to Wenzel. (See note 94, p. 315.)

Richter observed that upon mixing solutions of two neutralsalts, the neutrality is maintained, even when double decom-position takes place, and from this he concluded that thequantities a and b of two bases, both of which are neutralisedby the same quantity c of an acid, are both likewise neutralisedby the same quantity d of another acid ; and, conversely, thatthe weights of two acids which are neutralised by the same

4 Richter, Ueber die neueren Gegenstande der Chemie. 6 Berzelius,Essai sur la the'orie des proportions chimiques et sur rinfluence chimiquede l' lectricite, Paris (1819), 2. 0 Memoir of John Dalton, etc., 160.7 Wenzel determined the proportions in which base and acid combineto form salts. He found, however, exactly the opposite of what Berzeliusmakes him say. Compare Wenzel, Ueber die Verwandtschaft der KoYper,Dresden (1782), especially 450 et $eg>


quantity a of a base> require for neutralisation the samequantity b of another base. The mode of expressing thiswhich Richter employs is remarkable. He says :—8

u If Pis the mass of a deter mining element, where the massesof the elements determined by it are a) /;, c, d) <?, etc., and Q isthe mass of another determining element, where the massesof the elements determined by it are a, /$, y, S, €, and so on, butwhere a and a, b and /3, c and y, d and 8, c and e1 in each caserepresent one element, and the neutral masses P + a andQ + /3 ; />+5andQ + y ; -P+c and Q +a; P+a and Q + y,andso on, decompose by double affinity in such a way that theproducts obtained are again neutral, then the masses a, /;, c1

d) c1 etc., have exactly the same quantitative ratio amongstthemselves as the masses a, /?, 7, 8, £, etc., and conversely."

I must observe that by the determining element, and theelement determined, Richter understood the quantities of acidand of base that mutually neutralise each other. Richter wellunderstood the importance of the foregoing statement. Heremarks :9 "This rule is a true touchstone of the experimentsinstituted with regard to the relations of neutrality ; for, ifthe proportions ascertained empirically are not of the naturethat the law of decomposition by double affinity requires,where the decomposition actually taking place is accom-panied by unaltered neutrality, they are to be discarded asincorrect without further examination, since an error hasthen occurred in the experiments tried."

Richter tabulated the quantities of the bases which areneutralised by the same weight of sulphuric acid, of hydro-fluoric acid, etc., and these he calls neutrality series, or seriesof masses ;10 he also determined the quantities of the acidswhich are saturated by. the same quantity of different bases.11

In doing this, he thought he had discovered certain regulari-ties, but this subsequently proved to be fallacious. Thus,according to him, the series of masses in the case of the basesformed an arithmetical, and that in the case of the acids, a

8 Neuere GegenstHnde (i79$> 4, 67. 0 Ibid. 4, 69. w Ibid.4,70. » Ibid. 4, 92; 6, 176.

52 HISTORY OF CHEMISTRY [user, iv.

geometrical series. He wished, in fact, to establish a regularityin chemical compounds, such as had been assumed in the dis-tances of the planets from the sun, and, in order to accomplishthis, he had probably corrected many of his results.

Another department of Richter's " stochiometric investiga-tions "mustbe mentioned here; that is, his work upon metallicprecipitations. He determines the quantities of the metalsas they mutually precipitate one another from their solu-tions, and employs the numbers obtained to ascertain theproportion of oxygen in the oxides. Here again, his way ofexpressing himself leaves much to be desired. He says :—12

u When an aqueous solution of a metallic neutral salt is sodecomposed by an inflammable metallic substrate, that is, byanother metal in the metallic state, that not only does themetal which was dissolved separate out in a wholly metallicstate, but also that neither the dissolving acid solvent nor thewater associated with it, is decomposed, then the masses of thevital air which must unite with equal masses of the metallicsubstrates, in order to make their solution in acids possible,are inversely proportional to the masses (or weights) of theseparating and of the separated metallic substrates from themetallic neutral salts." And at another place:—13UThequantitative order of the specific neutrality of the metalstowards vitriolic acid does not by any means follow the usualorder in which one metal is separated by another from the solu-tion in the acid ; on the contrary, it is wholly analogous to theinverse quantitative order of the removal of the inflammablematter and of the respective combination with vital air,"

It is worthy of mention that Richter introduced the nameStochiometry, which signifies the measurement of the pro-portions in which substances combine.

Fischer, on the other hand, deserves credit for havingcombined Richter's various tables into a single one. In thisconnection, he expresses himself as follows :—u" It is only

12 Neuere Gegenstande. 8, 83. m Ibid. 8, 127. u Berthollet,Statique Chimique. German translation with explanatory comments by-Fischer, I, 135.


necessary to determine the proportional quantities of one acidagainst the different alkaline bases ; afterwards, it is sufficientto ascertain the proportional quantity of a single compound ofevery other acid with one alkaline basis, when, by means of aneasy calculation, the proportional quantities of the acids in allthe other compounds may be determined." It may be said,indeed, that Fischer's table was the first table of equivalents.The numbers attached in it to the different bases, representequivalent quantities, since these quantities are neutralised bythe same quantity of acid ; and conversely with respect to thedifferent acids. The conception of the equivalent was, in thisway, established about the year 1803, although the word itselfwas not in use at that time. The discovery of the law ofmultiple proportions, and the formulation of the atomictheory by Dalton (which were first announced in the thirdedition of Thomson's System of Chemistry), both took placealmost at this same time.

I wish to deal, in a few words, with the questions of prioritywhich were called forth by these important experiments andviews.1'"' The idea of the atomistic point of view is an old one,and at the beginning of this lecture I have named a few G reckphilosophers who advanced and upheld the theory. Theopinions pass down through all the centuries ; they are con-tested ; but they always find supporters. The chemists of theeighteenth century appear to have embraced pretty generallythe atomistic way of regarding matter. I may here adduce theviews of Lavoisier with respect to the constitution of matter,which I have already stated at length ; and those of Berthollet,who frequently speaks of molecules. In one word, thesewere the opinions of the day, and they were preferred to thedynamic hypothesis, chiefly, it is almost certain, because theassumption of discreet particles of matter, separate from oneanother, furnished a simple explanation of the diminution ofvolume with the lowering of temperature.

Dalton, on his part, did not claim that he had introduced

18 Compare the work by Smith, already merjtiqueclt


these opinions into the science. In this connection he saysthat the observation of the existence of different states ofaggregation has a led to the conclusion which seems univer-sally adopted, that all bodies of sensible magnitude, whetherliquid or solid, are constituted of a vast number of extremelysmall particles, or atoms of matter bound together by a forceof attraction, which is more or less powerful according to cir-cumstances, and which as it endeavours to prevent theirseparation, is very properly called in that view, attraction ofcohesion."1® And with regard to the "agency of heat" hesays:—u An atmosphere of this subtile fluid constantly sur-rounds the atoms of all bodies, and prevents them from beingdrawn into actual contact.''17

Dalton shows, however, in the course of his most interest-ing work how the relative weights of these particles can beascertained ; and it will remain as his imperishable merit tohave shown the possibility of determining the atomic weights.Higgins, it is true, tried to prove that he also participated inthis important discovery;18 but even if it must be admittedthat Higgins employed the atomic theory as early as 1789,19

still his way of expressing himself is not, by a long way, soclear and definite as Dalton's, and, so far as I know, he doesnot mention atomic weights.

Dalton turned the atomic hypothesis to account as thebasis of chemical considerations, after he had found that whentwo substances unite in several proportions, these proportionsare always expressible in simple multiples by whole numbers.He examined the two hydrocarbons, marsh gas and ethylene,and found that, for the same weight of hydrogen, there wastwice as much carbon combined with it in ethylene as in marshgas. He then examined to see whether any such regularitywas to be found in other compounds, employing in particularfor this purpose the oxides of nitrogen, and he thereby ob-

16 Dalton, A new System of Chemical Philosophy, 1, 141-142 ; AlembicClub Reprints. 2, 27. " New System, x, 143-144. 18 Experimentsand Observations on the Atomic Theory, by W. Higgins (1814). 19 AComparative View qf the Phlogistic and Antiphlogistic Theories,

LKCT. iv.] HISTORY OF CHEMISTRY 55

tained confirmation of the law. The law is to this effect:—Iftwo substances, A and B) form several compounds, of whichthe compositions are all calculated with respect to the samequantity of A, then the quantities of B combined with this,stand to each other in a simple ratio.20 Dalton sought in theatomic theory an explanation of this law, which was simplyan expression of the observed facts.

According to the atomic theory, chemical compounds areformed by the arrangement, in juxtaposition, of atoms of theelements, these latter being incapable of undergoing anyfurther decomposition. With regard to this Dalton says :—21

" Chemical analysis and synthesis go no further than to theseparation of particles one from another, and to their reunion.No new creation or destruction of matter is within the reachof chemical agency." By the fact that Dalton assigns adefinite unalterable weight to the atom of every element, andadmits the possibility of the combination of several atoms,his theory is brought into harmony with experiment, andbecomes, indeed, a necessary consequence of it. Accordingto t he number of atoms which enter into combination, theresulting atom may belong to a different order.

T h e atoms of elements are simple atoms, or atoms of thefirst order.

W h e n i atom of an element A combines with i atom ofan element By i atom of the second order is produced.

W h e n 2 atoms of an element A combine with i atom ofan element B) i atom of the third order is produced.

W h e n i atom of an element A combines with 2 atoms ofan element B, 1 atom of the third order is produced.

W h e n 1 atom of an element A combines with 3 atoms ofan element B, 1 atom of the fourth order is produced.

W h e n 3 atoms of an element A combine with 1 atom ofan element B, 1 atom of the fourth order is produced, etc.

30 It appears that Dalton never stated the law in this general farm.Compare Memoirs of John Dalton, by W. Henry, 79 et $eq> 2l

System, x, 212 j A.C.R. 2, 20,.


I have not been able to find any statement as to whether twoatoms of one element can combine withthreeatoms of another,but it appears as if Dalton regarded this assumption as un-tenable. Compounds which are most simply regarded in thisway, consist, according to him, of two composite atoms; he isobliged, of course, to make the assumption that atoms of thehigher orders are capable of combination with one another.22

I have pointed out, above, that Dalton's theory was inagreement with the facts; I shall now explain how, from hisexperiments, he determined the atomic weights. In order thathe might be able to do this, the first thing necessary was tosettle the number of atoms in a compound. According toDalton, this number is to be sought for, in general, in thesimplest possible ratios. In estimating it, he starts from thefollowing principles :—28

1. When only one compound of two elements is known,this is composed of an atom of the second order.

2. When two compounds are known, the one consists ofan atom of the second, and the other of an atom of the thirdorder.

3. When three compounds are known, one atom of thesecond and two atoms of the third order must be assumed.

How does Dalton now proceed to the determination of theatomic weights, i.e., the relative weights of the smallest par-ticles ? In the first place, he requires to choose a unit forcomparison. As unit he assumes hydrogen with the atomicweight = 1, and he refers all the other atomic weights to this.To fix the others, he then applies his first principle. At thattime, only one compound each of oxygen and of nitrogen withhydrogen was known, viz., water and ammonia respectively;therefore the atomic weights of oxygen and nitrogen can be de-termined directly from the composition of these compounds.In this way, Dalton finds them to be 7 and 5 respectively. Hechecks the numbers so obtained by the proportions of the

22 New System. 1, 213-215 ; A.C.R, 2, 30-31, tja New System. I,314; A.C.R.2, 30.


oxygen and nitrogen in the oxygen compounds of nitrogen.24

He is acquainted with four of the latter. In nitric oxide h efinds 7 parts of oxygen for 5 of nitrogen ; its atom is, therefore,the atom of the second order, derived from these elements.In nitric acid, according to his view, there are 14 parts ofoxygen for 5 of nitrogen, or two atoms of the former gas forone of the latter. In nitrous oxide, 7 parts of oxygen are com-bined with 10 parts of nitrogen, and in this he thereforeassumes two atoms of nitrogen and one of oxygen. Nitrousacid, however, is supposed to contain i o | parts of oxygen for5 of nitrogen, and in it he might have assumed two atoms ofnitrogen and three of oxygen. He prefers, however, to regardthis substance as a compound of nitric acid and nitric oxide.

Further, he finds in ethylene 5.4 parts of carbon for I ofhydrogen, and in marsh gas the same quantity of carbon for 2of hydrogen. On this account, he regards ethylene as consist-ing of atoms of the second order, and assumes the atomicweight of carbon to be 5.4. Carbonic oxide likewise consistsof atoms of the second order, since he finds in it 7 parts ofoxygen for 5.4 of carbon, while carbonic anhydride hasatoms of the third order, because it contains 14 parts ofoxygen for 5.4 of carbon.

But Dalton does not always adhere quite rigidly to hisown rules. Thus he regards sulphuretted hydrogen as con-sisting of one atom of sulphur and three of hydrogen, andsulphuric acid, of one atom of sulphur and three of oxygen,whereby he is led to 13 as the atomic weight of sulphur.

Hence there is, in my opinion, something haphazard inthese atomic weight determinations, quite apart from the rulesthemselves. I shall return afterwards to the question whetherthe latter are justifiable or not. The numbers advanced b yDalton were thus relative in a twofold manner, if I may so ex-press myself; they were affected by two unknown constants.In the first place they were all determined with reference to anarbitrary standard, and in the second, they were only relatively

w New System. 1, 215 j A.Q.R. g, 30-31,


accurate with respect to this standard. The atomic weightof carbon had in reality only been found to be a multiple ora submultiple of 5.4. Dalton himself does not appear tohave been aware of this arbitrary character of his numbers.

In spite of this, his theory met with very general recog-nition, and chemists were astonished at the simplicity withwhich it explained all the regularities which had been dis-covered immediately before. With the rapid progress thatthe science was at that time making, some new support wasnecessary. In order to avoid being left behind it wasessential to possess a general point of view from which theisolated facts and the different regularities could be con-veniently surveyed. It was soon to be shown that this theorywas capable of standing the test; that it was not only suffi-cient to connect the known phenomena, but that laws whichwere only discovered subsequently could also be explainedby means of it. This holds especially for the law of gaseousvolumes, which Gay-Lussac discovered in 1808, a fewmonths after the appearance of Dalton's ingenious book.

As early as 1805, Humboldt and Gay-Lussac, on theoccasion of their joint investigation of the composition ofthe air, had determined anew the proportions by volume inwhich hydrogen and oxygen combine.25 They found slightdifferences from the earlier observations, and arrived at thehighly interesting result that water is produced by the con-densation of 2 volumes of hydrogen and 1 of oxygen ; whileMeusnier and Lavoisier20 had found 23 volumes of hydrogenand 12 of oxygen, and Fourcroy, Vauquelin, and Seguin27

had found 205.2 of hydrogen for 100 of oxygen.Three years later, Gay-Lussac extended his experiments

to other gases.28 He had previously observed the law (oftencalled after him) of the uniform expansion of gases withincrease of temperature.29 He was also familiar with the

25 Journ. de Phys. 60, 129. "i6 Lavoisier, Oeuvres. 2, 360. ^ Ann.Chim. 8, 330 ; 9, 30. m Mivn. d'Arcuejl. %% 307, " A,nn. Qhim. 43,137'


so-called law of Mariotte, which was discovered by Boyle,and which states the relation between pressure and volume ;in short, he possessed all the data for reducing the resultsdirectly obtained to like pressure and temperature—a basisupon which to execute the experiments he had in view. Hisinvestigation is a model of accuracy, and, in this respect, it isvery markedly distinguished from other experimental inves-tigations of the period. The results obtained are extremelysimple ; and he states them somewhat after this manner.Two gases always combine in simple proportions by volume,and the contraction which they undergo, and, therefore,also the volume of the product formed, stand in the simplestrelation to the volumes of the constituents.

Thus Gay-Lussac found, for example, that 2 volumes ofcarbonic anhydride are produced from 2 volumes of carbonicoxide and 1 of oxygen ; that 2 volumes of nitrous oxide arecomposed of 2 volumes of nitrogen and 1 of oxygen ; thatequal volumes of nitrogen and of oxygen are combined in nitricoxide, while the product has the same volume as the two con-stituent gases separately ; and finally, that 1 volume of nitrogenand 3 of hydrogen are condensed to 2 volumes in ammonia.

Gay-Lussac, who was well acquainted with Dalton'stheory, shows at the end of his paper that the facts ascer-tained by him are in harmony with the theory ; that, by theassumption of a similar molecular condition in all gases, itwill explain their analogous behaviour towards changes ofpressure and of temperature; and that his law of gaseousvolumes is an important support of Dalton's view.

It might be supposed that the latter would have been highlypleased by so unexpectedly brilliant a confirmation of his views.This was not so, however. In the second part of his " NewSystem of Chemical Philosophy," which appeared in 1810, hepractically regards Gay-Lussac's experiments as erroneous. Ishall endeavour to explain the reasons that prompted him to dothis, especially as it has been stated that Dalton wished, fromjealousy or want of judgment, to dispute Gay-Lussac's merits.

In the first part of his book, Dalton had already speculated

60 HISTORY OF CHEMISTRY [LKCT. IV.

as to the volume relations of the gases. He says there :—30

41 In prosecuting my enquiries into the nature of elastic fluids,I soon perceived it was necessary, if possible, to ascertainwhether the atoms or ultimate particles of the different gasesare of the same size or volume in like circumstances of tem-perature and pressure. By the size or volume of an ultimateparticle, I mean in this place, the space it occupies in the stateof a pure elastic fluid; in this sense the bulk of the particlesignifies the bulk of the supposed impenetrable nucleus, to-gether with that of its surrounding repulsive atmosphere ofheat. At the time I formed the theory of mixed gases, Ihad a confused idea, as many have, I suppose, at this time,that the particles of elastic fluids are all of the same size ;that a given volume of oxygenous gas contains just as manyparticles as the same volume of hydrogenous." He after-wards became of a different opinion, to which he was ledby the following considerations :—One atom of nitric oxideconsists of one atom of nitrogen and one of oxygen. If, now,there were the same number of atoms in equal volumes, onevolume of nitric oxide should be formed by the combina-tion of one volume of nitrogen with one of oxygen, but,according to Henry's experiments, about two volumes areproduced ; hence nitric oxide could only contain half asmany atoms in the same space as nitrogen or oxygen.31

Dalton, in his reply, refers to these considerations, andthen, with regard to Gay-Lussac's il hypothesis that all elasticfluids combine in equal measures, or in measures that havesome simple relation one to another," he proceeds:—u

u In fact, his notion of measures is analogous to mine ofatoms; and if it could be proved that all elastic fluids havethe same number of atoms in the same volume, or numbersthat are as I, 2, 3, etc., the two hypotheses would be the same,except that mine is universal, and his applies only to elasticfluids. Gay-Lussac could not but see that a similar hypothesishad been entertained by me, and abandoned as untenable."

ao New System. 1, 187-188 ; A.C.R. 4, 6, 7. :n Compare NewSystem, I, 70-71 ; A.C.R. 4, 5. '•** New System. £, 5$6 ; A.C.R. 4, 25,


Dal ton shows, moreover, the poor agreement between theresults of Gay-Lussac and those of Henry, and this confirmshim in his conclusion that the former had not observedaccurately. It cannot be denied that Dalton ?s contention iswell founded. If the atomic theory was chosen as the basisfor chemical speculation, the law of gaseous volumes, asGay-Lussac stated it, could only be brought into harmonywith it if the assumption were admissible that equal numbersof the smallest particles are present in the same volumes ofall gases. This assumption agreed with the physicalproperties of gases, but was, as Dalton quite rightly con-cluded from known facts, impossible. The three rulesadopted by Dalton likewise told against the accuracy of thishypothesis. In water, for example, it would have beennecessary to assume two atoms of hydrogen for one of oxy-gen ; and it may be that this was also a reason for Dalton'scoming forward so decidedly ii> opposition to Gay-Lussac.

It will be clear from these explanations that there was areal difficulty in bringing Gay-Lussac's law into harmony withthe atomic theory. Avogadro was the first to show how thisdifficulty can be got over.33 The Italian physicist distinguishesmolecules integrantes and molecules e'k'mentaires, which, forbrevity and simplicity, we shall translate by molecule andatom respectively. As a step towards the explanation of theobserved facts regarding the proportions by volume in whichgaseous substances combine with one another and of therelation of these proportions to the volume of the products, towhich Gay-Lussac's law gives expression, Avogadro advancesthe hypothesis that equal volumes of all gases, whetherelementary or compound, contain the same number of mole-cules. These molecules are not supposed, however, to con-stitute the ultimate particles of matter, but are assumed to becapable of further subdivision under the influence of chemicalforces. According to Avogadro,therefore, substances (elementsand compounds alike) are not converted, in passing into the

m Jourru de Phys. 73, 58 ; A.C.R. 4, 28.


gaseous state, into indivisible particles, but only into mole-cules integr antes, which in turn are composed of moleculeselementaires. He bases his view upon the following con-siderations :—If nitric oxide, which arises without contractionfrom equal volumes of nitrogen and oxygen, contains as manymolecules as the mixed gases do, then the combination cannotconsist in the union with one another of previously separatemolecules, since this would necessarily involve a diminutionin the number of particles ; but it must be brought about by anexchange. The molecules both of nitrogen and of oxygen mustsplit into two parts, and these then combine by mutualexchange.

While, therefore, before the combination^the gaseousmixture consists of dissimilar molecules, of which the one halfare composed of two atoms of nitrogen, and the other half oftwo atoms of oxygen, the product of the combination consistsof thesamenumber of molecules, butthey are all like moleculesand each has been formed by the union of one atom of nitrogenwith one of oxygen. Consideration of the volume relations inthe formation of ammonia, likewise points to a subdivision ofthe gas particles of elementary substances. All these discus-sions assume the simplest character when the word molecule{molecule integrante) is employed instead of volume, the twoconceptions being identical for the gaseous state, according toAvogadro'sdefinition. From Gay-Lussac's numbers, then, itappears that the molecule of ammonia consists of half a mole-cule of nitrogen and one and a half molecules of hydrogen ;that the molecule of water contains half a molecule of oxygenand one molecule of hydrogen, etc. If the simplest hypothesiswith respect to the divisibility of the molecule is adopted, sothat it is not necessary to introduce fractions of atoms, then themolecules, not only of hydrogen, but also of oxygen and ofnitrogen, must consist of two elementary atoms ; and theproportions, by volume, in which the gases combme, then givethe number of chemical smallest particles which go to form themolecule. Avogadro finds, for example, that two atoms ofhydrogen and one of oxygen are necessary for the formation of


water ; that three atoms of the former gas and one of nitrogenare present in ammonia, etc., and he thus arrives at resultsquite different from those of Dalton.

He points this out explicitly in his paper, and drawsattention to the fact that in his determinations, he starts fromlegitimate physical principles,whereas Dalton Js rules containedarbitrary assumptions. He lays stress on the fact that Dalton,in case he wishes to identify the physical with the chemicalatoms {molecules inicgr antes and elementaires), will be forced toassume that in those combinations which take place withoutcontraction, the composite atoms must be further removedfrom one another than the uncombined ones.

Avogadro is able, from their densities, to determine directlythe molecular weights of the elements known in the gaseousstate. This, however, is not sufficient for him, and he alsoattempts the determination in the cases of other elements.Here, however, he has recourse to more or less doubtfulhypotheses. He finds the atomic weight of carbon to be 11.3,and that of sulphur 31.3, referred to that of hydrogen assumed= 1; that is, he finds numbers which very nearly agree withthose-adopted at present. I shall not enter into the moreminute details of this most interesting paper, and shall onlyremark further that Avogadro admits the possibility of mole-cules of elements consisting of 4, 8, etc., atoms, and believesthat nature has, in this very way, equalised the differencebetween simple and compound substances.

Starting from similar views, Ampere writes a paper on thesame subject three years later (1814).84 His conclusions are,however, less simple, as he tries at the same time to explainthe crystalline form of substances by the position of the atomsin the molecule.

These speculations met, on the whole, with but littleattention in the chemical world. It seems as if a distinctionbetween atom and molecule was not regarded as justifiable,and accordingly neither Avogadro's nor Ampere's ideas

84 Ann. Chim. 90, 43.


exercised any immediate influence upon the science. This mayalso be explained by the fact that the hypothesis only led todecisive results as to the number of atoms contained in themolecule (and thus to the determination of the atomic weight)in the case of gaseous substances, and was not applicable tosolids and liquids. Chemists, therefore, looked for newgeneralisations, and the next impulse in this direction wasgiven by Wollaston.

Wollaston had carried out an investigation on the carbon-ates,in 1808,35 which appeared simultaneously with an examina-tion of the oxalates by Thomson.30 It was shown in the papersof these chemists that carbonic acid can form compounds withone and with two parts, and oxalic acid with one, with two, andwith four parts of potash. These experiments produced agreat impression, because at that time there were few facts ofthis kind known which had been minutely examined ; on thisaccount, they formed an important support to the law ofmultiple proportions. But if Wollaston, on the one hand, thusexerted an influence upon the rapid recognition which theatomic theory met with, and consequently came to be regardedeven by authorities as an adherent of the theory,87 still, by alater paper,88 he contributed to the abandonment of the atomby a section of chemists, as too indefinite a basis for chemicalconsiderations.

In 1814, Wollaston, not without justice,represents to Daltonhow uncertain and arbitrary is his estimation of the number ofatoms in a compound; and how, in consequence, the atomicweights are wholly hypotheticalnumbers,sothat,in his opinion,they should not be adopted. He advised, instead of the con-ception of the atom, the introduction of the equivalent, whichword he employs for the first time. Wollaston was wellacquainted with Richter's works,80 and he derives the concep-tion of the equivalent principally from his investigations. I

36 Phil. Trans. 1808, 96; A.C.R. 2, 34. m Phil. Trans. 1808, 63 jA.C.R. 2, 41. B7 Kopp, Geschichte. 2, 373. x Phil. Trans. 1814,I ; Ann. Chira. 90, 138. 2Q Even Wollaston remarks {loc. cit) thatWenzel's analyses do not agree with the law of neutrality.


must at once remark, further, that, with him, not only are thosequantities of two bases equivalent which are neutralised by thesame quantity of acid, and those quantities of metal equivalentwhich mutually precipitate each other (and therefore unitewith the same weight of oxygen), but that also, in his deter-minations, he extends far beyond theselimits, without,it wouldappear, ever clearly perceiving that, he falls into exactly i!it-same error that he points our to Dalton. I go farfhcr, ev.-n.and assert that the uncertainly was Incroaseil by h!i:i; -iiK.i; Ju.first employed the equivalent in the sense of the at*;in, amithereby attached to it the vague signification which remainedconnected with i t ; and so it was this very paper which princi-pally led chemists to the fusion ofthe two conceptions, andthereby to the tacit and inaccurate assumption that the atomswere equivalent, an error which gave rise to great confusion.

I shall here give an example of Wollaston's determinations,so that the reader may obtain at least an idea of his method,and may satisfy himself as to the accuracy of my opinion.Wollaston sets out from the equivalent of oxygen, which heassumes = io ; from this he determines the equivalent ofhydrogen to be 1.3, clearly because 1.3 parts of hydrogen(according to the estimations ofthe period) unite with 10 partsof oxygen to form water ; thus equivalent quantities are, withWollaston, the quantities in which substances combine. Buthow, we may ask, does he proceed in the cases of substancesthat combine in more than one proportion—in the case ofcarbon, for example ? Does he recognise several equivalentshere ? The answer is, no, he never appears to think that sucha thing can be possible. He adopts the equivalent of carbonas 7.5, determining it from that of carbonic anhydride, which,according to him, is 27.5. He does not give any reason,however, for choosing the latter number, and we are left tofind it out for ourselves. We might suppose that Wollastonregarded as the equivalent that quantity of carbonicacid whichsaturates a quantity of a base containing 10 parts of oxygen,whereby he would necessarily have adhered to the view statedabove, that the combining weight is identical with the equiva-

66 HISTORY OF CHEMISTRY [LECT. rv .

lent. He is obliged, however, as a consequence from his o w nfigures, to assume in carbonic acid, two equivalents of oxygen,for one of carbon. In this case, therefore, the combiningweight and the equivalent were no longer identical, but t h eone was twice as great as the other. That such results w e r eunavoidable in his method, ought to have been clear to h i mat once, since he was acquainted with the law of multiple p r o -portions. Wollaston does not, however, pause in the l eas ton account of this result ; he does not bestow a word u p o nit ; and he continues his determinations of equivalents uncon-cernedly, but we shall not follow him in these, as they possessno further interest for us.

I think I have now shown that Wollaston's equivalentspresent the same uncertainties as Dalton's atomic weights,and that the views stated by him must be called retrogradebecause he believed that he was dealing only with real u n -ambiguous conceptions, free from all hypotheses.

It may perhaps seern that the opinion stated here is severeand unjust, but when the further development of chemistry i sfollowed up, and it is seen how a more rapid progress during"the succeeding decade was prevented by this very confusion o fequivalent and atom (or combining weight), and how a m o s tvigorous struggle was necessary before the separation of t h econceptions could be re-introduced, the reader will probablyadopt my view. It is true that the blame does not rest w i t hWollaston alone, because a school arose in Germany a l s o ,probably stimulated by him, which represented the s a m eideas. This school was at first, no doubt, dominated by t h egreat influence of Berzelius, but afterwards, especially at t h ebeginning of the fifth decade of the nineteenth century, i ttook the lead itself. I do not intend to withhold the details o fthese highly interesting developments, but in the next l e c t u r eI must direct attention to the electrical phenomena which a tthis time begin to exercise a great influence upon our science.

L E C T U R E V.

DAVY'S ELECTRO-CHEMICAL THEORY—DISCOVERY OF THE ALKALIMETALS—DISCUSSION REGARDING THEIR CONSTITUTION —DOESHYDROCHLORIC ACID CONTAIN OXYGEN? — HYDROGEN THEORYOF ACIDS.

I F we look back to the period when the alkalies were regardedas simple and undecomposable substances, we can easilyunderstand the enthusiasm with which the chemical worldgreeted the discovery of potassium and sodium. We are allfamiliar with the remarkable properties of these elements—with their metallic appearance and low specific gravity, theirchangeability in the air, their easy inflammability upon water,etc. It can be understood, therefore, that when substancespossessing such properties had once been seen, illusions ofall kinds were entertained by some people. The idea wasarrived at that the substances hitherto known were onlycompounds, and that the aim of chemistry was now to dis-cover the true elements, which, it was supposed, wouldresemble potassium and sodium. It is likewise comprehen-sible why the agency which had accomplished such resultsshould be admired and overestimated. Everything wassupposed to be possible by means of it, and *the directionwhich chemistry would necessarily follow—that of electro-chemical development—was clear to every one. The galvaniccurrent, at that period an entirely new agent, had accom-plished this marvel, and it was itself a marvellous thing. Byits aid it had become possible to decompose compounds intotheir true elements ; hence it is not surprising that this agencywas regarded as identical with the one which gave rise to com-binations, i.en with affinity. It was believed that an explana-tion was thereby furnished for two things, both of which stoodin need of it—that is to say, for electrical and for chemical

68 HISTORY OF CHEMISTRY [LECT. V.

phenomena, and the connection between these two was nowshown. The further development of the electro-chemicaltheories appeared at that time to be the highest aim of ourscience ; at a later date we see these theories abandoned. Theextraordinary enthusiasm was succeeded by an indifferencejust as extraordinary. In those cases where it was formerlybelieved (as regards the structure of the most complicated com-pounds) that the real secrets of nature were being discovered,the ordinary phenomena of decomposition were afterwards re-cognised. A secondary importance only, was usually ascribedto the latter in determining the constitution of substances. Theelectrical properties of substances, which at that time also indi-cated their position in the system, afterwards became of lessconsequence in the determination of their chemical character.

This is just such an example as is met with in the history ofeveryiscience. Some great end is achieved, in comparison withwhich everything else becomes dwarfed into insignificance.Every endeavour is turned towards development in the newdirection, and a system is established which has the observedphenomena for its basis. Then facts appear which are inconflict with the views that have thus arisen. These facts area sufficient ground for some to abandon the theory ; but forothers they act merely as a stimulus to bring the new experi-ments into harmony with the theory, and this compels them tohave recourse to further hypotheses. In this way a controversyis developed which only ends when the supporters of the olderview have their eyes opened so as to recognise how greatly theoriginally simple and elegant theory has been disfigured. Thewhole system now falls to pieces, and it is no longer compre-hensible how any one could have permitted himself to beguided by such views. The greatest folly is now perceived inwhat had been previously recognised as the highest wisdom.So the times change!

The electro-chemical theories had the same fate. If weconsider the astonishing discoveries which were made by meansof the galvanic current, we can understand the importancewhich attached at that time to electrical phenomena ; and I

JLficT. v.J HISTORY OF CHEMISTRY 69

believe I may almost assume that we would do similarly if wewere placed in the same position. Listen, and judge !

In 1789 Galvani1 discovered that upon simultaneouslytouching a muscle and a nerve of a frog, by means of twodifferent metals which were joined together by a conductor,the frog was convulsed. Galvani's explanation of this fact wascontroverted and replaced by another, in 1792, by Volta.2

We find the question of the cause of the electrical currentdiscussed for a long period. Is the contact of the two metalssufficient to generate electricity, or must they also be separatedby a decomposable fluid conductor? The reader is, no doubt,aware of the answer which we must now give to this question,in accordance with our scientific principles, even although theopposite view is perhaps not wholly refuted.8 We cannotin this place further occupy our attention with these theories,which belong to physics, and not to chemistry.

Nicholson and Carlisle4 observed in 1800, that upon dis-charging the galvanic pile through water, the latter is de-composed into its constituents, hydrogen and oxygen. Manyendeavours were made to observe similar phenomena in thecases of other substances, but the first of the more importantinvestigations into the nature of the decomposition of chemicalcompounds by means of the electric current comes fromBerzelius and Hisinger, and was published in 1803,6 Thesetwo investigators studied the action of dynamic electricity uponsalt solutions, and, further, upon ammonia, sulphuric acid, etc.Their apparatus was so arranged that they were able to collect,separately, the constituents liberated at the different poles. Inthis way they arrived at the highly remarkable result, thatsubstances may be divided into two groups in respect to theirbehaviour towards the galvanic current; that hydrogen, themetals, the metallic oxides, the alkalies, the earths, etc.,

1 De Viribus Electricitatis in Motu Musculari Commentarius. Com-mentarii Acad. Bononiae. YoL 7 (1791). 2 Giowale Fisico-medico di L.Brugnatelli, 1794 ; Gren's Journal der Phj sik. [2], 2. a Compare Wiede-mann, Galvanismus. 1,25. 4 Nicholson's Journal (quarto), 4.(1801), 183.5 Ann. Chim. 51, 167.


separate at the negative pole of the battery, while oxygen, theacids, etc., separate at the positive pole. Besides this, theybelieved that they had discovered relations between thequantities of the substances decomposed, their mutual affini-ties, and the quantities of electricity derived from the battery.

With respect to the cause of the decomposition, they onlyexpress themselves in a very uncertain manner ; they believethat this decomposition may be explained by the greater orless attraction that the electricity exerts upon the differentsubstances.

I now turn to Humphry Davy's investigations, whith, as hesays himself, were begun in 1800.0 He commenced them with,apparently, a quite unimportant question. Even in the firstexperiments on the decomposition of water7 it was believed thatthe formation of alkaline and of acid substances as products ofthe electrolysis had been observed. Cruickshank8 and Brug-natelli9 con firmed this observation, and a belief was entertainedin the conversion of water into alkalies and acids under theinfluence of the electricity. Simon10 had already opposed thisview, and Davy refuted it by decisive experiments.11

Davy causes the decomposition to take place in vesselsmade of different materials—glass, agate, gold, etc.—and satis-fies himself that the nature and quantity of the substancesliberated are thereby varied. This leads hi m to the assumptionof the decomposition of the vessel. But even when he carriesout the decomposition in gold vessels, he observes the forma-tion of the volatile alkali (ammonia) and of nitric acid. These,he now concludes, owe their formation to the air (nitrogen)which was dissolved in the water. In order to satisfy himselfof the accuracy of this view, he causes the decomposition totake place in closed vessels, pumping out the air which is incontact with the surface of the water, and substituting for it anatmosphere of hydrogen. In this way he succeeds in provingthat pure water is decomposed by the action of the electric

6 Phil Trans. 1807, 2. 7 Nicholson's Journal (quarto), 4 (1801), 183.8 Ibid. 188-189. 9 Phil. Mag. 9, 181. 10 Gilb. Ann. 8 (1801), 36,11 Bakerian Lecture for 1806 (Phil. Trans. 1807, l).

LECT. v.] HISTORY OF CHEMISTRY 71

current into its constituents, hydrogen and oxygen, but thatno other kind of change occurs in the water ; and that allobservations of the apparent occurrence of such change areto be explained either by some action upon the vessel inwhich the experiment has been conducted, or by some im-purity in the water.

This enquiry is comparable, in many respects, with the firstof Lavoisier's investigations.12 In both cases an endeavour ismade to refute a statement based upon inaccurate observation,and that not simply by speculation, or by the assertion that itis in contradiction to general views. In opposition to theearlier superficial experiments, new ones are adduced whichhave been carried out with the most minute attention to all theconditions. In both cases the purpose is attained, and theolder inaccurate view is replaced by a correct one. Suchresults as those obtained in these cases by Lavoisier and byDavy are often called negative, but most people will agree withme when I assert that they may be of great positive value.

Davy, however, does not stop here. He next investigatesthe decomposition of salt solutions, and finds confirmation ofthe statements of Berzelius and Hisinger. But he proceedswith still greater circumspection, and endeavours to follow upthe phenomena more exactly. All the means are at his com-mand, and he does not fail to avail himself of them.

Direct observation shows Davy that hydrogen, the alkalies,the metals, etc., are separated by means of the current at thenegative pole, and oxygen and the acids at the positive pole.From this he concludes that the former substances possess apositive, while oxygen and the acids possess a negative electri-cal energy; that in this case,as usual, the oppositely electrifiedbodies attract each other; and that, in consequence, the posi-tive substances separate at the negative pole, and vice versa.In this assumption Davy had arrived at a conception, or, shallI say, an explanation of the phenomena of decomposition ob-served in the galvanic circuit. But he goes a step further, and

18 See p. 22.


tries to refer all chemical combination and decomposition tosimilar causes.

Volta had assumed13 that simple contact of two hetero-geneous bodies was sufficient to place them in opposite electri-cal conditions, and this hypothesis explained the generation ofthe electric current to himself and to his numerous followers.Davy identified himself with this opinion, and tried to proveits accuracy by direct experiments.14 He brought dry, in-sulated acids into contact with metals, and showed by meansof the gold-leaf electroscope that the acids thereby becomenegatively, and the metals positively, electrified. He observedsimilar phenomena on rubbing sulphur upon copper, whenthe former became negative and the latter positive. Davyfound further that these electrical energies which, in the lastcase, for instance, are only slight at ordinary temperatures,increase considerably on heating, and are very great at themelting-point of sulphur. On still further elevating thetemperature, the two substances unite with evolution of light,and the compound obtained is non-electric. Davy concludesfrom this that the combination consists in a mutual dischargeof the opposite electricities, and that heat and light, whichappear simultaneously, are consequences of this discharge.According to him, chemical affinity is produced by differencein electrical condition, and the affinity increases or diminishesthe greater or less this difference is. In cases where there isconsiderable difference of energy, equalisation is accompaniedby phenomena of fire; with feebly electrified substances, onlysmall quantities of heat are evolved; but if combination is totake place at all, the electrical energy must always be ableto overcome the cohesion of the substances. Davy tries toprove directly the dependence of chemical affinity uponelectrical condition, since he says :—w

a As the chemical attraction between two bodies seems tobe destroyed by giving one of them an electrical state different

13 Brugnatelli, Ann, di. China. 13 and 14; compare also Ann. Chim.,40, 225. u Phil. Trans. 1807, 32. w Ibid. 1807, 39.

LKCT. v.] HISTORY OF CHEMISTRY 73

from that which it naturally possesses ; that is, by bringingit artificially into a state similar to the other, so it may beincreased by exalting its natural energy. Thus, whilst zinc,one of the most oxidable of the metals, is incapable of com-bining with oxygen when negatively electrified in thecircuit, even by a feeble power ; silver, one of the leastoxidable, easily unites to it when positively electrified ; andthe same thing might be said of other metals."

He observes in another place that if there were no co-hesion, the chemical attraction would necessarily be propor-tional to the electrical forces. Both are, in his view, effects ofthe same power, which, if it extends to the mass of a sub-stance, gives rise to electricity, while, if it excites the smallestparticles, it produces affinity.16 By the action of the electriccurrent, the electricity which was liberated from the atomsupon their combination, is restored to them again, and de-composition is thereby effected. In this action the positivesubstance goes to the negative pole and vice versa.

It must be admitted that these views start from a simpleand a clear idea, and that by the application of this idea theyexplain the observed facts in a way that is easily understood.They therefore fulfil the conditions which are required in ascientific hypot hesis, and secure that their founder, Davy, shallalways be regarded as an investigator of undoubted originality.Davy's fame spread very rapidly, and, when he succeeded, ayear afterwards, in isolating the alkali metals, it appeared as ifhe had, for the first time, pointed out the proper line of ad-vance in chemistry. It is true that Davy's theory was illumi-nated by but short glimpses of sunshine, for ten years later wefind that it is abandoned. It was certain to fall as soon as thecontact with one another of heterogeneous substances was nolonger regarded as a source of the excitation of electricity; andthe affirmative view with respect to this matter was soon vigor-ously contested. Ritter, in particular, strove to show17 that

w Davy, Elements of Chemical Philosophy, 165. ^ Ritter, Elek-trisches System. 49 ; see also Gehlert's Physikalisches WOrterbuch. 4, 795.

li HISTORY OF CHEMISTRY [LECT. V.

galvanic currents are only produced simultaneously withthe occurrence of chemical decompositions. He assumedthat electrical phenomena are the consequences of chemicalprocesses, but that mere contact is not sufficient to pro-duce different electrical conditions.

Davy's theory could not be brought into harmony withthese views, and accordingly it was given up. This was not,however, on chemical but on physical grounds. A new systemhad in fact already arisen, which was able effectively to take theplace of the old one. This was the electro-chemical theory ofBerzelius. I postpone its discussion until the next lecture,because otherwise I should require to enter fully into the re-searches of Berzelius ; whereas I desire at this place to explainmore clearly the influence of Davy upon chemistry, by givingsome account of the discovery of potassium and sodium, andof the discussion regarding the nature of these substances.

In the course of his investigations respecting the con versionof water into acid and basic substances, Davy had had oppor-tunity of gaining a knowledge of the decomposing power of theelectrical current, since neither glass, agate, nor felspar hadproved able to resist its effects. He thus hit upon the idea ofexposing the alkalies also to this action, in order to separatethem into their constituents if any such were present in them.18

For these experiments he first employs concentrated aqueoussolutions of potassium and of sodium hydroxides ; and as hedoes not succeed in obtaining products of their decomposi-tion in this way, he next passes the current through thefused alkalies. He now observes the formation of smallmetallic globules, which burn, however, with great brilliancy,as soon as they come into contact with the air. Yet, bysuitable arrangements^ he succeeds in isolating small quanti-ties of potassium and of sodium, and in studying their mostimportant properties. I remark, in passing, that he onlyobtained potassium in the fused state. Gay-Lussac andThenard, who showed how to effect the reduction of the

18 Phil. Trans. 1808, 1 ; A.C.R. 6, 5.

v.j HISTORY OF CHEMISTRY 75

alkali metals by means of iron, in 1808,19 first proved thatpotassium is solid at ordinary temperatures. They usedpurer materials, and, by their method, they had muchlarger quantities of the new substances at their disposal.

I cannot enter here into the complete history of potas-sium and sodium, although the subject becomes highlyinteresting from the facts that all Davy's experiments are atonce checked by the French chemists ; that the latter thenbring forward independent results, which Davy doubts ;and so forth. But one point in this somewhat vigorouslysustained discussion seems to me to be of sufficient im-portance to deserve attention, namely, the views concerningthe constitution of potassium and of sodium, and thoseconcerning their relation to the alkalies.

In the decomposition of the alkalies, Davy had observedthat the potassium and sodium appear at the negative pole,whilst oxygen is simultaneously evolved at the positive pole.211

He had further found that the new substances possessed theproperty of reducing metallic oxides ;21 and he believed thatthe alkalies were reproduced when the metals were burned inoxygen.22 From these results he draws the conclusions thatthe alkalies are the oxides of metals, and that the substanceshe has discovered are the metals themselves.28 The physicalproperties of the substances, especially their metallic lustre,supported this view, although their low specific gravityseemed to be an argument, but not a sufficient one, againsthis conclusions. Davy accordingly proposes, for the sub-stances, the names Potassium and Sodium, in which thetermination is intended to indicate their metallic nature.

Davy now holds so firmly to this hypothesis regarding theconstitution of the alkalies, that he also regards as oxides manyother substances about whose composition the evidence is notyet by any means so clear. Thus, like his contemporaries, he

19 Ann. Chim. 65, 325. *> Phil. Trans. 1808, 6 ; A.C.R. 6, 10.21 Phil. Trans. 1808, 19; A.C.R. 6, 22. » Phil. Trans. 1808, 8 ;A.C.R. 6, n. >a Phil. Trans. 1808, 32; A.C.R. 6, 34.

76 HISTORY OF CHEMISTRY [LECT, V.

assumes the existence of oxygen in hydrochloric acid, and,inopposition to the experiments of Bertholiet,24 advances theopinion that ammonia is also an oxygen compound.25 Hesuspects the presence of oxygen in silicic acid, which he triesto reduce;2G in the earths, which, as we know, he succeededin reducing ;27 finally also in phosphorus and sulphur,2S aview which was refuted.by Gay-Lussac and Thenard.29

Some time afterwards, when engaged upon the investiga-tion of ammonium amalgam30 (which had been discoveredshortly before bySeebeck81 and more minutely studied by Ber-zelius and Pontin32), he finds its behaviour to be analogous tothat of the other amalgams, and assumes that it is producedby the combination of mercury with a hypothetical substance,ammonium, resembling the metals and itself containing hy-drogen and ammonia. In comparing now this ammoniumwith the metals, he is led to ascribe a similarity of constitutionto both ; that is, he assumes the existence of hydrogen in themetals also, whereby their combustibility could be explained.Davy states this as a possibility, which he- most correctlyrecognises as identical with Cavendish's phlogiston theory,and which he naturally extends to potassium and sodium.

Gay-Lussac and Thenard arrived almost simultaneously atthe same view.33 They had investigated the action of potas-sium on ammonia gas, and, in doing so, had observed the for-mation of a green substance (potassamide),with simultaneousevolution of hydrogen. In these experiments, which theycarried out quantitatively, they found the quantity of hydrogenevolved to be identical with that which the quantity of potas-sium employed would have liberated from water. They furthershowed that, in addition to the formation of potash, the whole

*•" Mem. de l'Acad. 1785, 324 ; Stat. Chim. 2, 280 ; E. 2, 238. <25 Phil.Trans. 1808, 35 ; A.C.R. 6, 37. 26 Phil. Trans. iSio, 59. 27 Ibid. 1810,16, 62, etc. iJ8 Ibid. 1809, 67, etc.; Ann. Chim. 76, 145. 29 Recherchesphysico-chimiques, 1, 187. :*° Phil. Trans. 1810, 37. 81 Gehlen'sJournal fur die Chemie, etc., 5, 482. J{2 Biblioth&que Britannique (1809),122, Nos. 323 and 324; Gilb. Ann. 36 (1810), 261. xi Ann. Chim.66, 205.


of the ammonia employed was generated anew on the de-composition of the green substance by the action of waterupon it. They explained these observations by the assump-tion that potassium consisted of potash and hydrogen ; andthat the latter was liberated by treatment with ammonia, aswell as with water, the alkali combining simultaneously withthe ammonia or with water. Thus, in accordance with theirview, potassamide was composed of ammonia and potash, andit was decomposed into these constituents by the action ofwater upon it.

Davy, meanwhile, had reverted to his first explanation,and he now attacks these latest results obtained by Gay-Lussac and Thenard.34 In so far as the accuracy of his experi-ments is concerned, however, he falls short of his opponents,although he is more fortunate and more masterly in the inter-pretation of them. In his view, the evolution of hydrogenduring the action of potassium on ammonia, arises from thedecomposition of the latter. According to him the greensubstance is composed of potassium and the residue from theammonia gas. On treating the green substance with water,the latter is decomposed into its constituents, regeneratingammonia, and giving its oxygen to the potassium which isthus converted into potash.85 In addition to this, since heconsiders fused caustic potash to be free from water, he findsin the conditions necessary for the preparation of potassium,a further argument against its containing hydrogen.

Gay-Lussac and Thenard persist, at first, in their previousopinion that potassium and sodium are hydrogen com-pounds,80 and in this they are supported by the contempor-aneous statements made by Berthollet37 and by D'Arcet*38

who regard fused potash as containing water. In 181 x, how-ever, they adopt Davy's view.39 The reason for their changeof opinion is to be found in the following facts. They hadobserved that the substance obtained upon burning potassium

u Phil. Trans. 1808, 365 (Note). ;J5 Ann. Chim. 75, 168 etseq., 264,m Ibid. 75, 299. *? M&n. d'Arcueil. 2, 53. ^ Ann. Chim. 68, 175.y9 Rech. phys. chim, 2, 250, etc.


is different from potash, in so far that it contains more oxygenthan the latter;40 and they point out that the fact of potas-sium containing hydrogen would necessitate that the newoxide should contain water, since the production of uncom-bined water is not observed during the combustion. Further,as the oxide is decomposed by dry carbonic anhydride withthe formation of oxygen and potassium carbonate, theirhypothesis as to the composition of potassium leads to theassumption of the presence of water in salts, in cases whereanalysis does not reveal any.

From this period onwards the non-decomposability of themetals was no longer seriously doubted. At the same timethe elementary nature of phosphorus and of sulphur wasestablished anew by decisive experiments of Gay-Lussac andThenard,41 and the view that ammonia contained oxygenwas recognised as an error by the younger BerthoUet.42

To the foregoing, I may add an account of another scien-tific discussion, between Davy on the one hand, and Gay-Lussac and Thenard on the other, which is also of importanceinasmuch as it led to the overthrow of Lavoisier's theory ofacids. The discussion in question touches the compositionof hydrochloric acid. In this acid,-as in all others, Lavoisierhad assumed the existence of oxygen. Its presence therehad never been established, it is true, but the general theoryrequired i t ; and as this theory was almost universallyadopted, the existence of oxygen in hydrochloric acid seemedto be unquestionable. I say it was almost universally adopted,because BerthoUet, for instance, was of a different opinion.In 1787 the latter had examined hydrocyanic acid, discoveredsome time previously by Scheele,43 and had found in itcarbon, hydrogen, and nitrogen only. It was likewise known,from other investigations by Scheele, that sulphurettedhydrogen contains hydrogen and sulphur only ; and conse-quently BerthoUet felt justified in regarding other elementsas acid-producing, as well as oxygen.44 He does not appear,

40 Rech. phys. chim. 1, 125. 4l Ann. Chim. 73, 229. 42 M<§m.d'Arcueil. 2, 268. 4S Ann. Chim. 1, 30. 4i Stat. Chim. 2, 8 ; E. 2, 8.


however, to have had many adherents in this view, and, sofar as hydrochloric acid is concerned, he also assumed theexistence of oxygen in it. Chlorine was looked upon asoxygenated muriatic acid, and was supposed to be producedfrom muriatic (hydrochloric) acid by its taking up oxygen.

These latter views were strengthened by the experimentsof Henry, and by the interpretation of them which hegave.45 He passed electric sparks through gaseous hydro-chloric acid, which was confined over mercury, and obtainedhydrogen, while the metal was simultaneously attacked bywhat he believed to be free oxygen. This led him to theassumption of the presence of water in hydrochloric acid, aview which found general approval since the investigationsof others appeared to be in agreement with it.

In 1808, Davy had decomposed hydrochloric acid bymeans of potassium,46 and in this way had obtained hydrogenand potassium chloride, the latter of which he had also pre-pared by burning potassium in chlorine. He showed, in1809, that the chlorides (muriates) of the metals are not de-composed by heating them with phosphoric glass,47 or withsilicic anhydride, but that decomposition at once beginswhen aqueous vapour is -passed over the mixture.48 Davywas of opinion that Henry's hypothesis furnished the ex-planation of these experiments, and that hydrochloric acidcould only be separated as soon as the quantity of waternecessary for its existence was supplied. Gay-Lussac andThenard further showed, about the same time, that water isproduced as well as silver chloride by the action of this acidupon silver oxide ; and, as formerly, they assumed that thiswater was already present in the hydrochloric acid.49 Theythen effected the synthesis of the acid, by exposing a mixtureof chlorine and hydrogen to sunlight.60 On this occasion,they advance a complete theory regarding hydrochloric acid

45 Phil. Trans. 1800, 191. « ibid. 1809, 91 ; A.C.R. 9, 7.47 Phosphoric glass is calcium metaphosphate j obtained by sufficientlystrongly heating monocalcrum phosphate. ** Phil. Trans. 1809, 93;A.C.R. 9, 9. m Rech. phys. chim. 2, 118. w Ibid. 2, :$9.


and chlorine, by means of which they are able to explainall their experiments.51 According to them, hydrochloricacid is a compound of an unknown radical, muriaticum, withoxygen and water ; chlorine, on the other hand, is anhy-drous hydrochloric acid combined with more oxygen, or,what amounts to the same thing, it is ordinary hydrochloricacid minus hydrogen. On this hypothesis the above-men-tioned experiment of the synthetic formation of hydrochloricacid is easily explained. The other facts connected withthe matter can be explained on the same hypothesis injust as logical a manner. It is true that the two Frenchphilosophers exerted themselves fruitlessly in trying to givedirect proof of the presence of the supposed oxygen. It wasin vain that they passed hydrochloric acid gas over red-hotcharcoal: no change was observable, and this negative resultmight well have led them to another explanation.52 Theypoint out that the hypothesis that chlorine (oxygenatedmuriatic acid) is a simple substance, and hydrochloric acidits hydrogen compound, could also serve as the basis for theexplanation of the observed facts ; they prefer, however, toadhere to the old view.

Davy, who arrived, independently it would appear, at thelatter assumption, declares himself a decided adherent ofit.53 He lays great stress on the fact that it agrees withScheele's original idea, in accordance with which chlorinewas dephlogisticated hydrochloric acid; and he tries tosupport this view by means of new arguments and newexperiments. He draws attention to the fact that chlorinedoes not become converted into hydrochloric acid by theremoval of oxygen, but only by treatment with substancescontaining hydrogen ; and, further, that chlorine is aneutral substance, which, adopting the old hypothesis, isnot in agreement with Lavoisier's theory, since it wouldthen be necessary to assume that a substance indifferent tolitmus had been obtained from an acid by the addition of

51 Mem; d'Arcueil. 2/339; Bulletin de la Soc. Philomatique, No. 18,May, 1809. 52 Mem. d'Arcueil. 2, 357-358. M Ann. Chira. 76, 112 and 129.

LECT. v.] HISTORY OF CHEMISTRY 8l

oxygen to the latter. Finally, he points out to chemists themany hypothetical substances to which it is necessary to haverecourse in order to preserve the older view, whereas the newone explains the facts in the simplest manner.

Gay-Lussac and Thenard do not admit this. In 1811,when they publish their investigations in full, under the title :Recherchesphysico-chimtques•, they place the two theories sideby side, and show how bothsuffice to explain the facts.54 Never-theless, they declare against the new system, and the reasonthey give for doing so, deserves to be mentioned. If chlorinewere a simple substance, dry sodium chloride would necessarilydecompose water when it dissolved in it, in order that muriateof soda might be produced ; and this they consider more thanunlikely. Moreover they held Lavoisier in too high esteem toabandon lightly any such doctrine as the one advanced by himthat all acids contain oxygen.

But their resistance did not avail them long ; the force offacts was stronger than they were, and Gay-Lussac was far tooclear-headed a thinker to be blind to them. The facts thatmade him an adherent of Davy's view, in 1813, were especiallyhis own experiments upon iodine/35 which had been discoveredby Courtois and described by Clement,50 and whose analogywith chlorine he recognised and emphasised ; and further, thediscovery of hydriodic acid. From this time onwards, the newtheory gains ground, and even Berzeiius is unable to alter thecurrent of opinion, although he puts himself to every conceiv-able pains, in a paper published in 1815, to deter chemists fromthe threatened step.67 After pointing out that the hypothesisof munaticum is still in a position to explain the facts, hedraws attention to the fact that it, alone, is in agreement withthe general theory of acids ; whereas, on the assumption thatthere is no oxygen present in hydrochloric acid, a distinction isdrawn between this compound and the other acids, to which,nevertheless, it shows the greatest resemblances. The salts

u Rech. phys. china. 2, 94. m Ann. Chim. 91, 5. m Moniteur.1813, Nos. 336 and 346. s7 Ann. Phil 2, 254; Schweigger's Journal.14, 66.

F

82 HlSTOliY OF CHEMISTRY [LECT. V.

also, would then necessarily fall into two classes ; or in otherwords, it would be necessary to assume differences in con-stitution in a series of substances whose behaviour is similar inevery respect. He thinks, further, that he is justified in con-cluding from the laws of combination, that chlorine is not anelement. I do not enter more minutely into the matter sincehis arguments had little effect. They came too late. Gay-Lussac's investigation of hydrocyanic acid in the same year58

indisputably proves the acid nature of this compound, and thefact that it does not contain oxygen ; and hence even Ber-zelius cannot maintain Lavoisier's definition of the acids andof the acidifying principle.

Some other cause which might furnish the acid characterobserved in certain substances was now sought for. Theconception of an acid seemed so definite at that time, and thesubstances included in this class were so distinctly separatedfrom all other substances, that it was necessary to enquire intothe cause which occasioned this difference. Besides, it cannotbe denied that Lavoisier, and even the chemists of the begin-ning of the nineteenth century were still influenced, in a certainrespect, by the ideas of the Greek philosophers. Just as thelatter ascribed general properties to the presence of a commonconstituent and identified the particular property to a certainextent with a particular constituent—just as they explainedcombustibility, for example, by the presence of a fire-material—so Lavoisier an d his adherents believed that in oxygen they haddiscovered the acidifying principle.

In a similar manner we find Davy, after he was satisfiedthat hydrochloric acid contains hydrogen and chlorine only,stating the view that the chlorine is the acidifying principle init, and the hydrogen its basis or radical.59 At a later dateGay-Lussac60 introduces the name '" hydracids " for the acidsfree from oxygen, and places hydrochloric acid, hydrocyanicacid, sulphuretted hydrogen, and hydriodic acid in this class.

60

58 Ann. Chim. 95, 136. m Phil. Trans. 1810, 231 ; A.C.R. 9, 21.Ann. Chim. 91, 148 ; 95, 162.


Even if the cause of the acid nature was to be found inthe chlorine, iodine, etc., rather than in the hydrogen, still thelatter was the common constituent of them all,, and hencebetter adapted to the formation of a name. Davy's investiga-tions upon chloric and iodic acids lead him to much moregeneral views. a Acidity does not depend upon anypeculiar elementary substance, but upon peculiar combina-tions of various substances."61

At this time he seeks to prove that it is not oxygen whichdetermines the peculiar character of an acid. Thus, forexample, when oxygen is united to common salt, the neutralityof the substance is not disturbed ; whereas, on the other hand,the saturating capacity of chloric acid is not altered when allthe oxygen is removed from it. This obliges Davy no longerto regard chloric acid (in accordance with Lavoisier's view) asan oxide of the radical chlorine, which, combined with water,forms the hydrated acid. He finds that, without water, chloricacid cannot exist; and for this reason he regards it as a ternarycompound of hydrogen, chlorine, and oxygen. Again, theexistence of euchlorine, which he obtains from chloric acidand hydrochloric acid,62 provides him with a reason againstLavoisier's hypothesis regarding acids.

The principles of a new theory of acids are included inDavy's discussions ; but he did not follow them up sufficiently,otherwise they might have prevented the distinction whichnow began to be drawn between acids which did and thosewhich did not contain oxygen. The same remark holdslikewise with respect to Dulong, who had read a paper beforethe French Academy in 1815, in which he had stated his viewregarding acids. This paper, unfortunately, does not appearto have been printed in extenso) and therefore I am able togive but little account of it.63 Dulong on this occasionexamined oxalic acid. The behaviour of some of its salts,which give off water when heated, led him to the opinion that

61 Phil Trans. 1815, 219. 62 Ibid. 1811, 155 ; A.C.R. 9; 63. « Mem.de l'Acad. 1813*1815, Histoire, p. exeviii. ; see also Schweigger's Journal,17, 229; Ann. Phil 7, 231.


the acid might be regarded as a hydrogen compound ofcarbonic acid—hydro-carbonic acid. On saturation with ametallic oxide, the oxygen of the oxide combines with thehydrogen of the oxalic acid to form water which can then bedriven off, and a compound ofcarbonic acid with metalremainsbehind. Here we find the view for the first time represented(by Dulong and Davy) that the water produced during saltformation was not already contained in the (oxygenised) acid,and that, in a salt, it is not a metallic oxide but the metal, assuch, that exists.

It is stated that Dulong made a similar assumption for theother acids, but unfortunately the development of his ideashas not been handed down to us. Besides, hypotheses of thiskind met with little approval at that time ; voices were loud intheir condemnation from the most different sides. Gay-Lussacdeclared himself emphatically against them ;C4 and Berzelius(who was then beginning to exercise a predominant influence)seeing that he was obliged to admit the existence of acids freefrom oxygen, introduced a strict distinction between the latterand the acids containing oxygen, and so between the haloidand the amphid salts.

This single point in the system of Berzelius must not,however, be treated of separately, but the system must be dealtwith as a whole. His views are of the utmost importance,since they dominated theoretical chemistry for twenty years,I shall devote the next lecture to their consideration.

w Ann. Chim. [2] 1 (1816), 157.

L E C T U R E VI.

BERZELIUS AND HIS CHEMICAL SYSTEM—DULONG AND PETIT'S LAW—ISOMORPHISM—PROUT'S HYPOTHESIS—DUMAS' VAPOUR-DENSITYDETERMINATIONS—GMELIN AND HIS SCHOOL.

BERZELIUS adopts dualism as the basis of his system. Evenbefore his time, the majority of compounds had been lookedupon as consisting of two parts. A uniform mode of regard-ing them in this aspect became possible to a still greaterdegree in the light of the electro-chemical phenomena, andit is the great merit of Berzelius to have introduced this intothe science and to have established it.

In his view, compound substances are produced by thearrangement of the atoms side by side.1 Compounds of thefirst order are formed in this way from the smallest particles ofthe elements; these compounds give rise in turn to the forma-tion of compounds of the second order; and so on. Berzelius,like his predecessors, seeks, in affinity, the reason for the com-bination of two atoms, but this again is for him, as it was forDavy, a consequence of the electrical properties of the smallestparticles. He differs from Davy very essentially, however,in the manner in which he assumes the electrical distribution.But, quite apart from this, the two theories are not to becompared as regards their importance for our science. Davy,no doubt, advanced ingenious ideas as to the mode in whichhe considered the chemical and electrical phenomena to beinter-related ; but from these hypotheses, by means of which a

1 Essai sur la throne des proportions chimiques et sur 1'influencechimique de I1 Electricity, 26. See also Berzelius, Lehrbuch der Chemie,First Edition, Vol. 3, part I. Compare also Schweigger's Journal. 0(1812), 119.

86 HISTORY OF CHEMISTRY [LECT. VI.

number of facts could be most excellently explained, he neversucceeded in producing a theory which might serve as thefoundation of a chemical system. Berzelius was the first todo this. He made it his life's task to establish in chemistry auniform system which should be applicable to all the knownfacts; and he accomplished it. Hence his views are of fargreater importance in the development of chemistry thanthose of Davy are.

According to Berzelius, it is not only when two substancesare brought into contact that electricity is generated, but itis a property of matter; and in every atom, two oppositelyelectrical poles are assumed.2 These poles do not, however,contain equal quantities of electricity. The atoms are unipolar,the electricity of the one pole predominating over that of theother; and thus every atom (and therefore every element)appears to be either positively or negatively electrical. In thisrespect it is possible to arrange the elementary substances intoa series, so that each member is always more electro-negativethan the next succeeding one. Oxygen stands at the top, andis absolutely electro-negative,3 while the other substances areonly relatively positive or negative according as they are com-pared with elements which come before them or after themin the electrical series. This series does not constitute a tableof affinities in the Geoffroy-Bergman sense ; and it does notexpress the affinity of the individual substances for oxygen, forexample. Berzelius has not forgotten Berthollet's teaching,that affinity is not of a constant character and independent ofthe physical conditions, as he supposes this unipolarity to be ;and he is also well aware that oxygen can be removed frommetallic oxides by carbon or sulphur, that is to say, by otherelectro-negative substances. With him, affinity depends prin-cipally upon the in tensity of the polarity, i.e., upon the quantityof electricity which is contained in the two poles. This isvariable, however, especially with changes of temperature.

2 Essai etc. 85. 3 In Schweigger's Journal. 6, 129, where he states hiselectro-chemical theory in detail JJerzelius calls oxygen electro-posJtive?

LECT. vi.] HISTORY OF CHEMISTRY 87

Generally speaking, it is increased by supplying more heat,and this explains why certain combinations only take placeat a high temperature.4

During the combination of two elements, the atomsarrange themselves with their opposite poles towards eachother, and mutually discharge their free electricities, wherebythe phenomena of heat and of light are produced. The olddoctrine is explained at the same time— Corpora non aguntnisisohita (substances do not interact unless dissolved), since freemotion of the smallest particles is only possible in the liquidstate. When a substance is subjected to the action of theelectric current, the latter restores to the atoms their originalpolarity, whereby the substance breaks up into its constituents.

A compound of the first order is not electrically (nor yetchemically) inactive, since, during the combination, only onepole of each atom is neutralised ; it is still unipolar, and it canenter into further combinations (of the second order) which arelikewise endowed with electrical forces; but the intensities ofthese forces diminish, the higher the order of the compoundbecomes, since the stronger poles are, in general, neutralisedfirst. According to Berzelius, the specific unipolarity of theoxides depends solely upon the radical or element combinedwith the oxygen. The latter gives ris,e to the most powerfullyelectro-positive and electro-negative substances (alkalies andacids) ; and as it cannot, therefore, be itself the cause in bothcases, it cannot be the cause in either.5

All chemical reactions, and, consequently, the phenomenaof heat and light that accompany them, are, according toBerzelius, produced by electricity, which " thus seems to bethe first cause of the activity all around us in nature.'70

If a substance C is to decompose the compound AB1 sothat B may become free, then C must be able to neutralise agreater amount of the electrical polarity of A than B can.Further, a mutual exchange between AB and CD only occurs

4 Lehrbuch. Second Edition, Vol. 3, Dresden (1827), part I, 73-74.% \h\d. 76. « Tbid. 77.

SS HISTORY OF CHEMISTRY [LECT. VI.

if the electrical polarities are better equalised in AC and BDthan they were previously. In reactions of this kind, Ber-zelius, like Berthollet, assumes an influence upon the resultingphenomena, of the quantities of the substances present andof cohesion; but he differs from Berthollet in regarding theaffinity as a function of the electrical polarity, and as inde-pendent of the saturating capacity of the substance.

This theory constitutes the basis of the dualistic theory ofchemical composition. Berzelius establishes it as follows:—T

LiIf the electro-chemical views are accurate, it follows thatevery chemical combination depends wholly and solely upontwo opposite forces, namely, the positive and the negativeelectricities, and that every compound must be composed oftwo parts, united by the effects of their electro-chemical reac-tions, since there is not any third force. From this it followsthat every compound substance, whatever the number of itsconstituents may be, can be divided into two parts, of whichthe one is positively and the other is negatively electrical.Thus, for example, sulphate of soda is not composed of sulphur,oxygen, and sodium, but of sulphuric acid and soda, each ofwhich can, in turn, be separately divided into an electro-positive and an electro-negative constituent. In the same way,also, alum cannot be regarded as immediately composed of itselementary constituents, but- is to be looked upon as the pro-duct of the reaction of sulphate of alumina, as negative element,with sulphate of potash, as positive element j and thus theelectro-chemical view justifies what I have said with respect tocompound atoms of the first, second, third, etc., orders."

As may be perceived from this, Berzelius had formed adefinite opinion as to the composition of compounds, and hew,ent so far with this opinion, that he regarded as inadmissiblethe assumption, which lies readiest to hand, that the substanceis composed of its elementary constituents. He thought heknew the arrangement of the atoms in compounds so minutely(principally from the decompositions which they underwent

7 Lehrbuch. 3, part I, 79-30.


when subjected to the influence of the electric current), thathe only regarded one particular view as possible.

As an appendix to the compounds, Berzelius takes solu-tions into consideration. He does not place these in the sameclass with the compounds, because a disappearance of heat isobserved during their formation, and therefore no electricaldischarge can take place in the operation.8

Before proceeding to the further statement of the systemof Berzelius (especially before turning to his interesting andextremely important method of atomic weight determina-tions) I wish to say something about the nomenclature9 andthe system of notation,10 which he had proposed some yearsbefore. In doing so, I can be all the more concise, as theformer is merely the perfecting of the system introduced byGuyton, Lavoisier, Berthollet, and Fourcroy,11 and both are,no doubt, well known. I can, therefore, confine myself towhat is of essential importance, or is characteristic of thepoint of view of Berzelius.

Substances are divided into ponderables and imponder-ables. Amongst the latter we find electricity, magnetism,heat, and light. The former are divided into elements andcompounds, solutions and mixtures. Amongst the simplesubstances, Berzelius places the metals and the metalloids.He uses a word here which Erman had employed before himfor designating the metals of the alkalies and of the earths;12

but Berzelius is the first to give it the meaning that we stillattach to it.

The oxygen compounds are either called oxides or acids.The substances of this class which possess neither basic noracid properties, and contain relatively little of the negativeelement, are called sub-oxides. The basic salt-forming oxygencompounds are designated oxides ; when an element or aradical forms two substances of this kind, these are distin-guished by the terminations of the specific name. This is very

8 Lehrbuch. 3, part I, 80. 9 Journ. de Phys. 73, 253. 10 Essai etc.j\X. J1 Compare p, 32, n Gilb. Ann. 42, 45.

90 HISTORY OF CHEMISTRY [LF.CT. VI.

easy in the Latin nomenclature (which Berzelius proposes toemploy) ; for example, oxidum ferrosum (with the smallerratio of oxygen) and oxidum ferricum. Finally, superoxidesare also distinguished. These contain a relatively large propor-tion of oxygen, and must be reduced before they form salts.

What Berzelius says respecting compounds with water, is ofinterest. According to him, water may occur in compounds,combined in three different ways. It plays the part, either ofan acid, as in the caustic alkalies, or of a base, when it uniteswith acids. In both of these cases it is called water of hydra-tion, and is distinguished from water of crystallisation whichunites with salts, and can be separated from these again, with-out their being, thereby, essentially changed in their nature.

The system of notation of Berzelius is original with himself,and, up to the present, it has always proved so thoroughlypractical that we have retained his proposals almost withoutalteration. In his system, the atom of an element is repre-sented by the initial letter of the Latin name of the element;and by placing the symbols side by side, the atoms of com-pounds are obtained. When several atoms of one elementoccur in a compound, a figure giving the number of these isplaced after the letter, and above (or below) the line. The so-called double atoms (i.e., two atoms of an element, which occurtogether) constitute an exception to this ; in these the symbolfor the atom is " barred."13 Thus, for example, i i = H2, or twoatoms of hydrogen ; H 0 = H20, or one atom of water, con-sisting of two atoms of hydrogen and one of oxygen, etc.

In the cases of more complicated compounds, several lettersare separated from others by the sign + ; and the mode ofdivision is dependent on the dualistic view. For the sake ofshortness, the atom of oxygen is often represented by a point,and that of sulphur by a vertical stroke ; and in compounds,these marks are placed above the symbol of the element withwhich the oxygen or sulphur is combined—a method of writingformulae that has now, however, been abandoned.

¥ Lejirbuch. j , part I, i<?8,

,ECT. vi.] HISTORY OF CHEMISTRY 91

With these indications, we shall now leave this matter, andDass on to a much more important subject in the system ofBerzelius, that is, to the mode by which he determined thelumber of atoms in a compound. He was the first who tookaccount of purely chemical facts in these determinations. Herejects Barton's rules entirely, pointing out, with justice, theirgroundlessness:—" When only one compound is known, thereis surely something arbitrary in assuming that it consists ofone atom of each element, altogether regardless of the otherrelations of the compound.14

Berzelius tries to establish the view, however, that regu-larities must exist, which determine the number of the atomsthat mutually combine with one another.15 He argues that ifan unlimited number of atoms of one element could combinewith an unlimited number of atoms of another, there would, inthis way, be produced an infinite number of compounds whichwould differ so little in composition that even our best analyseswould fail to show any difference. The view that substancesconsist of indivisible atoms, and that chemical compounds areproduced by the arrangement of these atoms side by side, isnot sufficient to explain multiple proportions. In the com-bination of atoms, special laws must prevail which limit thenumber of the compounds ; and it is upon these laws, inparticular, that chemical proportions depend.

He finds his first basis in Gay-Lussac's law of gaseousvolumes.

This law appears to him to permit of an unequivocaldecision of the question, since, with him, atom and volume areidentical in the case of simple gases. a We know, for example,with certainty the relative number of atoms of nitrogen and ofoxygen in the different stages of oxidation of nitrogen ; that ofnitrogen and of hydrogen in ammonia ; that of chlorine andoxygen in the different stages of oxidation of chlorine ;" l candso on. Amongst the gases, the law of multiple proportions

14 Lehrbuch. 3, part I, 88, 15 Essai etc. ?8. ™ Lefcrbuch. 3,part I, 89,


finds confirmation in Gay-Lussac's rule ; and, by measuringthe volumes, Berzelius can here determine the number ofatoms which mutually combine with one another. Forexample, since two volumes of hydrogen unite with onevolume of oxygen, water consists of two atoms of hydrogenand one atom of oxygen. He cannot conceive how anyonecan be of a different opinion, and he engages in controversywith Thomson, who only assumes half as many atoms in onevolume of hydrogen as in one volume of oxygen.

li It has been assumed that water is composed of an atomof oxygen and an atom of hydrogen ; but since it contains twovolumes of the latter gas for one of the former, it was concludedthat in hydrogen and in inflammable substances generally, thevolume weighs only half as much as the atom, while in oxygen,volume and atom have the same weight. As this is only anarbitrary assumption, the accuracy of which cannot even betested, it appears to me much simpler and more conformablewith probability to assume the same relation of weight betweenthe volume and the atom in the combustible substances as inoxygen ; because there is nothing which should make ussuppose a difference between, them. If water is regarded ascomposed of two atoms of radical and one atom of oxygen,then the corpuscular (atomic) and the volume theories coin-cide, so that their ^difference only consists in the state ofaggregation in which they present the substances to us.?? 17

It must be mentioned that Berzelius does not extend to thecompound gases, his view as to the identity of volume andatom, but considers that the atoms of these neither occupy thesame space as the atoms of the elements nor show uniformityof volume amongst themselves. That this is so, follows fromhis atomic weight estimations. With him, H = I = i volume* ori atom of hydrogen ; H2O = 18 = 2 volumes or i atom of water;

73 = 4 volumes or i atom of hydrochloric acid, etc.18

17 Lehrbuch. 3, part I, 44-45. 18 It is true that he calls HCl-36.5 anatom of hydrochloric acid and says HC1 is the double atom (see Lehrbuch),but for the most part he actually employs the formula H€£. I return tothis agjain, however, further on.

LECT. VI.] HISTORY OF CHEMISTRY 93

Berzelius does not adopt the distinction between physicaland chemical atoms, introduced by Avogadro and Ampere;and he endeavours to surmount the difficulty which hadled Dalton to regard Gay-Lussac's law as inaccurate, by com-pletely separating from one another the elementary and thecompound gases.

It is clear that the law of gaseous volumes, together withthe conclusions that Berzelius draws from it, is insufficient. It•can only be used to determine the relative number of atoms ina very few compounds, and the founder of the first chemicalsystem is, therefore, obliged to seek for other generalisationsof more universal validity. He advances the followingrules,19 which are intended, however, to apply to inorganiccompounds only.

I. One atom of an element combines with i, 2, 3, etc.,atoms of another element.

He does not state the limit. In 1819, he thinks thatmore than four atoms of one element seldom combine withone atom of another ; afterwards (1828) he drops this limita-tion.

II. Two atoms of an element combine with 3 or with 5atoms of another element.

This rulS leads him to a discussion of the question as towhether a compound of 2 atoms of one element with 4 orwith 6 of another element is identical or not with the combina-tion of 1 atom of the first element with 2 or with 3 of thesecond. In his Text-book (18 2 8) he leans to the latter opinion ;by this time, isomeric compounds were known.

The laws of combination of compound atoms of the first,second, and third orders are quite similar, but certain limita-tions here come into play, arising from the fact that whencompound atoms combine, they have either the electro-negative, or else, less frequently, the electro-positive constitu-ent common to both, and the proportions in which theseatoms then combine are determined by the common element

19 Essai etc. 29-30.

94 HISTORY OF CHEMISTRY [>ci\ vi.

in such a way that the quantities of the latter in the one con-stituent stand to those in the other as I to i, 2, 3, 4, 5, 6,etc. ; as 3 to 2 or 4 ; or finally, as 5 to 2, 3, 4, 4!-, and 6.2U

It is interesting to see how, by the aid of these rules,Berzelius determines the number of atoms contained in acompound. As an example, I choose the oxygen compounds,which are unquestionably of the greatest importance.

Berzelius believes he has discovered (especially from theconsideration of the proportions by volume in the case of gases)that it is usually the electro-negative constituent of whichseveral atoms occur, and, therefore, in the case to be examinedhere, the above mentioned rules take the following form.21

I. When an element or radical forms several oxides, andthe quantities of oxygen in these, as compared witha given quantity of the other element, st and to eachother as 1 to 2, it must be assumed that the firstcompound consists of 1. atom of radical and 1atom of oxygen ; the second, of 1 atom of radicaland 2 atoms of oxygen (or 2 atoms of radical and4 atoms of oxygen). When the proportions areas 2 to 3, the first compound consists of 1 atomof radical and 2 atoms of oxygen; the second, of1 atom of radical and 3 atoms of oxygen, and so on.

In conformity with this rule, Berzelius, in 1819, writessoda NaO2, and the peroxide NaO3 ; and his formulae for theother oxides are similar. Thus it comes about that the atomicweights which he proposes for the metals at this period, aredouble those which he definitely adopts afterwards (1828).2-Influenced by reasons which we shall learn immediately, hemakes, in his Text-book, the following addition to Rule I:—23

When the proportion of the quantities of oxygen intwo compounds is as 2 to 3, then in the first, 1atom of radical can also be combined with 1 atomof oxygen, and, in the second, 2 atoms of radicalcan be combined with 3 atoms of oxygen.

20 Lehrbuch. 3, part I. 40. 21 Essfai etc. 118. ** Berzelius, Jahres-bericht 1828, 73. n Lehrbuch. 3, part I, 90.

Lsct. vi.] HISTORY OF CHEMISTRY 96

II. When a positive oxide combines with a negative one(a base with an acid, for example), the oxygen inthe latter is a multiple, by a whole number, of thatin the former, and this number usually is, at thesame time, the number of the oxygen atoms in thenegative oxide.

These are the only two rules which Berzelius advancesin 1819 in his theory of chemical proportions. In the transla-tion of the second edition of his Text-book (theifirst Germanedition) new rules are added, called forth by Mitscherlich'sdiscovery of isomorphism, and by the relations which Dulongand Petit had found (1819) between the atomic weights andthe specific heats of solid elements.

Since both of these investigations are of the greatestimportance with respect to the views of Berzelius upon thequestion now under discussion, I shall here introduce theresults of these investigations, and shall then proceed with theconsideration of the atomic weight determinations.

Dulong and Petit proved, by exact experiments,24 that theproducts obtained by multiplyingthe specificheatsof bismuth,lead, gold, platinum, tin, silver, zinc, tellurium,copper, nickel,iron, cobalt, and sulphur, by the respective atomic weights ofthese elements, are almost identical; and from this fact theydrew the conclusion that the same regularity would hold withrespect to all the elements and would lead to the exactdetermination of their atomic weights.

In order to establish the law, Dulong and Petit hadassumed the atomic weights of most of the metals, withreference to that of sulphur, to be only half as great asBerzelius had stated them in 1819. They assumed 20r forthe atomic weight of sulphur (O = 100), as Berzelius had done,and then made Fe = 339, whereas Berzelius had adopted 693.According to their law, the atomic weight of silver was onlyone-fourth of Berzelius7 number. In the cases of telluriumand of cobalt, they arrive at results which are still further at

24 Ann. Chim. [2] 10, 395!

96 HISTORY OF CHEMISTRY [LECT. VI,

variance with his, but these do not merit any confidence, aslater observers (Regnault25 and Kopp20) have found other"numbers which agree better.

I may take this opportunity to state that Neumann showed,in 1831,27 that Dulong and Petit's law may also be extendedto compounds of analogous composition; that is to say, thespecific heats of these compounds multiplied by their equiva-lent weights (as Neumann calls them) give equal products.The law was proved, in particular, for the carbonates and thesulphates.

Before I pass on to Mitscherlich's interesting results, I wishto make some historical remarks by way of preface. WithHauy, the crystalline form (primitive form) was an importantcharacteristic for the determination of the nature of a sub-stance ; difference of form being, in his view, a ground for as-suming a different composition,28although Berthollet disputedthis.29 Gay-Lussac observed, in 1816, that crystals of potashalum increase in volume in a solution of ammonia alum, with-out altering in shape.80 Beudant31 also made very interestingstatements in this connection ; and, as early as 1817, J. N. v.Fuchs32 drew attention to the similarity of thecrystalline formsof arragonite, strontianite, and cerussite. Gehlen stated thathe had succeeded in preparing crystals of alum with soda.83

These were isolated observations, which were insufficientto overthrow Hauy's doctrine, and which only attained to anyimportance through Mitscherlich's discovery of isomorphism.84

In 1820, Mitscherlich established the fact that the corre-sponding phosphates and arseniates, with the same number ofmolecules of water, possess the same crystalline form, so thateven the secondary forms coincide. Even at that time, the samenumber of atoms was assumed to be present in both acids, and

-5 Ann. Chim. [2] 73, 5 ; ibid. [3] I, 129; 9, 322 etc. 2tJ Annalen.Supplementary Vol. 3, 291. <27 Pogg. Ann. 23, I. 28 Traite'de mineYalogie.29 Stat. Chim. I, 433 ; E. I, 432. 30 Kopp, Geschichte. 2, 406. 31 Ann.Chim. [2] 4, 72 ; 7, 399 ; 8, 5 ; 14, 326. 82 Schweigger's Journal. 19, 133-8S Ibid. 15, 383, Note. iu Ann. Chim. [2] 14, 172; 19, 350; 24, 264and 355*

LECT. vi.] HISTORY OF CIIKMISTKY '.'?

thus Mitscherlich arrived at the idea I h;it it w.i- f h»;- Miuil.u it vof atomic constitution which gave rise to tit- iilrniitv »»f f"*n>.And he really succeeded in confirming thN ««|>iiti«»u l»v a "*-«i'"-of facts. He called those substances isomnrplioiis wlm !i r\-hibit the same crystalline form in trnv*f>nn<J»V{! rMMp» 'in-.l-,and which, since they can crystalline U w ffirf i* }4 .«»another in indefinite proportions, f It pnintrfl« r? f i,« * M Iphism of selenic acid and sulphuric :u:'ul ; tiMt *»t II- ut .1 * ,zinc oxide, nickelous oxide, ferrnu* oxiclr, r h . n l!v JIneutral sulphates, etc.; that of alumina, frrnt ^\^h •<• f

manganic oxide. He also showed that Hciiil.u»ri «!r*pition, in accordance with which irnn vitrii»l *tn*I /ii^ %sni I(two salts of different crystalline form, and 4*«nt iiuin,1 »Ji«t 1ent proportions of water) crystalline toî'thri, ilrpfN/I, v*>t *,the fact that the proportion of water in tin* « nr r MJDJ <*»;«*"!is changed, and becomes the same a that in flit M'L* I,

Other observers confirmed M IIMJICI lk,h ;* \ it A \»\ u*i u •»•;manyobservations,35so that,at t\\n\ tinn , rnui h ^* ûpon the crystalline form of sub tanc ts; nitU U* nt 1 u^v Ithat they possessed, in this charm trr, ati r.\M ii* i «,* ^ «obtaining information regarding their dhmik % m *i';i>uu I?was Berzelius, in particular, who, in;»t;4iiflv f u ^ i . M , f}bearing of the great discovery, applied if m ih ruu*u i n * fhis system. Isomorphism led him to thr fulfilltu^ tuU !

III. When one substance is i*tom<>iphmi» wifti 4t?» ilutin which the number of atitim i UUUMI, \U*tithe number of atoms In bath i% \*w>i\u, *»M H<isomorphism is a nuu hain'f ai ^*n f i m n »?similarity of atomic ccm.<trut liuii.

Guided by these rules, Ber/c litHefirf«vs*,inii to.j,», UiUik

the number of atoms contained in a toiii|i< iî-l »<«!?•' s* |^^he can then deduce the atomic ivtitflit •• Iff 1 t^r * «a ,. '

m Literature in the article •« I w#*p!4%i i/* ,», iir< $f t / • *•• *derChemie, edited by Lkhi^ Vo^gmtlmll, tu<\ \\< h r \r iir,,,•u IsomorpWe" in the eecond edition of tIi»B lJitiir#t,»iy W? J i r t ! l f lticularly into the subsequent tlvwluymnd at llif *\u û ' r or, 430 Lehrbuch. 3, part I, 91. ' *


that his rules, in many cases, cannot lead him to a decisivedetermination, and that it is really in the cases of gaseouselements only that they can give unequivocal results. Butjust because he knows upon what shaky grounds he is pro-ceeding, he acts with the greatest caution ; and it is marvel-lous how often, guided by an acute judgment, he hits uponthe correct number, where almost every criterion is wanting.

In the case of the oxides, Berzelius constructs for himselfa series which furnishes him with the relative quantities ofoxygen with which certain weights of the metals combine. Indoing so, he does not require to construct a series of thiskind for every metal. By calling Mitscherlich's law to hisassistance, he is able to supply the places of any stages ofoxidation that are wanting in the case of a given element,by those of an isomorphous element. The series is :—37

Relative Proportionof Oxygen.

Cuprous oxide - - - - - - iCupric oxide, ferrous oxide, etc. - - 2Ferric oxide, manganic oxide, minium - 3Lead peroxide, manganese peroxide - - 4Manganic acid - - - - - - 5

I also give, below, a similar but more accurate tabula-tion, from the year 1835.38

Cuprous oxide - - - - - - 1Cupric oxide, ferrous oxide, etc. - - 2Ferric oxide, manganic oxide, etc. - - 3Lead peroxide, manganese peroxide, etc. - 4Nitric acid, chloric acid, etc. - - - 5Perchloric acid, permanganic acid, etc. - 7

In the compounds noted here, Berzelius assumes 1,2, 3, 4,5 (and 7) atoms of oxygen j thus making the simplest assump-tion possible. It is then only a matter of determining, inaddition, the number of atoms of the radical or element thatis united with the oxygen. His series does not furnish any

37 Lehrbuch. 3, part I, 97. m Lehrbuch. Third Edition, 5, 89.


information about this, and therefore he seeks for othergeneralisations. He now rejects the apparently mostnatural assumption of I atom of radical, which he had madein 1819, since it leads him to atomic weights that are notin harmony with Dulong and Petit's law. He finds a newstarting-point (except for silver, tellurium, and cobalt) byassuming the presence in compounds of two atoms of theelement concerned, and thus he obtains the following seriesfor the stages of oxidation of the most of the metals :—

R2O, RO, R2O3, RO2, R2O5, (R2Or),or RO, RO, RO3, RO2, RO5, (ROr),

where he writes RO instead of R2O2, andRO2 instead of R2O4.Berzelius advances several grounds which appear to tell in

favour of'the accuracy of his choice. The most commonlyoccurring oxides, such as cupric oxide, magnesia, lime, etc.,receive the simplest formula, RO ; further, the oxygen com-pounds of nitrogen and of chlorine, in which he knows,from the volumes, the number of atoms, can be made to fitin with his arrangement. On this account, he regards thisseries as one that occurs generally distributed, calls it thenitrogen series, and contrasts the sulphur series with it.

Berzelius finds the relative quantities of oxygen thatcombine with sulphur to be 1, 2, 2J, and 3. Hence, he writesthe stages of oxidation of this element: SO, SO2, S2O5, andSO3. He endeavours to arrange all the oxygen compounds,as far as possible, in the sulphur and nitrogen series, andassumes, for example, SiO8 as the formula for silicic acid,corresponding with sulphuric acid, an assumption whichafterwards gave occasion for many discussions.

The sulphur compounds (sulphides) are regarded as con-stituted in a manner analogous to the oxygen compounds.He writes sulphuretted hydrogen $£S, because water is HO.

In calculating the atomic weights which he deduces fromthese considerations, Berzelius starts from O=ioo , but, inorder to permit of comparison with earlier and with sub-sequent statements, and since it is merely a question of the


relative m a g n i t u d e s of the numbers, I shall give his values,calculated w i t h reference to oxygen as standard with atomicweight = 16. 3 9

Element and Symbol.

Arsenic - - AsCalcium - - CaCarbon - - CChlorine - - ClIodine - - II ron - _ FeManganese - MnMercury - - HgNitrogen - - NOxygen - - 0Phosphorus - PSilicon - - SiSilver - - AgSodium - - NaSulphur - - S

Atomic Weight(Berzelius).

75-3341.0312.253547

123.2054.3657.02

202.8614.1816.0031.4344.47

216.6146.6232.24

Atomic Weight(O = i6) (Inter-national Com-mission, 1905).

75.040.I12.03545

126.9755-955.o

200.014.0416.0031.028.4

107.9323.0532.06

Before I c o n c l u d e the consideration of Berzelius'system, Ishall add a few words with respect to the formulae of hydro-chloric acid a n d of ammonia. The atoms of these substancesarc r ep resen ted b y HC1 and NH3,

40 showing that Berzelius didnot Identify t h e conceptions of atom and equivalent in all cases,alt hough he e m p l o y s the names indiscriminately. This might,of course, b e regarded as no real exception, since, generallyspeaking, t h e d o u b l e atoms H€i and NTS3 are alone employed.Naturally, it is difficult to give an exact account of the views ofone who is n o l o n g e r alive ; but it is in every case necessary totake this d i f fe ren t periods into account. I believe then thatBerzeiius, a t f irst , and till about 1830, tried to extend thelaw of v o l u m e s a s far as possible (even to compounds as com-pared wi th o n e another) , and that this was a reason for assum-

m Berzelius, Jahresbericht 1828, 73; the values are also to be foundthen* calculated for H = i. 40 Lehrbuch. Third Edition, 2, 187 and 344.


ing the formulae HC1 and NH3 for hydrochloric acid andammonia; but that afterwards, influenced especially byDumas' investigations,41 he placed much less reliance upon thislaw, and applied it to the permanent (and elementary) gasesalone.42 He was then no longer prevented from believing inan agreement between equivalent and atom, even in these sub-stances, and he employed only the formulae HGi and N# 3 .

It follows, from the foregoing, that Berzelius did not admitthe distinction between the physical and the chemical atom,and he thereby establishes an essential difference between ele-ments and compounds. According to him, the atoms of theelementary gases occupy, in general, one half (or one quarter)the space occupied by the atoms of the compound gases.Whilst similarity in behaviour with respect to changes of pres-sure and of temperature was a sufficient reason for assumingthe same number of atoms in equal volumes of hydrogen andof oxygen, the same reason was insufficient to justify the sameconclusion with regard to chlorine and hydrochloric acid.There was an inconsequence in this, but it was of no materialimportance, since the experiments bearing most closely uponthe matter appeared to negative any general applicability toit of the law of gaseous volumes.

The chemical edifice which Berzelius erected was a wonder-ful one, as it stood completed (for inorganic substances) at theend of the third decade of the nineteenth century. Even ifit cannot be said that the fundamental ideas of the systemproceeded exclusively from himself, and if he was indebtedto Lavoisier, Dalton, Davy, and Gay-Lussac for a great deal,still it was he who moulded these ideas and theories into aconnected whole, adding also much that was original. Hiselectro-chemical hypothesis no doubt had points of similaritywith that of Davy, but, in spite of that, it was essentiallydifferent from it. Besides, the first method of atomic weightdetermination, of moderately general applicability, pro-ceeded from Berzelius ; and this method was so extra-

41 Ann. Chim. [2] 49, 210 ; 50, 170. 42 Lehrbuch. Third Edition,5, 82.


ordinarily serviceable that it rendered possible the fixing ofthese most important numbers, so that alteration wasnecessary in only a few cases.

It will thus be understood how the system of Berzeliusbecame the prevailing one, and why his judgment wasauthoritative. The publication of his yearly reports {Jahresbe-richte), which began to appear in 1821 and were employednot merely for reporting but also for criticising, contributedto increase his influence. Hence the ideas of others possessonly a subordinate interest, but still I wish to state theviews of some of his contemporaries, so that I may thebetter characterise the period under review.

British chemists had not yet come to a decision with re-spect to Dalton's conception of the atom and Wollaston's ofthe equivalent. Very little of much importance had mean-while been accomplished in Great Britain. The only thingto which I wish to refer is the hypothesis of Prout, whichwas the occasion of much discussion.

Prout, in 1815, thought it was possible to show that theatomic weights of the gaseous elements are multiples, bywhole numbers, of that of hydrogen.43 Stated in this way,the matter seems to be of small importance ; but it gainsinterest from the fact that, if it is admitted to be generallyapplicable, it almost necessarily leads to the assumption ofa primordial form of matter, and to the view that the mani-fold peculiarities of substances are explicable by the varyingdistribution of this matter in space. Thomson44 set him-self the task of extending the statement of Prout to all theelements, and, for this purpose, he carried out a largenumber of atomic weight determinations. His results areworthless, however, as Berzelius somewhat bluntly pointsout to him.45

At a later period, Prout's hypothesis was taken up again byDumas,46 after it had been shown that a more accurate deter-

43 Ann. Phil. 6, 321. u Thomson, An Attempt to establish theFirst Principles of Chemistry by Experiment, 2 vols. London (1825).45 Berzelius, Jahresbericht 1823, 40. m Ann. Chim. [3] 55, 129.


mination of the numbers told in its favour. In particular, theatomic weights of the best known elements, such as oxygen,hydrogen, nitrogen, carbon (chlorine?), bromine, iodine, etc.,appeared to be in harmony with it. Stas has proved, however,by means of experiments in which the highest degree of accu-racy and completeness was attained,47 that even in the case ofthose elements which appeared in accord with it, the hypo-thesis does not in any instance hold rigidly, and can onlybe looked upon as an approximation.

The highly important theory established by Newlands,Lothar Meyer, and, especially, Mendelejeff, dealing withthe periodic relation of the properties of the elements tothe magnitudes of their atomic weights, can only be enteredupon in a subsequent lecture.

In France, the law of volumes in its most extended sensebecame the basis of the atomic considerations. It was Dumas,especially, who took up a very decided position in this con-nection. He shows that the conception of the equivalentcannot be employed as the basis of a system, because itloses its significance when it is extended further than toacids, to bases, and to other substances which closely Tesembleeach other (oxides and sulphides) ; and, especially, that itbecomes quite vague when the attempt is made to identifythe equivalent with the combining weight,48 since verymany substances can combine in several proportions. Thus,for example, 8 parts of copper are combined with.i part ofoxygen in cuprous oxide, while for 8 parts of copper, 2parts of oxygen are contained in cupric oxide. Calculatedfrom these numbers, the equivalent (combining weight) ofcopper, referred to that of oxygen as unity, is 8 or 4.

Dumas believes that, by adopting Avogadro's hypothesisas a basis, he has obtained a sure guide in considerationsregarding atomic weights. He assumes that in equalvolumes of all gases (at the same temperature and pressure)

47 Recherches sur les lois des proportions chimiques etc, Bruxelles 1865,and Recherches sur les rapports re'ciproques des poids atomiques, i860.48 Dumas, Traite de Chimie appliquee aux arts. Paris (1828-46).


there is the same number of (physical) atoms, but that theseare still divisible by chemical means. " We call atoms, thegroups of chemical molecules that exist isolated in thegases. The atoms of the elementary gases always containa certain number of molecules which is unknown to us." 49

The ratio of the densities of the gases gives Dumas theratio of their atomic weights. In fixing the atomic weightsof the solid elements he makes use of the law of Dulongand Petit, which he regards, accordingly, as holding forgroups of chemically smallest particles—molecules, as weshould now say. Further, he employs for the same purposethe relative densities of volatile compounds, making assump-tions, from analogy, as to the volume relations of theunknown elementary gases contained in them. Thus hefinds the atomic weight of sulphur from the density of sul-phuretted hydrogen, which he assumes to be constituted likewater and to consist of 2 volumes of hydrogen and 1 ofsulphur vapour ; and that of phosphorus from phosphurettedhydrogen, which he supposes to be constituted like ammonia.His determination of the atomic weight of carbon is note-worthy. He deduces it from the relative densities of ethy-lene and of marsh gas. He assumes in the latter (as Gay-Lussac had also done previously) 2 volumes of hydrogen for1 of carbon vapour, and in the former, equal volumes of thetwo. He thus finds the atomic weight of carbon to be onehalf of what Berzelius had estimated it to be, that is 6, ifthat of hydrogen is assumed equal to 1. In general, how-ever, the values which he assigns to the atomic weights ofthe better known elements are the same as those of Berzelius.Mercury, silicon, etc., form exceptions. Dumas does notstate the weights of the chemically smallest particles.

Berzelius contested the principles of the system just con-sidered, although they were so closely related to his own.50

He thinks that it is absurd to assume fractions of atoms, andsays it was formerly the custom to abandon hypotheses as soon

40 Dumas, Traitd 1, 41. m Berzelius, Jahresbericht 1828, 80.

LECT. VL] HISTORY OP CHEMISTRY 165

as they led to absurdity. Dumas stands entirely isolated inhis views, but, in spite of this, he would probably have adheredto them, had he not himself discovered facts which causedhim to doubt the accuracy of Avogadro's hypothesis.

Dumas was not only an ingenious thinker, but he was alsoan excellent experimenter; and, since he had chosen the den-sities of gases and vapours as the basis of his atomic theory, hethought it necessary to increase our knowledge respecting thesedensities. He succeeds in elaborating a method for carryingout determinations of this kind at high temperatures, and em-ploys it for ascertaining the relative densities of the vapours ofiodine, phosphorus, sulphur, mercury, etc.51 His results, fromwhich he anticipated confirmation of his views, lead him toabandon them. He finds the density of phosphorus vapour tobe twice as great, and that of sulphur vapour to be three timesas great as he had previously assumed, whilst that of mercuryvapour is only one half of what he had supposed. In view ofthese facts he begins to doubt; in fact he declares that eventhe simple gases do not contain, in the same volume, the samenumber of chemical atoms. According to him, the assumptionmay still be made that there is the same number of molecularor atomic groups present in equal volumes of all gases ; butthat this is only a hypothesis, which cannot be of any service.52

Dumas is obliged to-admit that Gay-Lussacrs law, when ap-plied in the way he had applied it to the determination ofatomic weights, furnishes erroneous results. Hence hebelieves that it cannot be employed for this purpose, andhe now abandons Avogadro;s hypothesis.

Berzelius, too, can no longer maintain the identity ofvolume and atom in the cases of the elementary gases, and hasto confine his proposition to the incondensable elastic fluids.53

It must be admitted that the law, when so stated, was notcapable of any extended application, and was more thaninsufficient for the determination of the atomic weights of the

81 Ann. Chim. [2] 33, 337 ; 44, 288 ; 49, 210; 50, 170. 52 Lemons.268. and 270. K! See p. 92.


majority of the elements. And how did it fare with the othergeneralisations upon which his system rested? The hypo-thesis of Dulong and Petit was not without exceptions in itsapplicability, as I have already stated. The numbers deducedfrom it for silver, cobalt, and tellurium were not in harmonywith Berzelius? determinations, that is, with the atomic weightsrequired by the chemical analogies, and by isomorphism ; sothat even this hypothesis was not tenable when rigidly con-sidered. Mitscherlich's law still remained, and the majorityof chemists believed that it permitted of an unerring conclusionas to the atomic constitution. Other voices were heard, how-ever, which indicated doubts, especially after Mitscherlich hadshown that there are dimorphous substances, i.e., substanceswhich can occur in two crystalline forms.54 Attention wasdrawn to the fact that, as the occurrence of dimorphismproved, the crystalline form of a substance was not determinedsolely by the number of its atoms.55

Of all the physical laws that had been applied to thedetermination of atomic weights, there thus remained not oneupon which full reliance was placed. The conception of theatom was looked upon, in consequence, as uncertain andhypothetical. Chemists believed they would have to be con-tented with the combining weight or the equivalent, the latterof which had gained new support from Faraday's electrolyticlaw.66 At the end of the fourth decade of the nineteenthcentury, we thus find the atomic theory—the most brillianttheoretical achievement of chemistry—abandoned and dis-credited by the majority of chemists, as a generalisation oftoo hypothetical a character. A new school had arisen,which had adopted Woilaston's equivalents, and whichsought, successfully, to supplant the system of Berzelius.

At the head of this movement there stands L. Gmelin.The views of this chemist are of all the more importance fromthe fact that he expounded them in his excellent Hand-book;

54 Ann. China. [2] 24, 264. M Ibid. [2] 50, 17r. M ExperimentalResearches in Electricity, Series 3, § 377, Series 7 § 783 etseq. 1833-34.


which latter, on account of its completeness, had, at thattime, already become widely distributed.

With Gmelin, there is no strict distinction between mix-tures and compounds, and this proves that he does not believein the real existence of atoms. Two substances, especiallywhen they possess only a weak affinity for each other, cancombine, according to him, in an infinite number of propor-tions ; but the greater the affinity, the greater is their tendencyto combine in few proportions only.57 These proportionsthen stand to each other in simple relations. " There cantherefore be assigned to every substance a certain weight inwhich it combines with definite weights of other elements.This weight is the stochiometric number, the chemical equi-valent, the mixture-weight or atomic weight, and so on.Compounds are composed in such proportions that onemixture-weight of one substance is united to J, J, |-, f, f, i, i | ,2> 2h 3) 4> 5) 6, 7, or more mixture-weights of the other."According to Gmelin, Gay-Lussac'slaw runs:—One measureof an elastic fluid substance combines with i, i | , 2, 2J, 3, 3J,and 4 measures of the other.

His table of equivalents is well known. It ran:—H= 1,0 = 8, S= i6 , C = 6, etc. Water was written HO, and informulae generally, the endeavour was made to replace bysimplicity what they had lost in conception and in purpose.Chemistry was to become a science confined to observation—indeed almost to description alone. Skill in manipulationwas all that was required ; speculation was banished asdangerous.

It had come to this then .-—Inorganic chemistry, in con-nection with physics, had not been able to maintain theconception of the atom. It is my business to show, in thenext lectures, how it was reintroduced into the science bymeans of organic chemistry.

57 Handbuch der theoretischen Chemie. Second Edition, 1821.

L E C T U R E VI I .

ORGANIC CHEMISTRY AT THE COMMENCEMENT OF ITS DEVELOPMENT-ATTEMPTS TO DETERMINE THE ELEMENTARY CONSTITUENTS OFORGANIC COMPOUNDS — ISOMERISM AND POLYMERISM—VIEWSREGARDING CONSTITUTION—RADICAL THEORY.

IN this lecture I shall endeavour to give an account of thedevelopment of organic chemistry. I have intentionally post-poned this subject until now because I wished to consider itin a connected manner, because it had almost no influenceduring the first three decades of the nineteenth centuryupon the perfecting of general theories, and also because theviews which constitute the basis of inorganic chemistry didnot, at first, seem to be capable of any application to theorganic branch of the science. Thus we find Berzelius, in1828, treating the subject of the organic compounds separ-ately. The electro-chemical theory, the law of multipleproportions, and the law of volumes did not appear todominate substances derived from the animal and vegetablekingdoms; these substances were subject to the so-calledvital force, the nature of which was wholly unknown andobscure. It only became possible to extend to organicchemistry also, the laws which held for inorganic substances,after the study of this part of the subject had further attractedthinkers to itself. The opinions and hypotheses to whichthe examination of the better known substances had led, hadnow to be turned to account in the younger branch of thescience. It was dualism, in particular, which was now intro-duced into organic chemistry also.

Lavoisier had already assumed, as has been stated previ-ously, that the acids consist of oxygen and a basis; and that, ininorganic compounds, the latter is an element, while in organic

108

LECT. vii.} HISTORY OF CHEMISTRY 109

compounds it is a compound radical. The latter name hadnot been lost. The chemical nomenclature had been basedupon it, and the dualism of Berzelius was a happy extension ofit. All observations seemed to be in agreement with it. Thusthe salts (at that time the best known class of substances) areformed from acid and base, and they can be decomposed againinto these constituents. Why should we not endeavour tolook upon organic compounds also as formed in a similar way ?Since organic compounds (according to the view held at thatperiod) consisted of at least three elements, any simple de-composition must still leave one part of a composite nature.Organic chemistry thus became the Chemistry of the Com-pound Radicals. In thus employing the term radical, its originaldefinition was retained. After the removal of the oxygen froma substance, the residue which remained, and which, moreover,played the part of an element, was called a radical. Wohlerand Liebig remodelled this conception. In their admirableresearch on bitter almond oil and the allied compounds, theyshowed that we may assume the existence, in these substances,of an oxygenated group which remains unchanged in themajority of the reactions, and therefore behaves like anelementary substance. On this account, they called it theradical of bitter almond oil.

By this departure the first great step had been taken ;organic chemistry had become independent; it had freed itselffrom the fetters which had been placed upon i t ; if, fromwithin its own limits, it had not produced a new idea, it had atleast given new and increased importance to one which wasalready held. From this time forward, it proceeds on its ownway, and pays no heed to the limitations which some desiredto place upon it. The very harmonious edifice of chemistrysuffers in consequence. Every endeavour is made to adapt itto the new ideas. But it is in vain—the breach is unavoidable.The young science, quite conscious of its own strength, daresto make an assault upon the foundations, and, in spite of staysand props, the structure begins to totter. The attack uponthe electro-chemical theory led to an embittered controversy

110 HISTORY OF CHEMISTRY [LECT. VII.

between its supporters, with Berzelius at their head, and theadherents of the substitution theory or the theory of types.This controversy was triumphantly passed through by thelatter, and led to the complete separation of organic and in-organic chemistry. At any rate the endeavour was still made toretain in the latter, as before, the dependence of the chemicalupon the electrical forces, whilst the newest facts in the domainof organic chemistry appeared to be incompatible with this.Our science thus fell anew into two schools, and the principleswhich guided the one school were rejected by the other.

Simultaneously with the abandonment of the electro-chemical hypothesis, the radical theory was also given up ;there was now no longer any actual need for it, and, in theform in which it had been advanced, it was insufficient. Agreat deal had been discarded as useless, and therefore it is byno means inadvisable to inquire into the principles which stillremained with the representatives of the new school. Theviews as to the preservation of the type, and as to substitution,although, of course, most valuable for the comprehension ofmany reactions, could scarcely be employed as the basis of acomplete system. But amongst the ruins left upon the battle-field, and found there when it was cleared up, there was ajewel, which, although little heeded during the controversy,was now capable of becoming of great significance when thequestion was no longer one of getting rid of old views, but oneof setting up new views in their stead. The atomic theory,despised by many, forgotten by some, was now destined toarise again in its original brilliancy, although a hard strugglewas necessary. New foundations for the determination of therelative masses of the atoms had to be obtained. It wasGerhardt, in particular, who insisted on the necessity of fixingupon comparable quantities for these determinations. Butwhence was the standard to be derived ? Liebig's polybasicacids, and Dumas' substitution, had at length taught chemiststhe difference between atom and equivalent, so that therecould no longer be any question with respect to the latter.Recourse was again taken to Avogadro's hypothesis ; but this

LECT. VII.] HISTORY OF CHEMISTRY 111

still proved insufficient, and chemical reasons were requiredin order to convince chemists. Gerhardt, who received verysubstantial support from Laurent, exerted himself in vain toadduce decisive proofs of the accuracy of his ideas. At thisjuncture, Williamson's investigations appeared, and they gavea real foundation to the thoughts that had flitted beforeGerhardt's mind. This gifted chemist had shown the way ;and, in imitation of him, it was extensively followed, since itpermitted of a direct comparison of the quantities enteringinto reaction. Thus there arose the conception of thechemical molecule. In Gerhardt's system, which was nowrapidly gaining recognition, this conception found its formalexpression in the theory of types.

I shall here conclude this sketch, in which I have indicated,in general outline, the different phases of the historical develop-ment ; and I shall now proceed to a detailed account.

As early as the second half of the seventeenth century,Lemery separated organic from inorganic chemistry. Hedivided substances, according to their origin, into threeclasses, viz., mineral, animal, and vegetable.1 The phlogis-tians occupied themselves chiefly with the first class. Scheeledeserves to be mentioned as the discoverer of an extensiveseries of organic substances.2 Lavoisier believed that com-pounds belonging to this class consisted of carbon, hydrogen,and oxygen ; Berthollet proved the presence of nitrogen insubstances of animal origin j 3 at a later date, it was recog-nised that all the elements can enter into organic combina-tions, but that carbon must never be absent.4

It is difficult to say what substances were regarded asorganic'compounds at the beginning of the nineteenthcentury.This class naturally included all substances occurring in the

1 Kopp, Geschichte. 4, 241. 2 See pp. 10-11. 3 Journ. de Phys. 28,272. 4 Gmelin, Handbuch der Chemie. Fourth Edition, 4, 3 ; E. 7, 4.

112 HISTORY OF CHEMISTRY [user. vn.

plant or animal organism, but from these there were prepareda large number of other compounds, whose position in thesystem had also to be determined ; and a decision as to the classin which they were to be enumerated was often an arbitrarymatter. Simplicity of composition was frequently a reasonfor placing substances in the inorganic class. With regard tomany substances, the views as to their nature had changedin course of time ; for example, in the case of the cyanogencompounds, which were first classed as organic and afterwardsas inorganic substances. Wherever it was possible, Lavoisier'sidea was upheld that in organic substances, the basis com-bined with oxygen, or the radical, consists of several elements.This gave rise, at a later date, to Liebig's definition of organicchemistry as the chemistry of the compound radicals.

The study of the compounds belonging to this class laggedconsiderably behind that of the others. The reason lay partlyin the easy alterability of these substances, and, thus, in thegreater difficulty which their isolation presented ; partly alsoin the scarcity of methods for their analysis. At the begin-ning of the nineteenth century, when qualitative analysis hadalready attained a high degree of accuracy, and even thequantitative method had found excellent exponents in Proust,Klaproth, and Vauquelin, Lavoisier's experiments withalcohol, oil, and wax, were the only ones in existence, designedto ascertain the composition of organic compounds ; andthese, as may be easily understood, were not very accurate.

It is thus explicable that Berzelius should still doubt, in1819, whether the law of multiple proportions held in organicchemistry also.6 He was well aware that when organic com-pounds unite with inorganic ones—organic acids with metallicoxides, for example—the same regularities are observed as ininorganic chemistry; but he believed the proportions in whichcarbon, hydrogen, oxygen, and nitrogen unite, to be so variedthat Dalton's law lost its significance, simply because 1, 2, . . natoms of one element could unite with 1,2, . . m atoms of

6 Essai etc., 96 ; compare alsa Lehrbuch. 3, part I, 151.


another. Nevertheless Berzelius himself afterwards assistedmore than anyone else in extending the laws of stochio-metry to organic chemistry, inasmuch as he materially im-proved the method of elementary analysis employed at thattime, and thereby provided himself and others with themeans of ascertaining the composition of organic substances.

It may not be out of place to give some account here ofthe history of elementary analysis, just because the viewsrespecting organic compounds were essentially changed inconsequence of its development.

I shall not again revert to Lavoisier's method, which I havealready indicated in an earlier lecture.6 There are almostthirty years between his experiments and those of his nearestsuccessors. I pass over the experiments of Saussure,7 Ber-thollet,8 etc., as well as the first labours of Berzelius9 uponthis subject. These furnished analytical processes that weresufficient, perhaps, in special cases, but cannot by any meansbe regarded as general methods. On the other hand, theinvestigation of Gay-Lussac and Thenard, in 1811, deservesour attention.10 They burned the organic substance withpotassium chlorate, by forming small pellets of the mixture andallowing these to fall into a perpendicularly placed tube, thelower end of which was heated red-hot. The tube was closedabove by means of a stop-cock furnished with a recess de-signed for the reception of the pellets. The gases from the com-bustion had to make their escape through a side tube into aeudiometer, and they were there measured. Gay-Lussacand Thenard then absorbed the carbonic anhydride formed,and determined the oxygen left behind. They knew,further, the quantity of substance burned and the quantityof potassium chlorate mixed with it. By the help ofLavoisier's equation :—Substance + Oxygen employed=Carbonic anhydride + Water

6 See pp. 28-29. 7 Journ. de Phys. 64, 316 ; Bibliothfeque britannique.54, No. 4; 56, 344; Ann. Phil. 4, 34. 8 M6n. d'Arcueil. 3, 64;M&n. de l'Acad. 1810, 121. 9 Gilb. Ann. 40 (1812), 246. 10 Rech. phys.chim. 2, 265.

114 HISTORY OF CHEMISTRY [LECT. vtu

they were then able to calculate the quantity of water pro-duced by the combustion, and, therefore, the compositionof the compound.

Gay-Lussac and Thenard carried out the analysis oftwenty substances in this way. Their results are moderatelyaccurate, but the method still left much to be desired.The combustion was very violent; it was accompanied byexplosion, and was, therefore, frequently incomplete.

The next great step in the development of elementaryanalysis was made by Berzelius in 1814.11 By carrying out thecombustion with a mixture of potassium chlorate and sodiumchloride, he secured the much more moderate progress of theanalysis. His method, also, differs very essentially andadvantageously from the earlier one, in so far that he didnot gradually introduce the substance intended for com-bustion into a red-hot tube, but instead, put the wholequantity of the substance, along with the oxidising material,into a tube which he gradually heated to redness in a hori-zontal position. Further, he was the first to weigh thewater directly, which he did after having absorbed it bymeans of calcium chloride ; whereas he determined thecarbonic anhydride either by volume or by weight.

This mode of carrying out the analysis already approxi-mates closely to the present method ; it was still further im-proved by employing cupric oxide instead of potassium chlo-rate. This was first used by Gay-Lussac for nitrogenous sub-stances,12 but a year afterwards it was employed byDobereinerin the combustion of substances free from nitrogen.13

Analyses were carried out by this process for more thanten years, until the method was modified by Liebig14 in1830, and brought into the form now employed. As a con-

11 Ann. Phil. 4, 330 and 401. 12 Ann. Chim. 95, 154; Ann. P,hil.7? 357- 13 Schweigger's Journal. 17, 369; compare also Chevreul, Re-cherches chimiques sur les corps gras d'origine animale (1823). u Pogg.Ann. 21, 1 ; more fully in his " Anleitung zur Analyse organischer Kfirper."Braunschweig (1837); English Translation, by Gregory: ** Instructions forthe Chemical Analysis of Organic Bodies," Glasgow (1839).


sequence of Liebig's investigations, elementary analysisbecame an easily accomplished operation, which, in so far asaccuracy was concerned, might be placed alongside of otheranalyses. A rapid advancement of organic chemistry datesfrom this period. Since a simple and sure means of deter-mining the composition of the substances was now available,investigations which had not previously been attempted onaccount of the endless difficulties associated with them, nowbecame possible and were actually carried out.

No doubt many analyses had already been carried out bythe method of Berzelius, and the conviction became more andmore settled that the law of multiple proportions was applicableto organic compounds also, and that formulae similar to thoseassigned to mineral substances could be assigned to them.But an important distinction was still drawn, in the thirddecade of last century, between these two classes of substances.It was supposed that the latter alone were producible artificially;while the synthesis of the former was wholly beyond our powerand was reserved for the living organism, in which it was per-formed under the influence of the Vital Force. From suchnaturally occurring substances chemists had, it is true, learnedto prepare, by dry distillation, by treatment with nitric acid,with alkalies, etc., othersubstances which were likewise classedamongst organic compounds, but these were, for the most part,simpler in composition, and the material existing in naturealways remained the starting-point. In this connection, anexcellent investigation for that period, by Chevreul, deservesto be mentioned,15 in which the author showed that t h e fatsconsist of an acid and of glycerine (a substance discovered byScheele), and that they should, accordingly, be placed inthe series of ethers, where all those substances were classedwhich could be separated by means of alkalies into anacid and an indifferent substance (an alcohol).

This and similar investigations could not, however, shakethe belief in a vital force under whose influence all organiccompounds originated. . As yet no one had succeeded in

116 HISTORY OF CHEMISTRY [LECT. vn.

artificially preparing any substance occurring in the organ-ism ; but even this great step had not to be waited for muchlonger. For this discovery we are indebted to Wohler, andwith it he opened his long and brilliant scientific career.

Wohler had discovered cyanic acid in i822,10 and was oc-cupied in its investigation when he made the observation, in1828, that urea, a known product of animal life, was formedupon the evaporation of a solution of its ammonium salt.17

It is true that the problem was not completely solved by thisdiscovery. The synthesis from the elements was not yet pos-sible, but still the most essential thing had been accomplished.From inorganic compounds (amongst which many at thattime classed cyanic acid)18 a substance had been prepared whichhad hitherto been found in the animal organism only. Inspite of this, the revolution of ideas proceeded but slowly j itwas still believed that the vital force could not be dispensedwith, and some decades afterwards, scientific discussions tookplace as to its existence. Nowadays, when the materialistictendency becomes more and more ascendant, there are few tobe found who ascribe the production of organic substancesto forces different from those that govern the production ofmineral substances. It is true that the experimental sciencehas made great progress in this respect also, since it hassucceeded in preparing from their elements many organic sub-stances. Thus Kolbe effected the complete synthesis of tri-chloracetic acid,19 and Berthelot the syntheses of formic acidand of alcohol. The latter chemist inaugurated, with theseresearches, his brilliant series of synthetical investigations.20

It may appear remarkable to many persons, into whosehands a treatise on organic compounds published in the thirddecade of last century, or earlier, may chance to fall, thateven at that time, when this department of chemistry was in

1(1 Gilb. Ann. 71, 95. 17 Pogg. Ann. 12, 253 ; Quart. Journ. Science.1828, 1, 491. 18 Compare, for example, Dumas, Traite. 1, 409. 1() Annalen.54, 145. tJ0 Berthelot, Chimie organique fondle sur la Synth se, Paris(i860) ; see also his more recent investigations, Bull. Soc. Chim. [2] 7,113,124, 217, 274, 303, 310, etc.

LECT. VIL] HISTORY OF CHEMISTRY 117

so backward a state of development, experiments were madein order to obtain some information as to the constitution, ormode of arrangement of the atoms, of compounds. A pursuitof this kind may be regarded as idle speculation, and yet scien-tific chemistry was directed, at an early period, towards suchconsiderations. This was owing to the phenomena of isomer-ism, into which, therefore, I must here enter with some detail.

After chemists had begun to pay attention to the quanti-tative composition of substances, and especially after they hadlearned to regard constant proportions by weight of theirconstituents as a real characteristic of chemical compounds,it was assumed as a matter of course that the same com-position per cent, always postulated the same properties- Ttwas, of course, known that very many, and indeed most,substances occurred in several states: solid, liquid, andgaseous ; crystalline and amorphous, etc. ; but the sensationthat the discovery of dimorphism made, shows us how greatthe tendency was at that time to regard physical andchemical properties as functions of the composition percent, (and of the temperature). It must naturally havecreated much surprise to see that sulphur can appear in twocrystalline forms ; to hear that arragonite is pure calciumcarbonate, and is therefore dimorphous with calc-spar, etc.21

It was to be shown, however, in the same year as thatin which the dimorphism of sulphur was recognised (1823)that even the chemical properties can change withoutalteration of composition. In the analysis of fulminic acid,Liebig obtained numbers which agreed exactly with thoseestablished for cyanic acid.22 This was at first believed tobe an error, but subsequent examination confirmed theobservation, and the great difference between the two sub-stances seemed wholly inexplicable. Two years later,Faraday discovered another fact of the same kind.23 Hewas engaged in the examination of oil gas, when he dis-

21 Ann. Chim. [2] 24, 264. ^ Ibid. [2] 24, 294; 25, 288;Schweigger's Journal. 48, 376. a} Phi]. Trans. 1825, 440 ; Ann. Phil. 27,44 and 95 ; Schweigger's Journal. 47, 340 and 441.


covered a hydrocarbon that behaved very like ethylene ; butit did not yield chloride of carbon when mixed with chlorineand exposed to sunlight, and it possessed, moreover, adensity double that of ethylene.24 Further, an investigationof phosphoric acid was made at this period by Clark, who,through neglecting the fact that the salts contained water,was led to the opinion that there were two phosphoric acids,with different properties but with the same composition.25

Berzelius had previously observed the same thing with respectto stannic acid.20 He also showed in 1830 that the acidproduced along with tartaric acid during the manufactureof the latter, had the same composition as tartaric acid.He calls the new substance racemic acid (Drufsyra. Trauben-schire), and introduces the word isomer to designate sub-stances of this kind. According to him, this word is onlyto be applied to compounds possessing the same compositionand the same atomic weight, but with dissimilar properties.27

A year later, Berzelius designates as polymerism the pheno-mena observed by Faraday respecting the hydrocarbons.This name embraces those cases where the same composi-tion is accompanied by dissimilar properties and differentatomic weights.28 Metameric substances, on the otherhand, are those which possess the same composition, thesame atomic weight, and dissimilar properties, when thedifference can be explained by a different arrangement of theatoms—ie.j by a different constitution.29 As an example,Berzelius very appropriately chooses stannous sulphate andstannic sulphite, which he writes: SnO + SO8andSnO2 + SO2.

At that time, the different modifications of an elementwere also regarded as cases of isomerism; and it was only

21 I mention here, in. passing, that Faraday on the occasion of thisinvestigation also discovered benzene. t2a Edinburgh Journal of Science. 7,298 ; Sehweigger's Journal. 57, 421. a8 Ibid. 6, 284. 27 Pogg. Ann. 19,305. 'M It appears from this that Berzelius at that time regarded thevapour densities of compounds also as a guide to their atomic weights*P P l i S j Jahresbericht 1833, 6jt

LECT. vn.] HISTORY OF CHEMISTRY 119

in 1841 that Berzelius introduced the word allotropy todesignate such cases.30 A number of examples belonging tothis class were already known ; one of the most interestingbeing carbon, in the forms of diamond, graphite, and soot.

It will be understood that the idea of metamerism couldonly be introduced after it had become possible to entertainany conception of the constitution of a substance j while, onthe other hand, the phenomena of isomerism necessarily ledchemists to hypotheses respecting the mode of arrangement ofthe atoms. It is well known that there was in existence at thattime a mode of regarding the facts which Berzelius endeavouredto extend more and more. I refer to dualism, which I havealready had occasion to mention several times, and the con-sequences of which I shall now state more definitely.

The phenomena of combustion had led Lavoisier toassume, in so far as it was possible to do so, that substancesconsisted of two parts. This way of regarding them wasvery advantageous and clear in the case of salts, whichwere looked upon as composed of base and acid. This viewwas in agreement with their whole behaviour, and renderedit possible to consider them all from one common standpoint.The arguments of Gay-Lussac and Thenard against theelementary nature of chlorine, which have been stated ina previous lecture,31 prove how deeply these ideas hadbecome rooted, and how firmly it was the custom to baseconclusions upon them.

After the existence of the so-called hydrogen acids (*'.#.,of acids which do not contain oxygen) had been generallyadmitted, various opinions arose respecting the nature of theirsalts. A few investigators (Davy and Dulong, for example)regarded them as compounds of metals, just as they regardedother salts ;32 but this view met with little approval at thattime. Others remained true to the earlier conception, andwith them common salt was still muriate of soda, which hadthe peculiarity, however, that it gave oflf its " water." Others,

80 Berzelius, Jahresbericht 1841, Part II. 13, in See p. 81, n Seep. 83,

120 HISTORY OF CHEMISTRY [LECT. VH.

again, no longer looked upon these substances as salts, butcompared them with the oxides. In this connection, the doublechlorides and iodides prepared in 1826 by Boullay, caused thelatter to develop his ideas more fully.33 In accordance withthese ideas, the chlorides, iodides, etc., of the alkali metalswere bases, from which true salts were only produced by com-bination with the chlorides and iodides of the heavy metals ;and the latter, in turn, were analogous with the acids. Othersstill, and amongst them Berzelius,34whose opinion at that timecarried the greatest weight, regarded common salt and similarsubstances as compounds possessing a salt-like character ; butthey separated them from ordinary salts. According to them,the whole group of salts consisted of two divisions—the am phidsalts, to which the oxygen, sulphur, etc., salts belonged, andthe haloid salts, which embraced the chlorides, iodides, etc.The latter were composed of two elements or radicals—ofa metal and a halogen, as chlorine, iodine, cyanogen, etc.,at this time came to be called. It remained, however,altogether unexplained why substances with such similarproperties as those of the amphid salts and the haloid salts,possessed such different constitutions.

If it was desired to regard the oxygen salts as compoundsof an acid with a base, then the establishment of these ideaswas further attained as follows. Taking nitre as an example,KO would represent the base, and N2Ofl

3f> the acid (or whatwe now call the anhydride). It thus came about that aceticacid was represented by C4HCO3, formic acid by C2H2O3, sul-phuric acid by SOS, etc. ; that is, instead of the actually exist-ing substances, others were represented, of which some wereimaginary. The free acids were held to contain u a proportionof water which we cannot separate except by combining theacid with another substance " ;30 and although Berzelius him-self had previously distinguished water of hydrat ion from watercontained in salts and not necessary for their existence,87 yet

88 Ann. Chim. [2] 34, 337 ; Journ. de Pharm. I2r 638. M Berzclius,Lehrbuch. Third Edition, 4, 6. m Berzelius' atomic weights: see p. 200.86 Berzeliusj Lehrbuch. Third Edition, 2, 4. m Journ, de Phys. 73, 253,


this water also was neglected, as if non-existent, in most of thediscussions as to the constitution of bases and acids. It mayhavebeenaconsequenceof this that the presence of water wasassumed even in substances which belonged to other classes,when hydrogen and oxygen were found in them in the propor-tions necessary to form water, and that this water was thenneglected in writingformube for these substances. Many thingsmight be adduced as having contributed to erroneous ideas ofthis kind ; such, for example, as the way in which Gay-Lussacand Thenard in 1811 interpreted their analytical results relat-ing to organic substances.38 According to these chemists,substances fall naturally into (1) those which contain just asmuch oxygen as is required in order to form water with thehydrogen present (carbohydrates); (2) those which containless (resins, oils) ; and (3) those which contain more (acids).

I regarded these perhaps seemingly detailed explanationsas necessary, before I could enter more minutely into theviews respecting the constitution of organic compounds. Inpassing on now to this most important question, I wish toshow how dualism was gradually introduced here also, andhow the radical theory arose as a consequence of this.

Berzelius explained in 1819 that his electro-chemical theorycould not be extended to organic chemistry, because underthe influence of the vital force the elements there possessedentirely different electro-chemical properties. In decay,putrefaction, fermentation, etc., he observes phenomena whichhe regards as demonstrating the tendency of the elements toreturn to their normal condition.39 He did not, at that time,as yet consider it possible to regard all organic substances asbinary groups. Dualism was, indeed, extended as far aspossible ; the oxygen compounds were looked upon as " oxidesof compound radicals, which, however, do not exist free, butare wholly hypothetical," 4G a mode of regarding the matterwhich was especially applicable to the acids. Accordingly,

38 Rech. phys. chim. 2, 265. m Essai etc, 96, 40 Berzelius, Lehrbuch.First Edition, 3, Part I. 149,


we now hear the radicals of acetic acid, C4H0, of benzoic acid,C]4H10, etc., spoken of, and these radicals are the remaindersof the acids after the deduction of their oxygen.

It is easily understood that endeavours should be madefrom other points of view, and in other directions, to establishhypotheses regarding the nature of organic substances ; but Imay pass over those which possessed no general significanceand were applicable to afew substances only. I must, however,adduce one example of this kind, because the idea involvedwas for a long time held in respect in chemistry. It has todo with a conception of oxalic acid, which was then writtenC2O3, the elements of water being neglected. Dobereiner,who carefully studied the behaviour of the oxalates in 1816,proved that some of them give off carbonic anhydride andcarbonic oxide when heated, and on this account he thoughthimself justified in regarding the "acid of sorrel " as carbon-ate of carbonic oxide.41 This was an attempt to refer backcomplex substances to simpler ones, and it possesses a certainsignificance, in so far that it is based upon facts.

More important by far is an observation of Gay-LussacJsconcerning the composition of alcohol and of ether, whichdates from about the same time,42 and which became thebasis of the so-called Etherin Theory. The discoverer ofthe law of gaseous volumes points out that the densities ofthe vapours of alcohol, ether, and water, and the density ofolefiant gas, stand to each other in such a relation thatether may be regarded as composed of half a volume of watervapour and one volume of olefiant gas, and alcohol as com-posed of equal volumes of the two.

Dumas and Boullay adopted this observation as the basisof the views regarding the ethereal compounds, which theyadvanced in 1828 on the occasion of a detailed investigationof these substances.43 In their view olefiant gas is a radical;that is, a group of atoms which enters into combinations in

41 Schweigger's Journal. 16, 105. 42 Apn. Chim. 91, 160; 95, 311,*> ibid. [2337,1^.

LECT. vri.] HISTORY OF CHEMISTRY 123

the same way that the elements do. They compare it withammonia, and are at pains to show that, just as the latter is theradical of the ammonium salts, defiant gas must be assumedin the ethers. In doing so, they try to carry the analogyso far that they even assert that ethylene possesses basicproperties, and that the reason why it does not colourlitmus tincture blue is merely because it is insoluble inwater ; and that its alkaline nature is proved, moreover, byits property of neutralising hydrochloric acid, whereby hydro-chloric ether, observed so long ago by Basil Valentine, isproduced. They then show, by means of a table, how theradical C4H4 or 2C2H2 (olefiant gas) may be assumed in theformulae of the ethers analysed by them ; whereby completeuniformity with the ammonium salts is attained :—44

Olefiant Gas - - - 2C2H2Hydrochloric Ether - 2C2H2+HC1

NH3 - - - - AmmoniaNH.. + HC1- - Sal-ammoniac

EtherAlcohol -Acetic Ether 4C2Ha + C8H6O3 +• H2O

Acetate of AmmoniaOxalic Ether 4C2H2 + C4O3 + HaO

Oxalate of Ammonia, etc.40

We find the opinion that the ethers are to be regardedas analogous to salts, first advanced by Dumas and Boullay,although it is true that they did not adopt the usual view, inaccordance with which salts do not contain any water. Theendeavour to classify the organic compounds in the same wayas the inorganic ones, constituted the basis of their views, how-ever ; and, since this idea was found to be applicable to a wholeclass of substances, it was thus of great importance. The pointof view was distinctly dualistic, but not quite in the formersense. Accordingly we find that Berzelius at first maintainsa very cautious attitude towards it;40 he finds in it at best a

44 Here, as in all other cases, I quote the formulae of the authors, andJience I employ in this case Dumas' atomic weights which are referred toH = i : O = i6, C = 6, etc. *" The table given in the paper quoted abovecontains obvious misprints; compare Dumas, Traite*. Organic Part, j^ 68,4s HerzeKus, Jahresberich^ 1829, 286,


symbolic mode of expression, which cannot be regarded asrepresenting the actual composition of the substances. Onlysome years afterwards does he revert, for a short time, toDumas' ideas, and he then calls the radical C4H8 Etherin.47

This appears to me to be the place to state the results ofan investigation of Gay-LussacJs into the cyanogen compounds,which had been carried out as early as 1815, and contributedmaterially to giving a more definite meaning to the con ceptionof a radical.48 Gay-Lussac repeated Berthollet's experimentson the composition of hydrocyanic acid and confirmed them,inasmuch as he established beyond all doubt that the acid isfreefrom oxygen, and contains carbon, nitrogen, and hydrogenonly. The examination of the salts led him to study thebehaviour of mercuric cyanide at high temperatures, and thusto discover cyanogen. What is of importance for us in Gay-Lussac's work, is the way in which he regards the substanceshe describes. These are, in his view, compounds of a radicalcontaining carbon and nitrogen (cyanogen) and identical withthe gas obtained from mercuric cyanide. The possibility ofpreparing radicals was in this way demonstrated, and, in con-sequence, the conception attained a more real significance.It is further to be remarked that Gay-Lussac, in calling theradical of hydrocyanic acid, cyanogen, permitted himself acertain freedom, since it was not actually " the residue of anacid which has been deprived of its oxygen." Obviously thegreat French scientist compares hydrocyanic acid with hydro-chloric acid, and with hydriodic acid which he had himselfdiscovered a short time previously. They are hydrogen com-pounds of elements or radicals, exactly as the ordinary acidsare oxygen compounds. If it was now desired to define aradical, and to include cyanogen in the definition, it was nolonger possible to say, with Lavoisier, t ha t u it is the residueof a substance which has been deprived of its oxygen " ; butit was the other half of the definition that was to be accen-tuated ; " a radical is a composite group which behaves like an

47 Annalen. 3, 282. m Ann. Chim. 95, 136.


element."49 As a consequence of Gay-Lussac's investigation,and of the isolation of cyanogen, this latter view had acquiredan increased significance. Reflections of a similar kind do notappear to have occurred further to the chemists of that period.Generally speaking, radicals were only looked for in oxygencompounds, and especially in acids ; whereas Dumas andBoullay's assumption of Etherin proves, on the other hand,that attention was not confined exclusively to these.

Wohler and Liebig's investigation of bitter almond oiland its derivatives, in 1832,50 had a pronounced effect on theviews respecting radicals. It led these two chemists to theassumption of a radical containing oxygen, and thus addedan entirely new significance to these ideas.

Wohler and Liebig first show that the conversion of bitteralmond oil into benzoic acid consists in the taking up ofoxygen, since they establish the formulas C14.H12O2 andC14H12O4 respectively for the two substances.51 In doing HO,they assume, however, an atom of water, H2O, in the latter ;but they neglect this and write the formula of benzoic acidC14H10O3. In this way they come to look upon both sub-stances as compounds of the radical Benzoyl, C14HK)O2, andto regard bitter almond oil as benzoyl hydride, and benzoicacid as an oxygen compound of the new radical. They show,in the course of the investigation, how the same radical maylikewise be assumed in an extensive series of substances.By treatment of bitter almond oil with chlorine and bromine,they prepare benzoyl chloride and bromide, C14H luOrClg

and C14H10O2.Br2. From these, by means of potassiumiodide and of potassium cyanide, they obtain the iodine andthe cyanogen compounds of the radical, C14H10OaJ2 andC14H30O2.Cy2. Finally, with ammonia and with alcohol,they obtain benzamide and benzoic ether.

This investigation is regarded even now as one of thegreatest achievements in the range of organic chemistry. The

49 Lavoisier, Oeuvres, 1, 138, 80 Annalen. 3, 249. BJ Berzcliun'atomic weights.


impression which it produced at the time can be well under-stood. It was the first instance where, starting from onecompound, an extensive series of well-defined substances hadbeen obtained, the relations of which could easily be explainedif the suggested mode of regarding them was adopted. Wethus find Berzelius, then at the zenith of his fame and onlyseldom in accord with the views of others, bestowing abundantpraise upon the investigation.52 He hopes that, as a conse-quence of it, a new day will dawn in chemistry, and he pro-poses to Liebig and Wohler to call the new radical Proinor Orthrin (break of day) because he believes that a clearerlight will be cast upon our science by the assumption ofternary radicals.

And Berzelius was right! For even although the chiefimportance of the investigation does not rest with the radicalcomposed of three elements, still the special part which hadbeen universally attributed to oxygen since Lavoisier's time,even in this branch of chemistry, was taken away from it.Further, that the true signification of the word radical was tobe sought for elsewhere, was demonstrated by the fact that,in the choice of a radical, the composition was left entirelyout of consideration ; and the justification of this proceedingwas found in the experimental results. Benzoyl was a radicalbecause, like an element, it combined with other elements,and because it could be transferred, without decomposition,from the compounds so formed, into others. It was the keyto the interesting reactions of Liebig and Wohler, and itformed the foundation of the benzoic acid series just ascyanogen was the basis of a large number of substances.

Cyanogen and benzoyl are the pillars of the radical theory,which received confirmation by the discovery of cacodyl. Icannot enter into the details of this extremely difficult andbrilliantly executed investigation of Bunsen's, but it is myduty to state the general results of his work.

In 1760, Cadet had obtained a fuming liquid, possessing a

82 Annalen. 3, 282.


nauseating smell, by distilling potassium acetate with arseni-ous anhydride.53 This liquid took lire spontaneously in air,and it was known to contain arsenic and to be poisonous.These properties appear to have deterred chemists from thestudy of it, for, with the exception of a few unimportantexperiments by Thenard, they had not occupied themselveswith it at all for seventy years, anti had been content tomention it in text-books as Cadet's liquid. Dumas had thenendeavoured, by distillation, to separate a pure compoundfrom the crude product, which was contaminated, amongstother things, with elementary arsenic. According to hisanalyses, it is represented by the formula C8H12As2 [C — 6,As = 75].54 Bunsen's first results appeared to confirm this,55

while later experiments eventually fixed it as C4H12As2O[C = i2].5e Bunsen called the substance cacodyl oxide, andassumed the existence in it of the radical C4H12As2. Hesucceeded in preparing the chloride, bromide, iodide, cyanide,and fluoride by treatment with the corresponding acids ; theaction of barium hydrosulphide produced the sulphide ; byoxidation Bunsen obtained cacodylic acid, C4H12As2.O3 + H2O;finally, he found it possible to isolate the radical cacodyl bydecomposing the chloride by means of zinc, and, naturally,this assisted very materially in procuring recognition for hismode of regarding the matter. We can understand howkeenly interest must have been aroused on hearing of theisolation of an organic radical containing a metal, and pos-sessed, besides, of the extremely remarkable property ofspontaneous inflammability.

I have intentionally introduced here the account of thisimportant research of Bunsen's (the completion of which fallsat a later date) in order to be able to make clear the idea of aradical as it now gradually came to be conceived. This ideais essentially different from what was formerly understood bythe term, and the new conception was brought to the front by

68 M6m. de Math, et de Phys. des savants Strangers. 3, 633. M Dumas,Traits. Organic Part, 1, 135; compare Annalen. 27, J48. m Annalen,24, 271. * Ibid 31, 175 ; 37, 1; 42, 14; 46, 1.

128 HISTORY OF CHEMISTRY [LECT. vii.

means of a series of investigations, of which I have shortlystated the most important. In doing this, I was at pains toexplain the development of the ideas, and I only desire now tobe permitted to define the meaning of the term as it eventuallybecame fixed in the minds of the chemists of the period.

I begin with the celebrated definition of Liebig : r>7 il Wecall cyanogen a radical," he says, in 1837, in his criticism ofLaurent's theory, " (1) because it is a non-varying constituentin a series of compounds, (2) because in these latter it can bereplaced by other simple substances, and (3) because in itscompounds with a simple substance, the latter can be turnedout and replaced by equivalents of other simple substances."

These three requirements, of which, according to Liebig,at least two must be fulfilled in order that an atomic groupmay have any claim to the designation of radical, prove thatonly a study of the nature of a compound could lead to aknowledge of the radical contained in it. The behaviour ofa substance towards elements and compound bodies requiredto be known, in order that its radical might be ascertained ;and from this it may be gathered what significance such adetermination possessed. The choice involved, to a certainextent, a resume of the whole investigation, since the de-composition products were known when the radical wasknown ; the latter was, of course, composite itself, but withits decomposition, those affinity relations ceased which con-nected with one another, substances containing the sameradical. That the radical behaved like an element, hadbeen confirmed over and over again. Not only did it enterinto combinations with elements, but it could also be isolatedfrom these combinations. How far this comparison wascarried, is shown by a quotation taken from a joint paperby Dumas and Liebig:58 u Organic chemistry possessesits own particular elements, which sometimes play thepart taken by chlorine and oxygen in inorganic chemistry jsometimes, on the other hand, the part of the metals.Cyanogen, amide, benzoyl, the radicals of ammonia, of fatty

87 Annalen. 25, 3* m Comptes Rendus* 5, 567*

LECT. vir.] HISTORY OF CHEMISTRY 129

bodies, of alcohols and analogous bodies, these are the realelements with which organic chemistry operates, and not theultimate elements carbon, hydrogen, oxygen, and nitrogen,which only appear when every trace of organic origin has dis-appeared." It will thus be understood that the atoms whichconstituted such a group were supposed to be held togetherby stronger forces than those which united the group toother atoms. The radical in this way attained a very realsignificance in the minds of the chemists of that period; itactually existed in the compound, and hence, in any particularsubstance, only a single radical could be assumed since therewas only one present. And thus, with the constantlyincreasing importance which the radical of a substanceattained in respect to views concerning its constitution,divergences necessarily arose in the choice of a radical,according to the decomposition products which were lookedupon as the most important. The discussions thus calledforth were very helpful in the further development of thescience. Everyone tried to support his own view by evidence,and this could only be found in the reactions of the substance.We are indebted to these discussions, therefore, for a veryintimate knowledge of certain classes of substances.

The foregoing explanations will not, I hope, prove super-fluous. Their purport is to render clear the importance ofthe radical theory, the further development of which will bediscussed in the next lecture.

L E C T U R E VII I .

FURTHER DEVELOPMENT OF THE RADICAL THEORY—VIEWS CON-CERNING ALCOHOL AND ITS DERIVATIVES—PHENOMENA OF SUB-STITUTION — DUMAS' RULE — THE NUCLEUS THEORY — THEEQUIVALENT OF NITROGEN.

IN the preceding lecture I endeavoured to explain the sig-nificance of the radical, and I shall now deal more fully withits nature. Enough has been said already by way of prepara-tion for the controversies which were called forth by the choiceof a definite atomic group as the radical of a compound ; andI now consider it my business to pass in review the mostimportant of the discussions. It was especially with respect tothe constitution of alcohol, and of the substances derived fromit, that differences of opinion arose. Since it cannot be deniedthat the opinions regarding these compounds exercised an im-portant influence upon general views, and, further, since themost prominent chemists took part in the discussions, I shallendeavour to show, with respect to this group of substances,how various and how contradictory were the conceptions as tothe arrangement of the atoms.

I dealt, in the preceding lecture, with the so-called etherintheory, which originated in the comparison of the ethers withthe salts of ammonia. In that comparison the radical NH8 wasassumed to be present in these salts ; and even although thedualistic tendency of the whole mode of regarding them cannotbe disputed, still the view is not in harmony with the otheropinions respecting salts. Even then there was anothertheory as to the compounds of ammonia, in accordance withwhich they did not occupy any exceptional position, butwere looked at from a purely dualistic point of view.

The radical ammonium, which, as has already been stated,ISO

vnr.] HISTORY OF CHEMISTRY 131

Produced into the science by Davy,1 constitutes the basishypothesis. Ampere 2 and Berzelius3 were afterwardsinstrumentalmprocuringits recognition. Thesuperi-f this view, with its purely dualistic basis, as comparedie other, is indisputable ; for the compounds of ammonianium compounds) now appear as analogues of the:y salts, as their behaviour necessarily requires thatiould. The analogy was illustrated thus :—4

KCI.2 - - Muriate of Potash- Sulphate ,,

oniac - - (N2Ha)CJ2of Ammonia (NoHB)0S0j

(N2H8)ONaO3 KON.2Or> - NitrateKOQH(iO3 - Acetate

i clear that this way of regarding substances could alsoied to the compound ethers, if the radical C4H10 werei instead of C4H8. Berzelius took this step in 1833.5

; led to do so not only by his predilection for thehim theory, but also on account of newly discoveredlich I shall state here.tng the same year, Magnus, by acting with sulphuricde on alcohol and ether, had discovered ethionic andic acids.0 The latter acid was obtained by the decom-of the former by means of water, and, according to the, it was isomeric with it. Its barium salt, in accord-th the prevailing etherin theory, had the formula5SO3 + BaO + H2O assigned to it; or, according to Mag-is to be regarded as a compound of ether and anhydrousc acid, with baryta. Liebig and Wohler's analysesn sulphovinate,7 which were confirmed by Magnus,8

d its composition as C4H8 + 2SO34-BaO-f-2H2O ;

>. 76. 2 Ann. Chim. [2] 2, 16, Note. 8 Gilb. Ann. 46, 131 ;Lehrbuch. Third Edition. 4 Berzelius adopts NH4 as an atomum ; and yet this does not occur in combination any more thanammonia NH3. Instead of it the double atom N~M^ (=N2H8)urs. The isomorphism of ammonium chloride and potassiumust have influenced him to write ffH^Cl and not (NH4)2C12.in. 28, 626. e Annalen. 6, 152. 7 Ibid. X, 37. * Magnus,

132 HISTORY OF CHEMISTRY [LECT. VIII.

that is to say, it seemed to contain an atom of watermore than the newly discovered compound. Berzelius nowdraws attention to the fact that the latter compound cannot beconverted into the barium salt of sulphovinic acid by boilingit with water, and that therefore the view underlying theseformulae (in accordance with which the substances containready formed water) is erroneous. According to him, alcoholand ether are the oxides of two radicals, C2H6 and 62$J5 =C4H10. The compound ethers are thus still regarded as com-posed of ether and of acid ; but there is no longer ready formedwater in them, and they now become comparable with salts:—Ether - - - C4H10O KO - - - PotashHaloid Ether - - C4H1OC12 KC12 - - Muriate of PotashAcetic ,, - C4H10O + C4H6O3 KO + C4H6O3 - Acetate „Nitric „ - C4H10O + N2O5 KO + N2O5 - Nitrate „

Berzelius very clearly perceived the importance of his sug-gestion. He had now attained what he had long striven for.The dualistic conception was now applicable to organic com-pounds, or at least to the most fully investigated group ofthem ; and he does not conceal the pleasure which this occa-sions him. He states that the organic substances are nowto be regarded (in the same way as mineral substances) asbinary groups, but that in them compound radicals aloneplay the part of the inorganic elements—a view whichDumas and Liebig afterwards develop fully in a specialtreatise.9 (Compare p. 128.)

A ground for discord was now provided, and it was not tobe long before the contention should begin. The next incite-ment to it is given by Liebig, who throws down the glove tothe etherin theory.10 According to him, this theory has nojustification, and all the grounds that can be advanced in itsfavour rest upon fallacious experiments. Amongst these thereis, in the first place, an observation of Hennell,11 according towhich sulphuric acid absorbs etherin (olefiant gas) and pro-

9 Comptes Rendus. 5, 567. 10 Annalen. 9, 1. u Phil. Trans. 1826,340; 1828, 365.

LECT. VIII.] HISTORY OF CHEMISTRY 133

duces sulphovinic acid directly. Liebig endeavours to provethat Henneirs etherin was contaminated with the vapours ofalcohol and of ether, and that the pure gas is not absorbedby sulphuric acid.12 He then attacks the formulae of Zeise'splatinochloride compounds, which, according to their dis-coverer, consist of etherin, platinous chloride, and potassiumchloride.13 Liebig thinks he is justified in concluding fromZeise's analyses, and from the reactions of the substances,that it is not etherin but ether that they contain. Finally,he disputes the existence of the ethyl-oxalate of ammonia(ethyl-oxamate) which Dumas and Boullay had preparedfrom oxalic ether and dry ammonia gas.14 According toLiebig, the same substance is formed by the action ofaqueous ammonia, and is identical with oxamide. By thisattack all the supports of the etherin theory seemed to bedestroyed. Liebig, in bringing the matter forward, admitshis adherence to the hypothesis of Berzelius regarding theethereal compounds, while he only differs from him in hisview with respect to alcohol. In the latter substance alsohe assumes the radical C4H10, which he calls ethyl; andwith him alcohol is the hydrate of ether, C4H10O,H2O. InLiebig's opinion it was no objection that half as many atomswere thus assumed in one volume of alcohol as were assumedin the same volume of ether. Chemists were now furtherthan ever from adopting Avogadro's hypothesis, as may begathered from the following assertions of Liebig :—16

"Apart from the contradiction which is involved, if etheras an oxide is deficient in the property of uniting with waterto form a hydrate, while, like other oxides, it is able to unitewith acids, and like the metals, its radical is able to unite withthe halogens, the specific gravity of alcohol vapour cannot belooked upon as any evidence for its constitution as an oxide ofanother radical. On the contrary, I believe the very circum-stance that ether and water vapour unite in equal volumes, and

12 Berthelot afterwards showed that absorption can be brought aboutby vigorous shaking. M Mag. f. Pharm. 35, 105; Pogg. Ann. 21, 533-14 Ann. Chim. [2] 37, 15. u Annalen. 9, 16.

134 HISTORY OF CHEMISTRY [LECT. vnr.

without condensation, is evidence in favour of the view thatthis compound, alcohol, is a hydrate of ether. . . . In theformation of benzoic ether from absolute alcohol and benzoylchloride, we perceive a simple decomposition of water, whichdoes not extend further than to the water of hydration."

Liebig went too far in his argumentation, and Zeise andDumas justly protested against it. The former repeats hisprevious examination of the inflammable platinochloride, andfinds his first results confirmed : there is no more oxygenpresent in the compound from which the water of crystallisa-tion has been removed, and, therefore, ether cannot beassumed to be present in it, but only etherin.10 Dumaslikewise upholds his earlier experiments.17 He points outthe difference in the action of aqueous and of dry gaseousamm.onia upon oxalic ether. In the first case only is oxamideformed, whereas ammonia gas gives rise to the substance hehad previously described, which he now calls oxamethan, andto which he assigns the formula C4O8,NHS,C4H4 [C = 6].18

As a consequence, Dumas also adheres to his old opinion.He draws attention likewise to the fact that it was withhim the idea originated according to which ordinary ether(" sulphuric ether ") is the base of the compound ethers ; andhe states that it is, in point of fact, the essence of the wholeethyl theory. He goes a step further, indeed, inasmuch ashe regards ether itself as composed of water and olefiant gas.

In this Dumas was right. There was one point, how-ever, which had previously been brought forward by Liebig,19

on which Dumas and Liebig differed. This was the assump-tion by Dumas that in ether two of the hydrogen atomsplay a different part from the others ; and Liebig contestedthis particular point. The discovery of the mercaptans byZeise20 furnishes Liebig with an opportunity to adduce new

16 Annalen. 23, 1. n Ann. Chitn. [2] 54, 225. Annalen. 10, 277 ;JS 52- 18 It seems as if Dumas, in accordance with his earlier experimentsassumed one atom of water more ; the analyses do not, however, afford anysharp decision respecting this. 19 Annalen. 9, 15. 20 Ibid. II, I.

LECT. vin.] HISTORY OF CHEMISTRY 135

proofs of the accuracy of his ideas.21 He regards these com-pounds as analogous to alcohol; they are composed of ethylsulphide, C4H10S, and sulphuretted hydrogen, H2S, and theirextremely interesting metallic compounds show that in themthere really are two hydrogen atoms which behave differentlyfrom the others. Two years later, in 1836, he collectstogether all the grounds for and against each view ;22 andfrom the most various facts (but especially from the pheno-mena of substitution, already discovered by Dumas), heconsiders himself justified in drawing the conclusion thatether is not a hydrate but an oxide.

The matter was not yet settled, however. In his ex-amination of wood spirit (carried out in conjunction withPeligot), Dumas had found new support for his views.23 Hesucceeded in settling the composition of this substance, amatter that different chemists had attempted to settle, butwithout success. He showed that in its whole behaviour ithad the closest resemblance to alcohol, forming, as the latterdoes, ethereal compounds with acids ; and he assumes in itthe radical C2H4, methylene, with which etherin is polymeric.The advantages of this way of regarding the matter, whichdoes not lead to the assumption of hypothetical radicals,are again pointed out.24

The discussion between Berzelius, Dumas, and Liebig,of which I have given some examples, was of much serviceto our science. The facts were illuminated from the mostdifferent points of view, and this was far more favourablefor progress than if a single theoretical opinion had cometoo prominently to the front. The chemists just mentionedwere the chief exponents of chemistry at that time, andround them the other investigators gathered. Of these,only a few represented independent opinions, and chemistswere divided, accordingly, into three camps. It is true thatin 1837 a s o r t °f armistice was concluded. At a personal

21 Annalen. II, 10. <22 Ibid. 19, 270, Note. ^ Ann. Chim. [2] 58,5 ; Annalen. 13, 78 ; 15, 1. ^ Dumas and Peligot believe that they canisolate methylene.

136 HISTORY OF CHEMISTRY [LECT. VIIL

interview, Liebig converted Dumas to his opinions, and wefind the two savants conjointly publishing a scientific treatise25

in which they announce that thenceforth they wish to elabor-ate organic chemistry with united energies and from the samepoint of view ; to have analysed in their laboratories all thesubstances not yet examined by them ; to open up, by the aidof their pupils, additional lines of investigation in the mostvaried directions ; and to subject the work of others to arigid criticism and check. But the union only endures fora short time : it is terminated in a year, and each returns tospecial paths, which diverge more and more. In 1840 the twoare again hostile towards each other, although perhaps morecareful in their assertions and more polite than previously.

Dumas had made observations in the meantime whichcaused him to break away from all traditions, to abandondualism and the electro-chemical theory, and to express viewsthat Berzelius especially attacked most vigorously. Thelatter, who up to this time had taken the most importantpart in the development of the science, holding his opponentsin check by means of his theories, and who had struggledfor the ascendency with Liebig and Dumas, now tries,ineffectually, to oppose his ideas to those of Dumas and ofLaurent. He relies upon unestablished hypotheses, whichonly later, at Kolbe's hands, receive a real foundation, andacquire thereby a scientific significance.

Before turning to this period, with its theories of sub-stitution, of nuclei, and of copulae, we must still consider afurther development of radicals in the earlier sense, withwhich the various notions respecting alcohol and the com-pounds derived from it are brought to a close.

Regnault, in his examination of the oil of the Dutchchemists, had found that this substance loses the elementsof hydrochloric acid when it is distilled with potassiumhydroxide, and that it yields a new substance of the com-position C4H6C12.

26 This he regards as the chloride of theradical aidehydene, C4H6, and confirms his view by preparing25 Comptes Rendus. 5, 567. a Ann. Cbim. [2] 59, 358 ; Annalen. 15, 60.


the bromine and iodine compounds of this radical, which healso assumes to be present in aldehyde and in acetic acid.He writes :—

Hypothetical radical AldehydeneChloraldehydeneBromaldehydeneChloride of Hydrocarbon (Ethylene

chloride)Bromide of Hydrocarbon (Ethylene

bromide)

C4H6,C12

C4H6,Br2

C4HG,C12 -f-H2Cl2

C4H6,O AldehydeAcetic acidC4H6,O3

This investigation of Regnault's, carried out in 1835, wasprompted by Liebig, and was intended to prove to Dumasthat the radical etherin is not present even in ethylenechloride ; it was intended to overthrow the etherin theory,and it may have had much influence in moving Dumas togive up his former views. I t is true that in consequence ofthis investigation Liebig also abandoned the radical ethyl,and tried to explain the ethereal compounds on the assump-tion of a radical acetyl, C4HG.27 These substances are againcompared with the salts of ammonia, but in the latter theradical amide is now assumed.

AcetylOlefiant gasEthylEtherEthyl chloride -Alcohol

Mercaptan -

Isethionic acid -

Acetic acidAldehyde -

- Ac = C4H6- AcH2- AcH4

-AcH4OAcIi4Clo

AcH4O + H2O

AcH4S + H2S

-AcH2 + 2SO3

AcO + Oo

27 Annalen.

Ad = N2H4 -AdH2 -AdH4 -AdH4O -AdH4Cl2AdH4O-hH2O

AdH4S + H2S

AdH2 + SO3 -

30, 139-

AmideAmmoniaAmmoniumAmmonium oxideSal ammoniacCompound in sul-

phate of ammoniaHydrosulphuret of

ammoniaRose's anhydrous

sulphate of am-monia.


Three views respecting the ammonium salts and the com-pound ethers had thus been advanced, and these differed fromone another merely in the number of hydrogen atoms whichwere assumed in the radical. These were :—

1. The ammonia theory of Lavoisier, corresponding tothe etherin theory of Dumas and Boullay;

2. The ammonium theory of Davy, Ampere, andBerzelius, corresponding to the ethyl theory ofBerzelius and Liebig ; and

3. The amide theory of Davy and Liebig, correspondingto the acetyl theory of Regnault and Liebig.

Liebig believed that, by his new theory, he had overcomeall difficulties, and had settled all contentions with respecteither to the ethyl theory or the etherin theory. He closes hispaper with the following words : "From this point of view,both of the formerly opposed theories possess, as may easily beobserved, the same kind of basis, and all further question asto the truth of the one view or of the other is thereby settled."

In a certain respect, Liebig was right; the question whetherethyl or etherin was present in alcohol was no longer discussed ;not, perhaps, because acetyl was preferred, but because chemistsnow began to attach another signification to the radicals. Thephenomena of substitution, which were already known, gradu-ally became of general applicability, and, after the discovery oftrichloracetic acid, the hypotheses which Dumas and Laurenthad advanced, acquired great influence. Not only was theradical theory, in the form in which it was then stated, threat-ened by these hypotheses, but dualism andthe electro-chemicaltheory—the very foundations of the entire mode of viewingchemical matters—were attacked by them, and were eventuallydriven out of the science. These hypotheses led to the recog-nition of the radicals as variable and of chemical compoundsas possessed of an individual or unitary character; and to theabandonment, as arbitrary, of the division of the latter into twoparts. At a later period, in conjunction with the conceptionsderived from the theory of the polybasic acids, they led to arevision of the size of the atoms in the case of compounds, to

LECT. vni.] HISTORY OF CHEMISTRY 139

the establishing of the chemical molecule, and to the theoryof types. Simultaneously, the conception of the equivalentassumes a more fixed form, and is distinguished from that ofthe atom ; it is recognised that the atoms are not equivalent,but are of different values in combination ; the theory ofatomicity is developed and this stimulates the determinationof rational constitution as we now understand it.

Let us next make something more than a mere bird's eye in-spection of this periodjrich as it is in discoveries and hypotheses.We shall now subject it to a minute examination. In doing so,we find that the development of chemistry during the last sixtyyears is not inferior in interesting and important episodes tothat of any period in the science. The participation in thisdevelopment is a constantly increasing one and it is a difficulttask to seek out from the enormous mass of material which waselaborated during this period, the things that were importantand conducive to progress, to state the development of the ideasin such a way that they shall be at once logical and in accord-ance with the actual facts, to do justice to every one, and yetnot to lose the thread over details or questions of priority.

The history of this epoch has never yet been described ina connected manner.28 In venturing upon the attempt to dothis, I am well aware that an objective representation of theperiod is scarcely possible, and that I play the part rather ofcritic than of historian. Still I have endeavoured to make myexposition of some value from the fact that I have always beencareful to arrive as nearly as I could at the truth, and not topermit myself to be led by prejudices or personal matters.

The conception of equivalence might have led to that ofreplacement or substitution, since the quantities of two acidswere equivalent when they saturated the same quantity of abase. The acid in a neutral salt could thus be replaced by itsequivalent, without the neutrality being interfered with. Theword u replacement" received further justification after Mit-

28 Wurtz's Histoire des doctrines chimiques only appeared during theprinting of the first edition of these lectures, and Kopp's Entwickeluog derChemie in der neueren Zeit appeared some years afterwards.

UO HISTORY OF CHEMISTRY [LECT. vin.

scherlich had studied the phenomena of isom orphism. It couldthen be said that certain elements in a crystal might be replacedby others, without alteration of the crystalline form. Such sub-stitutions possessed the peculiarity, however, that they werenot connected with any proportions by weight, and it maythus appear ail the more remarkable that they should renderimportant assistance in the determination of atomic weights.The hypothesis underlying the phenomena of isomorphismwas that one atom could only be replaced by one other; thatis to say, that the numbers of the atoms in isomorphous com-pounds must be identical. Since chemically similar substanceshad alone been compared, an extension of the prevailingviews, based on the phenomena of isomorphism, would havebeen quite possible ; but this class of phenomena had neverled to any attack upon the system.

Such an attack nowtookplace, however, and it was foundedupon a series of facts which I must relate here. In the bleach-ing of wax by means of chlorine, Gay-Lussac had observed thatfor every volume of hydrogen eliminated, an equal volume ofchlorine was taken up.29 He had also found the same thingin the action of chlorine on hydrocyanic acid. In the courseof their investigation of the benzoyl compounds, previouslyreferred to, Wobler and Liebig, when acting with chlorineuponbitter almond oil, had discovered benzoyl chloride; and theyexpressly remark that this substance is produced from thebitter almond oil by two atoms of chlorine taking the placeof two of hydrogen.30 In 1834, -Dumas examines the actionof chlorine on oil of turpentine,81 and in this case also eachvolume of hydrogen eliminated is replaced by the samevolume of chlorine. Then when he studies the products ofthe decomposition of alcohol by means of chlorine and ofbleaching powder, in order to clear up the nature, and themode of formation of chloral and of chloroform, he statesthe empirical rule, observed in a single case by Gay-Lussac,in the following general form :—82

29 Gay-Lxissac, Lemons de Chimie. *° Annalen. 3, 263. m Ann. Chim.[2356,140. ™ Ibid. [2] 56, 113.


1. When a substance containing hydrogen is exposed tothe dehydrogenising action of chlorine, bromine, or iodine,for every volume of hydrogen that it loses, it takes up anequal volume of chlorine, bromine, etc.

2. When the substance contains water, it loses thehydrogen corresponding to this water without replacement.

The second rule was advanced chiefly to explain theformation of chloral, and at the same time to justify theformula C8H8 + 2H2O [C = 6] adopted for alcohol by Dumassix years previously. According to Dumas, the phenomena ofsubstitution furnish a new proof of the difference of the hydro-gen atoms, eight of which are united to carbon, and four tooxygen. In the case of the former alone does replacementoccur, whilst the others are removed without replacement.

Thus we have :—

(C8H8 + 2H2O) + 4C1 = C8H8O2 + 4HCIAldehyde.

C8H8O2 +12CI« C8H2Cl0O2 + 6HC1Chloral. "

By means of various examples, Dumas further endeavoursto prove the general validity of the laws which he advances.In establishing the correct composition of the Dutch oil, hepoints out that the chloride of carbon obtained from it bymeans of chlorine, and examined by Faraday,38 supplies anew argument in favour of the accuracy of his views. Healso finds similar support in the action of chlorine onhydroc3'anic acid, on bitter almond oil, etc.

But Dumas is not satisfied with this. He goes a stepfurther still, and regards oxidations as-cases of substitution,as, for example, the conversion of alcohol into acetic acid.84

In this case every volume of hydrogen eliminated is replacedby half a volume of oxygen. Accordingly we have :—

(C8H8 + H4O2) + O4 = (C8H4O2 + H4O2) + H4O2 ;Alcohol. Acetic Acid.

33 Phil. Trans. 1821, 47. ** Ann. Chim. [2] 56, 143.


and the formation of benzoic acid from bitter almond oil isexplained in the same way :—

C28H10O2.H2 + O2 = C28H10O2.O + H 2 O.Bitter Almond Oil. Benzoic Acid.

In order to be able to include the action of oxygenwithin his rule, he states the latter as follows:—When acompound is exposed to the dehydrogenising action of anysubstance, it takes up a quantity of this substance equivalentto the quantity of the hydrogen eliminated.

The doctrine of Dumas seems to me to be of most im-portance in this connection. He shows us that equalvolumes of hydrogen, chlorine, bromine, and iodine areequivalent, while they possess only half the value of thesame volume of oxygen. The difference between the two isclearly visible here, and this is the beginning of the separa-tion of atom and equivalent.

The phenomena of substitution, or of metalepsy, asDumas called it, were followed up further in the succeedingyears by Dumas himself, as well as by Peligot,35 JRegnault,36

Malaguti,37 and, especially, Laurent; and in particular, it isthe independent extensions which the latter gave to Dumas'rule, that we shall now consider.

Laurent has enriched chemistry with a very large numberof experimental investigations, but these, in many cases, areunfortunately wanting in the necessary accuracy. He hadat his command only very limited resources, and insteadof confining himself, on that account, to a few branches,Laurent, who was very fertile in ideas, preferred t o start a greatmany things and to carry them out in a superficial manner.He destroyed, in this way, his reputation as an experimentalist,and was met with hostility at the very outset of his scientificactivity ; while he was afterwards treated with unnecessaryseverity, especially by Berzelius and Liebig. This naturally

85 Annalen. 12, 24; 13, 76; 14, 50; 28, 246. 30 Ibid. 17, 157; 28,84; 33' 3io; 34, 24, etc. & Tbid. 24, 40 ; 25, 272 ; 32, 1$ ; 56, 268, etc.


reacted upon him, and he went his own way, and becamegradually more incomprehensible, particularly in conse-quence of a nomenclature which was used almost ex-clusively by himself. Many of his clever and originalideas were thus lost to our science, or are now ascribedto the services of others. A great deal, on the other hand,was supplied to us first by Gerhardt, who was a friend andcollaborator of Laurent for many years, and who combineda clear mode of expressing and of regarding chemicalmatters with probably more perspicuity and less genius.

At an early period, Laurent had begun to occupy hisattention with the phenomena of substitution. He first studiednaphthalene and its derivatives ;38 then, simultaneously withEegnault, the derivatives of ethylene chloride;89 afterwardsthe action of chlorine upon compound ethers,40 and upon theproducts of the distillation of tar, especially on phenol,41 etc.

He very soon satisfied himself, by means of these variousinvestigations, that the form which Dumas had given to the lawof substitution was not a generally accurate one. He foundthat in very many instances there are more or fewer equivalentsof chlorine or of oxygen taken up than there are of hydrogeneliminated, and vice versa; and that this is the case even withsubstances which do not contain oxygen, so that the exceptionscannot be explained by Dumas' second rule.4'2 At the sametime, however, Laurent points out that the substituted product,when it is produced by replacement of equivalent by equivalent,still exhibits certain analogies with the original substance; andhe asserts that the chlorine introduced takes the place, and to acertain extent plays the part, of the hydrogen eliminated. Hisopinion maybe stated somewhat in the following manner:—43

Many organic substances when treated with chlorine lose acertain number of hydrogen atoms, which escape as hydro-

88 Annalen. 8, 8 ; 19, 38 ; 35, 292 ; 41, 98 ; 72, 297 ; 76, 298, etc.39 Ibid. 12, 187 ; 18, 165 ; 22, 292. 40 Ibid. 22, 292. 41 Ibid. 22, 292 ;23, 60 ; 43, 200, etc. ^ See p. T41. « Laurent, Me'thode de Chimie.242 ; E. 199 ; These de docteur, Paris, 20 Dec. 1837,11, 88, and 102 ; Ann.Chim. [2] 63, 384; Comptes Rendus. io, 413 ; Revue Scientifique. I, 161.


chloric acid ; chlorine atoms equal in number to the hydro-gen atoms eliminated are substituted for the latter, so thatthe physical and chemical properties of the original sub-stance are not essentially altered. The chlorine atoms thusoccupy the space left vacant by the hydrogen atoms. Inthe new compound the chlorine to a certain extent playsthe same part as the hydrogen did in the original substance.44

Laurent endeavours to give expression to the observedfacts and to the hypotheses based upon them, in the so-called nucleus theory.45 This theory is of importance inour science (although it never obtained any general recog-nition in it) because we have adopted, even if in anotherform, many of the ideas embraced by it, and also because itwas adopted by Gmelin as the basis of the organic portionof his excellent handbook. On this account I shall statethe chief points of Laurent's doctrines.

According to Laurent, all organic substances containcertain nuclei, which he calls either fundamental {radicauxfondamentaux) or derived. The former are compounds ofcarbon with hydrogen, in which the mutual proportion ofthe number of the atoms is a simple one (i to 2, 3, 4, etc.,2 to 3, etc.). For any definite proportion, several nucleiexist which are polymeric amongst themselves. Besides,these fundamental radicals are so chosen that the hydrogenand carbon atoms contained in them occur in pairs.

Subsidiary nuclei are formed from the fundamental nucleiby the substitution of other elements for the hydrogen ; forexample, chlorine, bromine, iodine, oxygen, nitrogen, etc.Laurent afterwards assumes replacement by radicals or groupsof atoms. In such reactions Dumas' rule always holds, thatthe hydrogen turned out is replaced by equivalent quantities

44 I wish to recall the fact here, simply because a question of priorityarose with respect to it (compare Comptes Rendus. 10, 409 and 511), thatLiebig and W5hler, in their investigation of bitter almond oil, had alreadyassumed that during the formation of benzoyl chloride the chlorine takesthe place previously occupied by the hydrogen (compare p. 140). *• Ann.Chim. [2] 61, 12$ ; compare also Gmelin, Handbuch. 4, 16; E, 7, 18.

LECT. VIII.] HISTORY OF CHEMISTRY US

of other elements. But this is not the only kind of change thatthe nucleus can undergo, and here Laurent differs from Dumas.Thus an indefinite number of atoms may attach themselves tothe radical, and be removed from it again without being re-placed ; whereas, as already stated, an atom cannot be re-moved from the nucleus without the entry of its equivalent,since the destruction of the whole group would otherwiseensue. Such destruction infallibly occurs as soon as carbon,in the form of carbonic anhydride, carbonic oxide, etc., is re-moved from the compound ; in which case either a completedecomposition takes place, or a fresh nucleus is formed. Therelation of this nucleus to the first one is not further defined,however. According to Laurent, the subsidiary nuclei show agreat resemblance to the fundamental radicals in their physicaland chemical characters ; the derived nuclei, obtained byattachment of new atoms, have, on the other hand, acquireda different character. Thus a union with hydrogen andoxygen (water) usually brings about the formation of alcohol ;a neutral oxide is formed by the taking up of two atoms ofoxygen, a monobasic acid, by the taking up of four atoms,and a dibasic acid by the taking up of six atoms ef oxygen.

Laurent further adopts a geometrical conception respectingorganic compounds. In accordance with this conception thenuclei are prisms in whose angles the carbon atoms are located,whilst the edges are formed by the hydrogen atoms. Theseedges can be taken away and replaced by others, without thefigure undergoing any considerable changes. But should theplace be left unoccupied, the internal connection would cometo an end and the whole would fall to pieces. Atoms can stillbe added to the prisms so as to form pyramids ; and the wholefigure may be surrounded in this way, by which means itsform is, of course, altered. These pyramids can be removed,whereupon the original prism makes its appearance again.

In our matter-of-fact science we are not accustomed tosuch imaginative views, and so it may appear as if nothing ofany value for chemistry lies hidden behind them. In orderto disprove this, however, I shall translate the hypotheses of

K


Laurent into our ordinary language. It will then be possibleto obtain, moreover, a better grasp of Laurent's ideas.

The nucleus theory clearly sprang from the radical theory,but only by an essential reconstruction of the latter. Theradical of Laurent is not an unalterable group of atoms, but itis a compound, which can be altered by substitution in equiva-lent proportions and does not lose thereby its characteristicproperties. Thus Laurent is able to derive all his radicals fromhydrocarbons, a proceeding which is, of course, in completecontradiction to the older ideas. These radicals can unite withother atoms, and in the substances so produced the nuclei arepresent as such ; the nuclei pre-exist in the substances, andLaurent, therefore, entirely agrees on this point with his prede-cessors. By means of these two hypotheses, he is able to explainall the facts—not only the cases which follow Dumas' rule, butthose also which are at variance with it, and of the latter he hadfound a large number. At the same time, his point of view fur-nishes reasons why both kinds of reactions are possible. On theassumption of the alterability of the radical, it may easily beunderstood that a group included far more compounds thanwas possible with the older radical theory. Laurent was thusable to discover far more of what we should now call "genericrelationships," and that was an unquestionable advantage.Since he assumed the number of carbon atoms in the nucleusto be constant, substances were arranged into series accord-ing to the number of atoms of carbon they contained, and thissupplied the basis of an excellent systematic classification.With him there was no connecting link between the seriesso formed ; and in this respect Laurent's division differsfrom the classifications of the present day, which accentuateall relationships as much as possible. No such mode oftreatment could, indeed, have been carried out at that time.

After these explanations, I may state that much that wasnew and good was advanced in Laurent's nucleus theory* Itsimportance lies principally in the fact that it was capable of ageneralapplication,and,as Gmelin proved,could be admirablyemployed as the basis of a detailed text-book. In this respect it


is distinguished by very marked advantages over the radicaltheory, which, owing to the very definite form that had beengiven to the radical, could only be of service in certain direc-tions, while it wholly overlooked various relationships.

I think it may be advantageous to show, by means of afew examples, the mode in which Laurent applied his theories,and also what his formulae were for different compounds. Indoing this I shall choose familiar groups of substances.

Nucleus, Etherene C4H8 [C = 12] *«Etherene HydrochlorateChloretherase -Chloretherase Hydrochlorate -Chloretherese -Chloretherese Hydrochlorate -Chloretherise - -Chloretherise Hydrochlorate -Chloretherose -Etherose Chloride -Chloral - - - - -Bromal - - - - -Chloracetic Acid (then unknown)

C4H8 + HOC12C4H(JC1, 'C4HXI2 + HX12

C^HXl,,. (then unknown)C4HX14 + HX12

C4H2CJ0 (then unknown)C4HXIO + H X 1 ^c x i 8

CX18+C14

CX16O + H2OC4Br0O + H2OC4H,C1<O + O2

Nucleus, Methylene C2H4

Chloroform -Brornoform -Cyanogen -Hydrocyanic AcidCyanic Acid

CX14 + HX12

C2Br4 + H2Br2

CjAzgC2Aza + H s

C2Az2 + o"48

Nucleus C14H14

Bitter Almond OilBenzoic AcidHydrobenzam ide

C14H10O2 + H 2

CuH10O2 + 0

CuH10A2V, + H2«46 Ann. Chim. [2] 63, 388 ; Annalen. 22, 303. 47 The principle of

the nomenclature employed originated with Dumas (Ann. Chim. [2] 57,305). Laurent almost always used this nomenclature. ^ The symbolAz (Azote) was written for the nitrogen atom in France at that time (as itstill frequently is). I have used it here for a reason that will be perceivedfurther on. *» Ann. Chim. [2] 62, 23.


I have intentionally chosen these particular examples.They lead us to a point in Laurent's views which we have,up to the present, only very superficially touched upon. Sincehe assumed substitutions of hydrogen by nitrogen, we mayinquire what the equivalent of nitrogen was. We recognise,from his deriving cyanogen from methylene, that Laurentassumed one atom ( = 1 4 parts by weight) of nitrogen asequivalent to two atoms or parts by weight of hydrogen. Thishypothesis did not accord with fact in the case of hydroben-zamide, which Laurent had obtained by treating bitter almondoil with ammonia. If the new substance was to be referred tothe same nucleus as that from which he derived benzald ehyde,then two-thirds of an atom, 0^.33 parts by weight of nitrogen,must be equivalent to two parts by weight of hydrogen.Laurent did not know how to get out of this dilemma. Thequestion was decided by Bineau.50 In a detailed paper thelatter endeavoured, in 1838, to give a solution to the problemof the determination of the equivalent of nitrogen. After adiscussion of the fact that the common method of fixing thisnumber is very arbitrary (considering that the quantity of asubstance which, in its lowest stage of oxidation, unites with100 parts by weight of oxygen is usually assumed to be equiva-lent to that quantity of oxygen, whereas it would be quite asjustifiable to start from any other stage of oxidation), he adoptsother considerations in respect to the matter. He obtains hisevidence chiefly from the hydrogen compounds. He com-pares ammonia with water, and asks how many atoms ofoxygen are necessary in order to oxidise completely the hydro-gen united to one atom of nitrogen. It is known that i j atomsare required for this, consequently Bineau finds 14 parts byweight of nitrogen to be equivalent to 24 of oxygen and to 3 ofhydrogen ; in other words, the equivalent of the former, com-pared with 16 parts by weight of oxygen, is 9.33 = Azf. He in-troduces for nitrogen the symbol N, and points out that hydro-benzamide now accords with Dumas' rule. It can very easilybe understood that Laurent accepted Bineau's determination.

50 Ann. China. [2] 6% 22$


On scarcely any side did the nucleus theory meet withacceptance. Dumas made use of much of it in advancingthe theory of types, and although in doing so he mentionsLaurent, still opinions which unquestionably were first statedby the latter, were ascribed to Dumas. No doubt, they metwith acceptance earlier on account of the superior authorityand position of Dumas.

Liebig, however, expressed himself very forcibly againstLaurent,51 and hewas not altogether unjustified in his charges.In the application of his theory Laurent incurred blame formany arbitrary proceedings, which Liebig knew how to pointout with much effect. Liebig further attacks the facts dis-covered by Laurent and employed by him in support of hisopinions ; and these are likewise not always able to with-stand the keen criticism applied to them.

Much more violent still were the attacks of Berzelius,62

which he erroneously directed against Dumas. The view thatnegative chlorine could take the place of positive hydrogen,without altering the nature of the product, was wholly inad-missible with the author of the electro-chemical theory. Hemakes every conceivable endeavour to bring the constantlyincreasing number of substitution products into harmonywith his theories. I shall postpone, however, the detailedconsideration of his views until a subsequent lecture, and shallclose this one with an observation of Gerhardt's,53 whichenables us to recognise the clear and intelligent perception ofthis chemist, who at the time was still quite a young man.

Laurent's formula for Dutch oil was C4H6C12 + H2C12.By treatment with chlorine this substance was said to beconverted into chloride of carbon, C4C112.

54 According toGerhardt, Laurent's formula is inaccurate because it assumesthe decomposition of hydrochloric acid by means of chlorine,accompanied by the re-formation of hydrochloric acid.

51 Annalen. 25, I. m Compare Comptes Rendus. 6, 629; andBerzelius' Jahresbericht 1840, 361, m J. pr. Chem. 15, 17. M Phil,Trans. 1821, 47.

L E C T U R E IX.

GRAHAM'S INVESTIGATION OF PHOSPHORIC ACID—LIEBIG'S THEORY OFPOLYBASIC ACIDS, AND HIS VIEWS WITH RESPECT TO ACIDS INGENERAL—ADOPTION OF THE DAVY-DULONG HYPOTHESIS—DIS-COVERY OF TRICHLORACETIC ACID—ATTACK UPON THE ELECTRO-CHEMICAL THEORY—REPLIES OF BERZELIUS—COPULA.

I WISH to begin this lecture with some general remarks whichmay serve as an appendix to what was stated in the preced-ing lecture respecting the phenomena of substitution, andespecially to the conceptions of them which were entertainedby Dumasl and by Laurent.2

I desire to point out how, in consequence of the observedphenomena of substitution, the conception of the equivalentassumed a more definite form. Thus in assuming, with Dumas,that the quantities replacing one another are equivalent (andthis was an assumption that had some justification, accordingto Laurent's views which rendered possible a direct com-parison between the original and the final products), a series ofexperiments was all that was necessary in order to determinethe equivalents of the substances so replacing one another.One section of chemists actually did work in this direction,and, in consequence of this, it is necessary to observe par-ticularly to which school the author of any paper publishedat that time belongs ; for at this very period Gmelin's school,which likewise wrote or desired to write in equivalent formulae,began to acquire much influence. .Whilst the adherents of thesubstitution theory (who made use of the equivalents), in spiteof numerous shortcomings and mistakes, always endeavouredto separate from each other the conceptions of atom and

1 Dumas, Trait<£, Organic Part, I, 75. 2 Annalen. 12, 187.150

LECT. ix.] HISTORY OF CHEMISTRY 151

equivalent, and to follow up both of them consistently, almostthe opposite might be said of their opponents. The latterknew that one atom of alumina, A12O3, requires three times asmuch sulphuric acid to saturate it as one atom of potash,KO ; they also considered that one atom of phosphoric acid,P2O5, required three times (in reality twice) as much base toform a neutral salt as one atom of hydrochloric acid ; andyet they did not hesitate to employ the name " equivalents "for these quantities.

Just because our chemistry of to-day is based essentiallyupon the difference between the conceptions of atom andequivalent, everything that could lead to a distinction betweenthese conceptions must be specially emphasised. I wished,therefore, to point out that a new means of determining equiva-lents was furnished, in the fourth decade of last century^ bythe phenomena of substitution, and that this involved a realadvance in the question which is especially interesting us atpresent. But it was further shown, from an entirely differentpoint of view, that the atoms of compound substances are notnecessarily equivalent. Decisive reasons were advanced toestablish the diiferences that exist, in this respect, in the caseof the acids, which formed one of the most fully investigatedclasses of substances. The experiments relating to this matterwere carried out at an earlier period than the advancementby Dumas of the theory of types, which was the next stepin the development of the substitution theory, and on thisaccount I think it should be treated of first. Even if bothmatters seemed at that time to be extremely diverse, still theexercise of some influence is not only conceivable, but it canactually be observed ; and, for this reason, the chronologicalorder must not be altogether lost sight of.

It is seldom that, in order to produce any great result, sofew investigations have been necessary as was the case in thefounding of the theory of polybasic acids. The experimentand the idea whereby this wide field of investigation wasopened up to experimental science, while new and securefooting was afforded to theory, are alike elegant and precise.

152 HISTORY OF CHEMISTRY [LECT. IX.

Only a few persons took a part in this important crisis inchemistry, but they were valiant champions who conqueredfor us this domain. And, once set foot upon, the ground wassecure, despite the contradiction of an authority whose words,although they had been observed in other cases in a scrupu-lously conscientious manner, were now spoken to the winds.

It is to Graham thai the first impulse towards an altera-tion in the views regarding acids was due. Graham's investi-gation of phosphoric acid, and the way in which he stateshis results—the former free from preconceived notions andhypotheses, and the latter clear and definite—show us thatwe have to do with an acute and clear-minded thinker.When regard is paid to the ideas that were necessarily intro-duced as the direct result of the investigation, and when thoseintellectual advances are considered which were occasioned—not exclusively, it is true, but still to a great extent—byGraham's labours, it must be conceded that so much hasseldom been accomplished by a single investigation.

As a consequence of Clark's investigation of phosphoricacid,8 the view had been arrived at that this acid existed intwo isomeric conditions, which, in their salts in particular,were stated to present great differences.4 Common sodiumphosphate gave a yellow precipitate with neutral silver salts,and the liquid possessed an acid reaction; the pyrophosphate,on the other hand, precipitated white silver pyrophosphateand the neutrality remained. It was known, of course, thatthe one sodium salt crystallised with more water than theother, but this was regarded as water of crystallisation, andno importance was attached to i t ; so that the two acidswere looked upon as isomeric modifications.5 Grahamrectified this mistake. He succeeded in throwing lightupon this hitherto obscure subject by proving that thewater which was contained in the hydrated acids shouldnot be disregarded as inessential to their constitution, but

3 Edinburgh Journal of Science. 7, 298 ; Schweigger's Journal. 57, 421.4 See also Stromeyer, Schweigger's Journal. 58, 123. 5 Compare,Berzelius, Lehrbuch, Third Edition, 2? 60,


that, on the contrary, it assumed the function of the base.6

Graham showed, in 1833, how ordinary phosphoric acid andall its salts may be regarded as compounds of one atom ofphosphoric acid, P2O5, and three atoms of base which can becompletely or partially replaced by water. Thus, accordingto him, ordinary (neutral) sodium phosphate consists of oneatom of phosphoric acid combined with two atoms of sodaand one of water : on mixing its solution with silver nitrate,the silver salt with three atoms of silver is precipitated,whilst sodium nitrate and nitric acid remain in the solutiontogether. Exceptions to Richter's law (already observed insimilar cases by Berthollet), where the mixture becomes acidwhen the solutions of two neutral salts are mixed,7 werethus explained. In this case two atoms of soda and one ofwater were exchanged for three atoms of silver oxide.

Another very important result of Graham's investigationis met with in the analysis of pyrophosphoric acid and itscompounds. Graham shows that on heating the sodiumsalt mentioned above to over 3500, the water it contains isdriven off, and that sodium pyrophosphate, identical withthe salt already known, is produced in this way. This saltis not, however, isomeric with the original one, as had beensupposed, but differs from it by containing one atom ofwater less ; and this is of essential importance in regard tothe nature of the acid. The white precipitate, too, whichis produced by silver salts, only contains two atoms of silveroxide ; and it is thus a quite general property of pyrophos-phoric acid to saturate only two atoms of base (or of water),which distinguishes it very sharply from ordinary phos-phoric acid. In the latter the ratio of the oxygen in thebase to that in the acid is as 3 to 5 ; in the other acid itis as 2 to 5.

Graham finds further that on heating the acid sodiumphosphate, which consists, according to him, of one atom ofphosphoric acid, one atom of soda, and two atoms of water

6 Phil. Trans. 1833, 253; A.C.R. No. 10; Annalepu 12, 1. 7 £qr-thollet, Stat, Cl im, 1, 117; E. 1,8$,

154 HISTORY OF CHEMISTRY [LKCT. IX.

(as base), both water atoms are driven off, and a hithertounknown salt, sodium metaphosphate, is produced. Theacid contained in this salt is characterised by being saturatedby one atom of base, whilst, in the free state, it containsone atom of water. The silver compound was again differentfrom either of the others. In this case the ratio of thequantities of oxygen in base and acid was as I to 5.

Finally, it was shown in the investigation that meta-and pyro-phosphoric acids, as well as the majority of theirsalts, pass ,into ordinary phosphoric acid or a salt derivedfrom it, when boiled with water, or still better, when fusedwith sodium carbonate.

Two important theoretical conclusions can be directlydeduced from Graham's investigation.

(1.) In acids there is a certain number of atoms of water,and salts are formed by the replacement of these.

(2.) The atoms of the acids are not always equal innumber to the atoms of the bases, and in some, even theratio is variable. Thus Graham showed how, from the samephosphoric anhydride, to prepare three hydrates which wereable to take up quite different quantities of base.

Liebig, in 1838, stated these conclusions with great clear-ness and precision.8 A man of his genius could not, how-ever, rest satisfied with publishing thoughts that were merelyconclusions drawn from the experiments of others. We areindebted to Liebig for an excellent investigation of a seriesof organic acids, from which it appeared that phosphoric aciddoes not stand alone with respect to its behaviour towardsbases, but that in the cases of certain other acids, one atomlikewise possesses the property of saturating several atomsof base. Founding, as he did, upon a broader basis, hewas then able to introduce the idea of the polybasic acids.

Liebig's experimental investigation embraces fulminic,cyanic, meconic, comenic, tartaric, malic, citric, and other acids.He finds relations amongst the salts of each of these acids,

8 Annalen. 26, H3 ; compare Comptes Rendus. 5, 863,


which are similar to those in the case of phosphoric acid. Buthe endeavours, especially, to range the three cyanogen acids,i.e.) cyanic, fulminic,9 and cyanuric, side by side with the threephosphoric acids. In the former, as well as in the latter, thereis present, according to him, a group of atoms which has thepower of saturating sometimes one, sometimes two, sometimesthree atoms of base. But whilst the atomic weight is notaltered in the case of the phosphoric acids, it increases in thatof the cyanogen acids in the same ratio as the saturatingcapacity, so that the resulting salts are polymeric with oneanother. In the latter case, the quotient obtained by divid-ing the quantity of oxygen in the acid by that in the base,remains unchanged, whereas this, according to Graham, isnot the case with the varieties of phosphoric acid.

Liebig writes :

3MO.P2O5 Phosphate. 3MO Cy6O3 Cyanurate.2MO.P2O5 Pyrophosphate. 2MO Cy4O2 Fulminate.MO.P2O5 Metaphosphate. MO Cy2O Cyanate.

Of distinctly greater importance are the considerationswhich lead Liebig to propose a separation, from the otheracids, of those which behave in a way analogous to phosphoricacid. The course of his argument in this matter is approxi-mately as follows : The relations are not so complicated inthe cases of all the acids which share with phosphoric acid thecharacteristic property of neutralising several atoms of base byone atom of acid, as they are in the case of phosphoric aciditself; and hence it is not so easy to establish in all cases thatthey belong to this category. In the case of phosphoric acid,no matter what number may be chosen as its atomic weight,it can never be shown that one atom of acid saturates oneatom of base in all three modifications.10 What, now, arethe characteristics that enable us to recognise that we haveto do with a substance belonging to this group ?

9 Liebig assigns to fulminic acid the formula 2H20.Cy402 [H = r,C=I2, O = i6]. 10 Here, as in the foregoing, the word acid is to beunderstood as referring to the anhydride.


Liebig has recourse to experiment in order to decidethis highly important question. He compares the behaviourof phosphoric acid with that of sulphuric acid, a compoundconcerning which he has no reason for reckoning it in thisclass. In doing so he says :—u

"If to acid sulphate of potash, we add another basewhich is not isomeric with potash and which forms withsulphuric acid a salt free from water of halhydration,12 sodafor example, the acid salt separates into two neutral ones,Glauber's salt and sulphate of potash, which crystallise apartfrom each other.

" If, on the other hand, a certain quantity of potash isadded to acid phosphate of soda, phosphate of soda andpotash is formed, wholly analogous in its composition tothe acid salt. It contains three atoms of base ; two of theseare soda and potash ; one of the two atoms of waterpreviously contained in it, is replaced by potash, the secondatom remains in the composition of the new salt.

" This behaviour distinguishes phosphoric acid andarsenic acid from the great majority of all other acids :their power of forming salts of the same class with differentbases, differing from those which are called double salts,depends essentially upon their property of combining withseveral atoms of base. I regard this character as decisiverespecting the constitution of these) and of all acids whichform compounds similar to those of phosphoric acid.11

A criterion is thus found for separating phosphoric acidand its analogues from the other acids, and Liebig employs itin order to establish the fact that all the substances examinedby him belong to this class. The grounds upon which he alsodecides to include tartaric acid in this group are very interest-ing and important. This acid was at that time written C4H4O5,so that its atom saturated only one atom of base. The exist-

11 Annalen. 26, 144-145. 12 Liebig regards as water of halhydration,that water in salts which can be separated and replaced by equivalents ofneutral salts,


ence of Rochelle salt and of tartrate of potash and ammonia,which can be obtained from the acid potassium compoundby neutralising with the corresponding bases, proves toLiebig that tartaric acid also possesses the property ofneutralising two atoms of base; and this leads him todouble its atomic weight, i.e., to write it C8H8O10. Thetalented author of this famous paper thus understood verywell that the considerations here advanced furnish a newaid to the determination of the atomic weight.

Liebig justifies the separation of the acids into differentgroups, in the following words:—13

"The acids might be divided into monobasic, dibasic, andtribasic. By a dibasic acid there would be understood onewhose atoms unite with two atoms of base in such a way thatthese two atoms of base replace two atoms of water in theacid. The conception of a basic salt thereby remains un-changed. . . . Accordingly, when two and more than twoatoms of base combine with one atom of an acid, and onlyone atom of water is separated during the operation (fewertherefore than the number of equivalents of the fixed base),then a really basic salt is produced/' . . .14

This great step was thus taken. The way- had beenprepared for it by the labours of Graham ; the change wascarried through and established by Liebig's investigations.If we desire to be strictly just (and we set some value uponthis), we must not suppress the fact that Liebig publishedthe first paper on this subject along with Dumas in 1837.15

This was the only fruit of the proposed association of thesetwo chemists.

In the same paper in which Liebig develops, in detail, thetheory of the polybasic acids (in accordance with which theacids fall into several classes), he endeavours, by means of a

13 Annalen. 26, 169. u In the property of forming pyro-acidsLiebig also finds a ground for classing acids as polybasic [he. cit. 169).15 Cornptes Rendus. 5, 863. According to a letter which Liebig addressedto the FrencrTAcademy in 1838 (Comptes Rendus. 6, 823 ; Annalen. 44,57), it appears that the share of Dumas in this investigation was veryunimportant.


" hypothesis/' to get rid of the division, which had subsistedup to this time, into hydrogen acids and oxygen acids. Thishypothesis is a reversion to the ideas of Davy and ofDulong.16

A similar attempt had previously been made by Clark,although much less elaborated. According to Griffin,17

Clark stated views of this kind in his lectures as early as1826. As he himself wrote to Mitscherlich in 1836,18 hefinds grounds for his opinion in the isomorphism of sulphateof soda and permanganate of baryta. At that time, theformulae assigned to these compounds were :

NaOSO3 and BaOMn2O7,according to which they contained an unequal number ofatoms. Clark proposes to double the atomic weight ofsodium (that is, to assume it to be four times as great as atpresent) and to assign to it the number that Berzeliusadopted in 1819.19 Since he further regards the acids ashydrogen compounds, from which salts are produced by thereplacement of the hydrogen by metals, sulphuric acid, withhim, is H2SO4 and permanganic acid HMnO4 ; and thereforesulphate of soda is NaS2O8, and permanganate of baryta isBaMn2O8; and by this means he attains similarity in thenumber of atoms in both compounds.

Quite different grounds, which are of much superior value,and are more numerous, lead Liebig to revive again the Davy-Dulong hypothesis. Graham had shown that pyro- and meta-phosphoric acids can exist in aqueous solution without at oncepassing into ordinary (tribasic) phosphoric acid. Liebig con-sequently inquires whether these three acids really differ fromone another by an atom of water in each case, and whether itis the gain or loss of water which brings about the changes inthe basicity of phosphoric acid. He does not believe thatconvincing grounds can be found for the adoption of thishypothesis; so that the contrary supposition, in accordance

16 Compare p. 83. w Griffin, The Radical Theory in Chemistry,London-(i858), 4 et seq. 18 Annalen. 27, 160. 19 Compare p. 94.


with which the salts are formed by the replacement bymetals of the hydrogen of the (hydrated) acid, is not to berejected unconditionally. The accuracy of this idea beingpremised, the acids would not contain any ready-formedwater, and they could not be any longer regarded as con-sisting of anhydride and water, any more than the saltswere compoundscomposed of acid (anhydride) and base.

Liebig finds an important support for the latter hypothesis(in accordance with which the metals, as such, should beassumed in salts) in the behaviour of tartar emetic at ahigh temperature. According to the analysis, the formulaC8H8KSb20M represents the compound dried at ioo°. Itwas assumed to contain an atom of anhydrous tartaric acid, anatom of potash, and an atom of oxide of antimony, so that itsformula was written C8H8O10 + KO + Sb2O3 (assuming that theformula of tartaric acid was doubled). According to Liebigthis substance, on being heated to 3000, loses two more atomsof water, a property which it does not share with any other saltof the same acid. The assumption of the presence of water inthe acid, hitherto regarded as anhydrous, appears objection-able to Liebig on account of the consequences which wouldfollow from it, and so he believes that nothing remains butto ascribe the formation of water to the reduction of theoxide of antimony* The actual existence of a base, in themetallic condition, combined with an oxygen acid (even ifonly for certain compounds) would no longer require to beregarded as a mere supposition.20

On another occasion, when discussing these relations,Liebig writes the formula for tartaric acid, C8H4O12.H8,and that of tartar emetic which has been heated to 3000,

f KC8H4012j gk , and T must not omit to draw attention to the

fact that the displacement of three atoms of hydrogen byone atom of antimony, is here assumed.21

20 Annalen. 26, 159- 21 The paper in question is by Dumas andLiebig, Coroptes Rendus. 5, 863*


Liebig admits that it is difficult to understand how potashis reduced by means of sulphuric acid, an assumption whichmust be made in case sulphate of potash is to be regarded asa potassium compound ; but he instances a case in which ahypothesis of the kind is indispensable to the explanation ofthe facts. The decomposition of thiocyanate of silver bymeans of sulphuretted hydrogen, with the formation of sulphideof silver and free acid, would be contrary to all views regardingaffinity if the salt corresponded to the formula AgS + Cy2S ;whereas the reaction becomes a normal one, assuming theformula to be Ag.Cy2S2. Unsatisfactory as it is at first sight,the hypothesis of the reduction of the oxides by means of acids,further gives an explanation of the behaviour of many acidswhich exhibit a higher capacity for saturation towards silveroxide than they do towards soda, although the latter isendowed with more strongly basic properties.

Finally, Liebig points out that by the assumption of thehypothesis of Dulong, the grouping of the hydrogen acids andof the oxygen acids into one class, which is almost enforcedby the similarities in their reactions, is attained. Thus limealways gives up the same quantity of water no matter whetherit is neutralised with sulphuric acid or with hydrochloric acid.The mode of explanation then adopted, in accordance withwhich the water, in the one case, was present in the acidready-formed, and in the other case was produced during theaction, according to Liebig takes no account of the analogyof the two cases. He tries to pull down the barrier, and hiswords are sufficiently significant to deserve a place here.22

" We employ, therefore, two modes of explanation for oneand the same phenomenon ; we are forced to ascribe to waterthe most varied properties ; we have basic water, water of hal-hydration, water of crystallisation ; we observe its entrance intocompounds in which it ceases to assume any one of these threeforms ; and all this is for no other reason than that we havedrawn a distinction between haloid salts and oxygen salts,

23 Annalen. 26, 179.

LECT. ix.] HISTORY OF CHEMISTRY 1G1

which we do not observe in the compounds themselves ; inall their relations they possess properties of the same kind."

Liebig then returns to Davy's ideas, and in doing so hestates his views as follows :—23

lt Acids are, accordingly, certain compounds of hydrogen,in which the hydrogen can be replaced by metals.

" Neutral salts are those compounds of the same class, inwhich the hydrogen is replaced by the equivalent of a metal.Those substances which we at present call anhydrous acids,acquire the property of forming salts with metallic oxides, forthe most part, only on the addition of water; or they are com-pounds which decompose the oxides at a high temperature.

11 On bringing together an acid and a metallic oxide, thehydrogen is separated, in the majority of cases, in the form ofwater. It is a matter of complete indifference for the constitu-tion of the new compound, in what manner the formation ofthis water is conceived: in many cases it is formed by thereduction of the oxide ; in others it may be produced at theexpense of the elements of the acid—we do not know which.

" We only know that, without water, no salt can be pro-duced at ordinary temperatures, and that the constitution ofthe salts is analogous to that of the hydrogen compoundswhich we call acids. The principle of the theory of Davy,which must be kept especially in sight in criticising the theory,is that he makes the capacity of saturation of an acid depend-ent upon the hydrogen or upon a part of the hydrogen whichit contains; so that, if the other elements of the acid, collec-tively, are called the radical, the composition of the radicaldoes not possess the most remote influence upon this capacity."

These statements are recognised, on the whole, as correcteven at present. Together with what I have stated above con-cerning the polybasic acids, they constitute the basis of ourviews regarding acids. No doubt the characters which dis-tinguish polybasic from monobasic acids were considerablyextended by Gerhardt and Laurent, so that the conceptions

28 Annalen. 26, t8r.


and definitions assumed a much more fixed and decided form.The distinction between the basicity and the atomicity of anacid was learned at a still later date, and rules were formu-lated whereby these also can be ascertained numerically.But these developments fall into a period which is too farremoved from the one now under consideration for us to beable to discuss them here at present.

It will easily be understood that Berzelius could not shareLiebig's opinions. To recognise them would have been toabandon dualism, the basis of his own theories. It is true thatthe new way of looking at substances was not purely unitary;the acids were supposed to consist of radical and hydrogen,and the salts, of radical and metal, so that there still existeda division into two parts ; but this was in a sense in whichBerzelius could not admit it. The mode of salt formation,as Liebig conceived it, must especially have been in opposi-tion to his views. There were no longer two compounds ofthe first order—an electro-positive and an electro-negativeconstituent—which united ; the formation of salts consisted,instead, in the replacement of hydrogen. How could this bereconciled with the electro-chemical theory, in accordancewith which compounds are only formed by the union ofatoms one with another ? Hence we find Berzelius also pro-testing24 against the theory of hydrogen acids, if I may thusdesignate Dulong's ideas. His reasons were not sufficient,however, to dissuade the greater number of chemists fromadopting this theory, and on this account I shall not entermore minutely into the matter, but shall again turn to thefacts which were to lead to a unitary system. I refer to thephenomena of substitution, or to the replaceability of hydro-gen by electro-negative elements.

Besides Dumas,and Laurent, Regnault and Malaguti wereespecially engaged in the investigation of this subject. Theresults obtained by these chemists—the theories o£ Laurent,as well as Liebig's views concerning acids—had not been

24 Berzelius, Jahresbericht 1839, 2^4 ; Annalen. 31, r.


without influence upon Dumas. An extremely interestingdiscovery, which he makes in 1H39, obliges him to expoundhis views at this time with respect to substitution ; and alsoto retract, partially at least, the statements previouslyadvanced and to introduce in their place new ones of fargreater significance. In this way, from the empirical rulesof substitution, the theory of types arises.

By the action of chlorine, in sunlight, upon acetic acid,Dumashad obtained a crystal line substance whose compositioncould be expressed by the formula C.jClnHl/)4,"'r' and whichcould, therefore, be regarded as acetic acid, C4HK()4, in whichsix atoms or volumes of hydrogen were replaced by six atomsof chlorine.**1 The interesting and important part of thisreaction lay in the properties of the new compound, whichDumas called chloracetic acid. Tins acid had the samesaturating capacity as acetic acid, so that Dumas was able toassert that by the entrance of chlorine in place of the hydro-gen, the chief character of the compound was not altered ;or, as he expresses himself, u that in organic chemistry thereare certain types, which persist even when an equal volumeof chlorine, bromine, or iodine is introduced into them inplace of the hydrogen which they contain.

It will thus be seen how Dumas, in consequence of his dis-covery of ehloracetie acid, is led to the same point of viewwhich had already been taken up by Laurent, but which theformer had at first put aside as extending beyond the limitsof fact**'7 It is, however, an injustice to Dumas to representhis theory of types as merely an application or perhaps anexpansion of Laurent's ideas. Laurent was a clever specula-tive thinker; but he did not hesitate to state a hypothesis forwhich a complete scientific proof could not, at the time, beadduced, and this, 1 think, was the case with respect to hisviews concerning substitution. That this, at least, was theimpression made upon his contemporaries in to be seen from

*M In the French papers Dumas retains the atomic weight 0 6,m Annalen. 32* tot* m Comptei Renclus. 6, 6S< At that period Duma?called Laureot*s theory an extension of hit i«lea§ which did ttot concern hint.


Liebig's criticism of Laurent's theories.28 The facts were stillwanting, which showed, in a definite and decisive manner, theanalogy between the original substance and the final product.Our science cannot advance by means of ideas alone : it isonly when an opinion is called forth, and to a certain extentnecessitated, by experiment that a further development isinvolved in it. It was not Dumas7 position and name alonethat now procured favour for the theory which, a year before,had scarcely been taken notice of. The chemists of thatperiod had no such respect for authority. The discovery ofchloracetic acid lies between the promulgation of the nucleustheory and that of the theory of types ; and even if u a theorycan be made up of words " still, in chemistry, greater valueis, fortunately, placed upon a decisive experiment than upondaring speculations.

An analogy between acetic and chloracetic acids could notremain unobserved ; and especially after Berzelius, who hadhis reasons for not admitting any similarity between them,had given prominence to their differences, and, with acertain amount of irony, inquired for their kindred relation-ships.29 Dumas shows the reactions which they undergo bythe influence of potash, and points out their similarity.30

We have—

Besides carbonate of potash, there is formed, in the onecase, marsh gas, and in the other, chloroform ; i.e., two sub-stances which again exhibit the same difference in composi-tion, the one from the other, as the two acetic acids do ; andof which the latter, as Dumas also particularly shows,82 canbe obtained from the other by the action of chlorine.

By the discovery of trichioracetic acid a basis is furnished28 Annalen. 25, r. ^ Berzelius, Jahresbericht 1840, 367 etc.

30 Annalen. 33, 179. 81 This latter reaction appears to have been dis-covered previously by Persoz (Introduction a 1'Etude de la Chimie mo\6-cXilaire), as was pointed out by Pelouze and Millon (Annalen. 33, 182).82 Annalen. 33, 187 and 275.

LECT. ix.] HISTORY Of CHEMISTRY 165

upon which Dumas erects his theory of types.33 Thus, accord-ing to him, all substances which contain the same numberof equivalents combined in the same way, and of which thechief characters are similar, belong to the same chemicaltype. These are, for the most part, compounds which can beobtained from one another by very simple reactions, suchas acetic acid and chloracetic acid; chloroform, bromoform,and iodoform ; ethylene and the products arising from it bysubstitution by means of chlorine.

Dumas thinks he has found, in the conception of thechemical type, the basis of a new classification, which includesthe recently observed facts ; but he employs, at the same time,the molecular type introduced by Regnault,34 which he callsalso the mechanical type. To this type the following com-pounds belong:—

Marsh gas - - - - - C2H2Hr)

Methyl ether C,0 H6

Formic acid C2H2O3

Chloroform - - - - - C2H2C1O

Chloride of methyl - C2C12HO

Chloride of carbon - C2C12C1G

These substances, which may be regarded as arising bysubstitution from one another, and which may possess verydifferent properties, are classed in one natural family. Thepoint of view which led to the establishment of Regnault'stypes, is a much more comprehensive one than that fromwhich Dumas was induced to advance his chemical types, thesubstances embraced under the latter heading constitutingmerely a subdivision of those which must be classed under thesame mechanical type. Dumas also sees this clearly, for hesays:35 " On every occasion when a substance undergoeschange without quitting its molecular type, it is changed inaccordance with the law of substitution. On every occasionwhen a substance passes, on undergoing modification, into

u Annalen. 33, 259; 35, 129 and 281; 44, 66. u Ibid. 34, 45.35 Ibid. 33, 279.


another molecular type, the law of substitution is no longeradhered to during the reaction." And further : " Alcohol,acetic acid, and chloracetic acid belong to the same naturalfamily ; acetic acid and chloracetic acid to the same species."It may therefore be said that, so far as the idea goes, mechani-cal type and nucleus amount to the same thing; both com-prise the substances which arise from one another, or whichcan, at least, be looked upon as arising from one another, byequivalent substitution.

Dumas, as may have been observed, has now arrived atthe opinion that his law of substitution is not applicable toall reactions, and that an equivalent of another element is notalways taken up even for the hydrogen removed. He is allthe more obliged to admit this, since he now no longer assumesthe existence of ready formed water in organic substances(alcohol, for example36) whereby his second rule ceases tohold.37 He is obliged, in consequence, to recognise, and hedoes it explicitly, that the phenomenon of substitution is nota general one; he even finds in this one of its most essentialfeatures.38

While he thus limits the applicability of the law of substi-tution, the validity of the law in another direction is enhanced.According to Dumas, it is not only the hydrogen of an organicsubstance that can be replaced, but all the elements which itcontains. True substitution of the oxygen, of the nitrogen,and even of the carbon may be accomplished ;89 and theseelements may be replaced, not only by others, but also by com-pound groups such as cyanogen, carbonic oxide, sulphurousacid, nitric oxide, nitrous acid, amide, etc. The assumptionof the replaceability of carbon, which at that time met withthe most vigorous contradiction as a silly hypothesis, and was,in Germany, made the subject even of ridicule,40 was a conse-quence of the experiments of Walter41 who had obtainedsulpho-camphoric acid by the treatment of camphoric acid

36 Annalen. 33, 261. 37 Compare p. 141. :w Annalen. 33, 264.89 Ibid. 33, 269. « I b i d 33i 3o8# « Ibid# 3(5) sg


with sulphuric anhydride, carbonic oxide being evolved.Dumas regarded this acid as camphoric acid in which oneatom of carbon was replaced by the group SO2.

If the conception of the molecular type is regarded in itswidest sense, it may be said that this idea of Dumas as to thereplacement of carbon, was wholly justified by subsequentexperimen ts. Wohler has pointed out a substitution of carbonby silicon ;42 and by means of reactions quite analogous tothose employed for converting a hydrocarbon into the corre-sponding alcohol, Friedel and Crafts have transformed ethylsilicide into silico-nonyl alcohol which, as the name indicates,they look upon as nonyl alcohol in which one atom of carbonis replaced by one atom of silicon.43 More recently, siliconcompounds have been discovered which are not only to beregarded as analogous to certain carbon compounds, but whichbehave in a manner similar to the latter. This is particularlythe case with triethyl silicol.44

I may also remark here that the view of Dumas concerningthe replacement of carbon was in contradiction to the nucleustheory of Laurent, and rendered difficult the classification oforganic substances according to the number of their carbonatoms. The ideas of the two chemists approach each othermore closely as regards the conception of the radical than asregards its composition. Dumas now expressly states also thatthe radical is not an unalterable group, but that in it, just as inall compounds, the atoms are replaceable by others. Gerhard thad, however, advanced similar views two years earlier, andwe shall, therefore, have to deal fully with this point after-wards in another lecture.

The first, and probably the most important consequence ofthe theory of types was that it demanded a unitary mode ofregarding substances. The compound was no longer to beregarded as consisting of two parts. It constituted, rather, a

42 Annalen. 127, 268. 43 Comptes Rendus. 61, 792 ; also Annalen.138, 19; compare further Friedel and Ladenburg, Annalen. 143, 118;145, 174 and 179; 147, 355; and Comptes Rendus. 66, 816. 44 Laden-burg, Annalen. 164, 300.

168 HISTORY OF CHEMISTRY [LECT. I.Y.

uniform whole, which might undergo change from the fact thatone atom could take the place of another. Dumas comparesit with a planetary system ; the atoms here represent the indi-vidual planets, and they are held together by affinity instead ofby gravitation. The atoms in this system can be replaced byothers: so long as the number of the equivalents and the relativepositions of the atoms are preserved, the system is unchanged.

According to the theory of types, the properties of a com-pound were affected far more by the arrangement than by thenature of the atoms ; and this doctrine which Dumas nowdefends as confirmed by experiment, leads him to an attackupon the electro-chemical theory. This is how he expresseshimself:—45

" One of the most immediate consequences of the electro-chemical theory is the necessity of considering all chemicalcompounds as binary substances. It is necessary to find out,in every on e of them, the positive an d the negative constituents,or the groups of particles to which these two distinctive char-acters are, ascribed. No view was ever more fitted to retardthe progress of organic chemistry." And in another place :46

" In general, when the substitution theory and the theory oftypes assume similar molecules, in which some of the elementscan be replaced by means of others without the edifice be-coming modified either in form or outward behaviour, theelectro-chemical theory splits these same molecules, simplyand solely, one may say, in order to find in them two oppositegroups, which it then supposes to be combined with eachother in virtue of their mutual electrical activity."

Dumas does not deny fhe influence of electrical forcesupon chemical reactions. On the contrary, chemical andelectrical forces might, according to him, even be identical.What he attacks is the electro-chemical theory of Berzelius,in accordance with which hydrogen is supposed to be alwayspositive, and chlorine always negative. He believes that inthe formationor decomposition of compounds he can recognise

46 Annalen. 33, 291. « ibid, 33 294.


the action of electrical forces; but what he declares to beerroneous, and irreconcilable with the phenomena of sub-stitution, is the assumption that the electrical state of theatoms is unchanging.

The fatal moment had now arrived ; it was a questionof defending dualism, and also the electro-chemical theory,which was in the fullest agreement with it and hadprevailed, almost unattacked, for nearly twenty years,against the views that had been stated in opposition to it.Ways and means had to be devised whereby the newly-dis-covered facts with respect to substitution could be broughtinto harmony with the electro-chemical ideas.

Before the storm actually broke, Berzelius had perceivedthe threatening clouds gathering about him and had takenhis precautions. As soon as Laurent had assumed, in his firstpapers, that the hydrogen of the nucleus (or fundamentalradical) could be replaced by chlorine, Berzelius, quite cor-rectly perceiving the danger to his theories which such viewsmight possess, energetically repudiated the statements ofLaurent.47 The entrance of electro-negative elements intoradicals is put aside as an untenable hypothesis ; and even theoxygen radicals, which he had greeted with so much pleasurea few years previously, are discarded. This assumption is,according to Berzelius, " of the same kind as that whichwould regard sulphurous acid as the radical of sulphuricacid, and manganese peroxide as the radical of manganicacid. An oxide cannot be a radical. The conception ofthe word radical is such that it represents the substancewhich, in an oxide, is combined with oxygen."

Berzelius now only recognises radicals which containcarbon and nitrogen, carbon and hydrogen, or carbon,nitrogen, and hydrogen.

Sulphur a cannot enter into the composition of a radicalany more than oxygen can." uThe ternary radicals" musttherefore be regarded "either as compounds of a binary

47 BerzeHus, Jahresbericht 1839,

170 HISTORY OF CHEMISTRY [LFXT. IX.

substance with a simple one, or as compounds of two binary-substances/' 48

The radical C14H10 is now constituted the basis of thecompounds discovered by Liebig and Wohler, and this isjustified by the analogy which benzoic acid, benzoyl, andthe hydrocarbon C14H10 exhibit with manganic acid, man-ganese peroxide, and manganese. Thus :

C14H10O3 Benzoic acid - MnO3 Manganic acid.C14H10O2 Benzoyl - - MnO2 Manganese peroxide.C14H10 - - - - Mn Manganese.49

Berzelius regards benzoyl chloride as similar to chromylchloride, adopting for the latter the formula of H. Rose.60

He writes:

2CrO3 + CrCl6 - - - Chromyl chloride.2C14H10O3 + C14H1OC16 - Benzoyl chloride. •

Quite analogous with this is the formula of phosgene, whichsubstance Dumas regarded as carbonic acid in which oneatom of oxygen is replaced by two atoms of chlorine.51

Berzelius writes CO2 + CC14, Phosgene.At this time Berzelius is putting dualism more pro-

minently forward than ever, and in his. view these formulaeare entirely justified. " Since, in accordance with our presentviews, the forces which bring about chemical combinationsdo not act between more than two substances, of oppositeelectro-chemical tendencies, all compound substances mustpermit of being split into two constituents, of which theone is electro-positive and the other electro-negative."52

In consequence of these views, all substances which, besidescarbon and hydrogen, also contain oxygen, chlorine, bromine,or sulphur, break up into several parts which often appear tobe chosen quite arbitrarily ; further the atomic weight is fre-quently doubled or trebled, so that the subdivision into binary

48 Annalen. 31, 13. ** Compare also Berzelius, Lehrbuch. ThirdEdition, 6, 205. 50 Fogg. Ann. 27, 573. 51 Dumas, Traite", 1, 400.52 Annalen. 31, 12.


radicals may be carried out. Highly complicated formulaeare thus obtained, of which I can only instance a few :

Malaguti's chlorinated ether:O C H C

Malaguti's chlorosulphuretted ether :(C4HCO3 + 2C4HcClfl) + (C4H0O8 + 2 C4HCS3) etc «

His conception of chloracetic acid is very important aswe shall find further on ; he regards it as a compound ofoxalic acid with chloride of carbon :

C2Cl8-f-C2O8

whilst acetic acid remains as the trioxide of the radicalacetyl, C4H0 or C4H8. Even in 1840 he still disputes thesimilarity in the constitution of the two compounds, anddoes not permit himself to be shaken in this by theiranalogous behaviour with potassium hydroxide.55

This view was not long tenable, however, in face of theconstantly increasing number of substitution products, verymany of which exhibited unmistakable analogies with theoriginal substances. When Melsens succeeded, in 1842, inreconverting chloracetic acid into acetic acid, by treatmentwith potassium amalgam/'0 and thus proved that chlorinecan be replaced by hydrogen again so that the original sub-stance is reproduced, even Berzelius was compelled to makean admission. He saysf'7—"If we recall to memory thedecomposition of acetic acid by means of chlorine withformation of ChlorkoJilenoxalsiiure (chloracetic acid), anotherview as to the composition of acetic acid presents itself aspossible. In accordance with this view it would be acoupled58 oxalic acid whose copula is C2H8, as the copula ofthe Chlorkohlenoxalsihirc is C2C18; consequently the actionof chlorine upon acetic acid would consist in the conversionof the copula C2H8 into C2C18."

83 Berzelius, Jahresbericht 1840, 375; here also Berzelius still writesU instead of Ha.

M Annalen. 31, 113 ; 32, 72. m Ibid. 36, 233.m Ann. Chim. [3] 10, 233. m Lehrbuch. Fifth Edition, 1, 460 and 709.m Berzelius here employs a word that had been Introduced into the scienceby Gerhardt.

in HISTORY OF CHEMISTRY [LECT. IX.

Perhaps Berzelius did not notice that he had thus con-ceded the chief point in the theory of substitution which, afew years before, he had vigorously contested. Chlorinecould replace the hydrogen of the "copula," and the con-stitution of the compound was not essentially alteredthereby. Berzelius now wrote :

C2O3+ C2C13 Chloracetic acid.C2O3 + C2H3 Acetic acid.

Was not a fundamental principle of the electro-chemicaltheory violated by this concession ? I think it was. It wasnow necessary to assume either that forces different fromthe electrical forces are present in the copula, or that theelectrical properties of the elements are altered in thecompound ; and both of these assumptions were much atvariance with the former ideas of Berzelius.

The substitution theory had thus come off victorious.Berzelius, it is true, never admitted his defeat, but, as amatter of fact, he had given in. The electro-chemicaltheory was now abandoned. In its last throes it had pro-duced the idea of copulse : had these latter any vitality? Atfirst it did not seem so; they were looked upon as the idleinvention of a wearied intellect. With this I partly agree ;but they still possessed a spark of life, otherwise they wouldnot have been capable of development, and it would nothave been possible even for a man like Kolbe to advancethem to what they afterwards became.

This subject will be dealt with in a subsequent lecture.

L E C T U R E X.

INFLUENCE OF THE SCHOOL OF GMELIN—THEORY OF RESIDUES—COUPLED COMPOUNDS—GERHARDT'S DETERMINATION OF EQUIVA-LENTS—DISTINCTION OF ATOM, MOLECULE, AND EQUIVALENTBY LAURENT—NEW CHARACTERISTICS OF POLYBASIC ACIDS—MOLECULES OF THE ELEMENTS.

THE battle was over, and the victory won. It had been shownthat, starting from the decompositions which substancesundergo under the influence of the galvanic current, weare not in a position to explain the manifold reactions oforganic chemistry, and in particular, the phenomena of sub-stitution. The foundations had been shaken by the factthat positive hydrogen was replaceable by negative chlorine,and the whole edifice—the electro-chemical theory—col-lapsed. Organic chemistry had shown that laws which hadbeen advanced without reference to the facts that it pre-sented, did not harmonise with the teachings of these facts.It was now a question, however, as to whether organicchemistry could also render any positive service—whetherit would be possible, starting from the facts which it hadalready furnished, or which it would furnish in the future,to establish new principles that might serve as the basis ofa chemical system.

The way of looking at substances from the electro-chemicalpoint of view, and dualism also, were maintained in the case ofinorganic compounds. In order to do this, a sharp distinctionbetween the latter and organic compounds became necessary,so that it might be possible to apply a doctrine to inorganiccompounds which had proved inapplicable to organic com-pounds. If this doctrine was to be completely supplanted,however, and if organic chemistry was to enjoy the fruits of itsvictory, it was necessary that thisyounger branch of the science

173

174 HISTORY OF CHEMISTRY [LECT. X.

should offer to the opposing party definite principles uponwhich it might be reconstructed. It was not in a position todo this at first, since, up to this time, more attention had beenpaid to overthrowing the old system than to building up a newone. It is true that endeavours were made on various sidesto embrace all organic compounds in one uniform conception.The radical theory, the nucleus theory, and the theory oftypes had arisen, and each had its supporters ; but the veryfact of their being so many views, proved the insufficiencyof any of them. We find, then, a great deal of confusion ;the adherents of the different systems were in continualstrife, and a becoming demeanour was not always maintained.

Consequently, it is difficult to say which were the prevailingideas at the beginning of the fifth decade of the nineteenthcentury. Even the views as to the principles of each mode of re-garding constitution were widely at variance from one another.The school of Gmelin had greatly augmented its adherents, andto the latter the atomic theory appeared too hypothetical.We cannot be surprised to find that chemists now begin toSean more and more in this direction, since even the expres-sion " atomic weight" is gradually supplanted by the " equiva-lent," and the latter is employed, as it had been by Wollaston,in the sense of combining weight.1 Upon the overthrow ofthe system of Berzelius (that is to say, of the only systemwhich, in any uniform sense, embraced the whole science),and with the origination of the most various hypotheses andtheories, which were not capable of any general applicationand did not seem to have any promise of a long existence,there arose in the minds of many a certain aversion to allspeculation, which was looked upon as premature and hurtfulto the science. Nothing was in keeping with the times exceptthe temperate consideration of observations, and Gmelin wasthe right man to represent a tendency of this kind. Heunited boundless industry with wide knowledge, and he under-stood how to turn both of these qualities to account in his

1 Compare Liebig, Annalen. 31, 36.

LECT. x.] HISTORY OF CHEMISTRY 175

Hand-book. In.adducing facts completeness and conscien-tiousness were his watchwords, and these were adhered to.

Since formulae, for Gmelin's school, merely represented thecomposition of substances by means of a contracted style ofwriting them, these chemists were at liberty to choose theirct equivalents " or u combining weights " at will from the pos-sible multiples. The guiding principle appeared to them tobe simplicity in symbolising, and therefore their numberspossess little real significance as regards development. Ishall merely remark that they adopted the formulae ofBerzelius for most compounds, and, in doing so, they re-garded the double atom as an equivalent. They arrived atthis by halving the atomic weights of oxygen, sulphur,carbon, selenium, etc., with reference to hydrogen, chlorine^bromine, iodine, nitrogen, phosphorus, and the metals.2

It must not be supposed, however, that, at the beginningof the forties, the atomic weights of Berzelius were no longerused. On the contrary, they were still employed by Liebig andhisnumerousand important followers,3 and it was only towardsthe end of the decade (after the appearance of Gerhardt's paper)that the latter also made use of Gmelin's equivalents. I havealready indicated in a previous lecture what the reasons werethat brought about this revolt from the atomic theory.4 Al-though I said there that none of the physical rules whichexpressed relations between the atomic weight and certainproperties of matter, appeared capable of general application,on the other hand, the law which had enabled Dalton toadvance the atomic theory, that is, the law of multiple pro-portions, was still unattacked. The examination of numerousorganic compounds whose investigation was already com-pleted, had 'only tended to confirm it. Nodoubt, ithad becomenecessary to admit that a much larger number of atoms canunite with one another than either Dalton or Berzelius had

2 Compare Gmelin, Handbuch. First Edition, 34. At this time Gmelinalso halves the atomic weight of phosphorus, and so writes phosphoric acidP2O5, as Berzelius did. 3 The sign for the double atom is not employedin Liebig's Annalen., and H,2O is therefore printed instead of HO, as Liebigobserves from want of the necessary types. 4 Compare p. 106.


considered possible, and the law had, on this account, obviouslyto some extent lost its definite character. Even at that timethe question might have been asked, whether the statementcould be called a law, since chemists were not in a position todetermine anything as to the limits of the power of combina-tion of atoms ; and also whether every compound could ulti-mately be referred to unvarying weights of the constituents,if any large multiple might be chosen at will. Ideas of thiskind do not appear, however, to have arisen at that period,5

and so there always remained this one generalisation for thosewho tried to retain the atomic theory and to advance specula-tions as to the constitution of compounds. Amongst these,Dumas had in the recent years played the most important part,by founding his theory of types. In this theory, which wasno doubt, partly borrowed from Laurent, there was much thatwas eminently suited for a classification of organic compounds;but still the use of it was only recognised more generallyafter its fusion with the radical theory, that is, after radicalshad been introduced into the types. This could only takeplace after the conception of the radical had been com-pletely transformed, and it is now my business to show howand by whom this development was brought about.

On studying the writings of the founders of the radicaltheory, one might be tempted to assert that it was they whonot only established the conception of the radical in its firstsignification, but that, at the same time, they had also donethe most important service in respect of the subsequentacceptation of the word. Thus, the following passage fromBerzelius is noteworthy :—6

11 We shall assume that, by means of any circumstancewhatsoever, we could clearly see the relative position of thesimple atoms in the compound atom of the salt [sulphate ofcopper]. It is clear that, whatever this may be, we shouldthen find in it neither oxide of copper nor sulphuric acid, for

5 Compare, however, Berzelius in Liebig's Annalen. 31, 17 j alsoDumas, ibid. 44, 66. 6 Jahresbericht 1835, 348.

LECT. x.] HISTORY OF CHEMISTRY ill

the whole is now a single coherent substance. We can pictureto ourselves . . . the elements in the atom of the salt ascoupled together in various ways ; for example, as one atom ofsulphide of copper combined with four atoms of oxygen, thatis to say, as the oxide of a compound radical • as one atom ofbinoxide of copper and one atom of sulphurous acid ; as oneatom of copper and one atom of a salt-former, SO 4 ; and,finally, as one atom of oxide of copper and one atom of sul-phuric acid. So long as the simple atoms remain together,one of these notions is as good as another. If it is a question,however, of the behaviour when the compound atom is decom-posed by electricity, or by the action of other substances (inthe wet way for instance), then the relation is quite different.The compound atom in that case never undergoes, decom-position in accordance with the first two views, but it doesaccording to the two latter. The copper can be exchanged forother metals according totheviewCu + SO4; but if the copperis taken away without being replaced, as is the case by the actionof electricity, then that part of the atom of the salt whichremains over, breaks up into oxygen and sulphuric acid. If,on the contrary, the salt of copper is decomposed either by avery feeble electrical force or by means of other oxides, intooxide of copper and sulphuric acid, both of these remain after-wards, and from them the salt can be compounded again. Theremust obviously be a reason for these circumstances, and thereason can scarcely be other than this, that when sulphuricacid and oxide of copper unite to form a compound atom of thesalt, the relative positions of the atoms in the united binarysubstances do not materially change, and the latter can thusbe combined or separated as often as is desired. . . . Fromthis, however, it easily follows that in decomposing to formother binary compounds of the elements, the atoms mustundergo a transposition of their relative situations, so thattheir capacity for combining anew is either diminished, or, asis usual, ceases entirely. Nitrate of ammonia, which is decom-posed into nitric acid, ammonia, and water, and is recom-pounded from these, can be decomposed by heat into nitrous

M

i7S HISTORY OF CHEMISTRY [LECT. *.

oxide and water, without our afterwards being able to recom-pound it from these substances. The reason for this factmust be that, in the latter mode of decomposition, the atomsof the elements are transposed into other relative situationswhich are obstructive to their reuniting."

These ideas are as clear, as impartial, and as unprejudicedas could possibly be desired. The same holds also for thefollowing statements of Liebig : 7

u A theory is the explanation of positive facts, which doesnot permit us, from the behaviour of a substance in variousmodes of decomposition, to make deductions backwards as toits constitution, with conclusive certainty, simply because theproducts vary with the conditions .of the decomposition.

" Each view as to the constitution of a substance, is truefor certain cases, but unsatisfactory and insufficient for others."

Even if I also admit that the principles of the newer radicaltheory are expressed in these statements of the two greatteachers,8 still it would appear to me presumptuous that anyone should declare the latter, on this account, to be the authorsof the views which are now to be discussed. By their activityin other directions in the domain of theoretical chemistry, theyhave shown that, with them, the radical is a definite, unchang-ing group, and that they only considered a single view as to theconstitution of compounds to be admissible. I would recallthe numerous discussions regarding the conceptions of alcoholand its derivatives. Would these have.been possible if opinionssuch as those quoted above had been guiding and predominantideas with Berzelius, Liebig, and Dumas ? Prior to the dis-covery of the phenomena of substitution, this certainly wasnot the case. In treating of Berzelius and of Dumas, we havealready discussed the influence which these facts exerted uponthe conception of the radicals. We have still got to consider

7 Annalen. 26, 176-177. 8 Thus Gerhardt, for example, begins theexposition of his theoretical views by directing attention to the variousformulae that are possible for sulphate of baryta ; that is to say, he adducesconsiderations quite similar to those quoted above as advanced by Berzelius.See Gerhardt, Traite' de Chimie. 4, 561*


Liebig's relations to them. His opinion concerning Laurent'snucleus theory must not guide us here. The discovery oftrichloracetic acid had not been without effect upon him also,and he not only admits the replaceability of hydrogen by nega-tive elements, but he also agrees with Dumas in his viewsrespecting these facts. This is seen from the following :9

" The remarkable observation has been made in inorganicchemistry, that the manganese in permanganic acid may bereplaced by chlorine without altering the form of the com-pounds which permanganic acid can produce with the bases.There can scarcely be a greater dissimilarity in chemical pro-perties than that between manganese and chlorine. . . .Chlorine and manganese can replace each other in certaincompounds without alteration in the nature of the compounds.I do not see why a similar behaviour should be impossiblewith other substances—with chlorine and hydrogen, forexample, and this very view of these phenomena, in theform in which it has been advanced by Dumas, appears tome to furnish the key to most of the phenomena of organicchemistry."

Dumas, it is true, goes too far for him. Liebig will notadmit, for example, the replaceability of carbon, and he printsin his journal the well-known letter of S. C. H. Windier10

which makes sport of Dumas in a somewhat harsh manner.However this may be, it might appear from these statementsthat Liebig had materially contributed by his views to thefurther development of the radical theory. This is not mybelief, and I find support for my view in a paper on thetheory of ether, which he published in the year 1839.11

Liebig here endeavours to solve the difficulties of the questionas to the constitution of ether, by the assumption of theradical acetyl; and, in doing so, he proves to us that radicals,with him, still retain their old signification. The whole planof his Handbook also shows the same thing.12

9 Annalen. 31, 119, Note. l0 Ibid. $$, 308. n Ibid. 30, 129 jcompare p. 137. n See Liebig's Handbuch der Chemie, Heidelberg(1843), especially 2, 1, etc


In my opinion, the labours of Laurent and of Gerhardtchiefly contributed to make the radical what it still is.Laurent, by advancing the nucleus theory, emphasised thevariability of the radicals, a matter that was afterwards broughtforward by Dumas also.13 But it was Gerhardt who firstindicated the possibility of the assumption of two radicalsin one compound, and thereby destroyed all idea of theactual existence of separated groups. It is with the historyof this part of the development of organic chemistry thatwe are now concerned.

The influences of his gifted teacher, Liebig, can scarcelyfail to be observed in Gerhardt's earliest publications. We areaware that Liebig disputes the presence of water in the acids.By a very happy extension of the idea, Gerhardt negatives thepre-existence of water in the majority of organic compounds.Its presence in alcohol, especially, appears to him just asunlikely as that of ammonia in the substances containingnitrogen from which ammonia is evolved by means of potash.He knows that there is a class of substances of simple com-position and of extraordinary stability, such as water, carbonicacid, hydrochloric acid, and ammonia, some of which are pro-duced in almost every organic decomposition, without, how-ever, our being able to recompound, from these substances,the substances originally decomposed.14

The formation of one substance out of another was not,with Gerhardt, any ground for the assumption that the firstwas present in the second, ready formed : substances do notrequire to contain any water in order that water may beseparated from them in certain reactions. The reason for thefrequent formation of water and similar substances is to befound in their stability, and in the great affinity which theirconstituents possess for one another. Now this view was ofessential significance, and it led Gerhardt, in 1839, to thetheory of residues and of coupled compounds.16 He says16

that when two substances react upon each other, an element13 Compare pp. 146 and 167, u J. pr. Chem. 15, 37. 1B Ann*

Chim. [2] 72, 184. 16 Cotnptes Rendus. 20, 1031.

LKCT. x.] HISTORY OF CHEMISTRY 181

(hydrogen) separates out from the one, and unites with anelement (oxygen) from the other to produce a stable com-pound (water), whilst the residues join together. Mitscher-lich's nitro-benzene17 is to be regarded, according to Gerhardt,as produced in this way out of a residue of benzene and aresidue of nitric acid. The hydrocarbon gives up hydrogen,and the nitric acid gives up oxygen. Sulphobenzide18 is alsoregarded in the same way ; it contains the residues C24H10

from the benzene, and SO2 from the sulphuric acid.19 TheSO2 in this case is not identical with sulphurous acid, as thelatter occurs, for instance, in sulphite of lead, but it is con-tained in the compound in quite a special form, namely asa substituted group.

Although this latter view is a very peculiar one, still it waseminently suited to supplant the belief in the pre-existence ofradicals. The residues were imaginary substances, which wereall the more completely deprived of any reality by the fact thatthey were assumed to be different from the similarly composedatomic groups which occurred in the free state.

About two years later (in 1841) Mitscherlich20 propoundssimilar ideas, which, however, he extends to a much largerclass of substances. According to him, also, compounds donot contain any ready-formed radicals which play the partof elements in decompositions ; in the appearance of water,he sees the reason for the observed direction taken by decom-positions ; and therefore he does not seek for this reason inthe constitution of the substances employed. The productswhich are obtained by the action of acids upon bases oralcohols (salts and esters), are likewise considered from thispoint of view, and it is shown that they decompose againinto their constituents by taking up water.

The idea of the residues was well adapted to explain thephenomena of substitution ; the latter, according to Gerhardt,obeyed the following rule.21 The element eliminated is replaced

17 P°gg* Ann. 31, 625. 18 Compare Mitscherlich, Pogg. Ann. 31,628. 19 Dumas' atomic weights : C = 6, O = i6, S = 3* etc. 20 Pogg,

, 95. 21 Ann, China. [2] 72, 196.020. *B jjuraas atomic weignis : o = o, LAnn. 53, 95. 21 Ann, Chim. [2] 72, 196.


either by an equivalent of another element, or by the residue ofthe reacting substance. The application of this rule was limited,however, for Gerhardt, besides substitutions, is also acquaintedwith additions, and these of two kinds. Firstly there are thosein which the saturating capacity is altered, and the formationof salts is reckoned amongst these ; and then there are addi-tions in which this is not the case. Gerhardt directs hisattention chiefly to the products of the latter kind of addition,and calls them coupled compounds {corps copulis). To thisclass there belong, in particular, trje substances produced by.the action of sulphuric acid upon organic compounds, such,for example, as benzoyl sulphuric acid and its salts, discoveredby Mitscherlich.22 The acid is produced by the action of sul-phuric acid upon sulphobenzide. According to Gerhardt, thetwo substances become coupled, whereby the saturatingcapacity of sulphuric acid, which was still regarded at thattime as monobasic, remains unchanged. Thus :—

C24H10(SO2) + SO3H2O = C24H10(SO2) . SO? . H2O.Sulphobenzide. Sulphuric acid. Hyposulphoben/idic acid.

Sulphovinic acid (ethyl sulphuric acid) is regarded as acoupled compound of sulphate of ethyl and sulphuric acid,and is written C8H10(SO2)O2. SO3H2O ; whereas sulphobenzoicacid, the basicity of which is supposed to be equal to the sumof the basicities of its constituents, and in the formation ofwhich the saturating capacity has remained unchanged, isreckoned amongst the conjugated acids, a class of substanceswhich were first distinguished by D umas.23 It may, however,also be regarded as produced by the coupling of a substitutedbenzoic acid, C2SH10(SO2)O4, with sulphuric acid.

The view announced above with respect to coupled sub-stances, is very soon abandoned by Gerhardt. He retains thename, but gives it a different signification. But I have inten-tionally stated the older notion, because the word was alsoemployed by-Berzelius and by Kolbe, who again bestowed a

22 Pogg. Ann. 31, 283 and 634,. >2S Dumas and Prria, Annalen. 44,66 ; Ann, Chim. [3] 5, 353,


special signification upon it. It appeared to me of interestto follow the historical course of an expression which has beenemployed in senses so numerous and so varied.

Gerhardt, in 1843, regards all compounds as coupled whichare prepared by the action of acids upon alcohols, hydrocarbons,etc., and in whose formation the substances unite with theelimination of water.24 Coupled compounds were, accordingly,no longer addition products, obtained by the union of two com-pounds, but they were formed by the joining together of tworesidues. They were, therefore, substitution products—a viewwhich Gerhardt, however, does not adopt. With him, theystill constituted a special class and were not compared withthe original substances, principally, no doubt, because theypossessed a different saturating capacity. With respect to thelatter also, Gerhardt has now become of a different opinion,and states that the basicity of the coupled compound is equalto the sum of the basicities of the substances coupled, less one.From this statement, which is advanced as an axiom, thedibasic character of sulphuric acid follows. Coupled withneutral substances, such as alcohols, or hydrocarbons, sul-phuric acid gives rise to monobasic acids, whereas acetic acid,nitric acid, hydrochloric acid, etc., do not possess this pro-perty, and are hence regarded by Gerhardt as also monobasic.

In 1845, Gerhardt endeavours to show the general appli-cability of the law of basicity mentioned above.25 He nowdesignates as coupled, all compounds which are formed by theunion of two substances with the elimination of water anddecompose into their constituents again by taking up water ;and, therefore, he reckons in this class, the neutral ethers, theacid ethers, etc., and formulates the law

where B represents the basicity of the coupled compound, andb and b' the basicities of the substances which take part in itsformation. Gerhardt expressly remarks here that this equation

24 Comptes Rendus. 17, 312. w Comptes rendus des travaux chimic uespar Laurent et Gerhardt, 1845, 161.


holds only for the coupling of a single equivalent,26 and that theequation must beapplied twice in order to as®ertain the correctbasicity of the product obtained by the coupling of two equiva-lents of one substance with one of another. Thus sulphuricacid, for example, which Gerhardt now regards as dibasic, canform with neutral substances both acids and neutral products.The ether of sulphuric acid belongs to the latter class of sub-stances ; it is produced from two equivalents of alcohol andone equivalent of acid. Its basicity, B, is obtained from thefollowing equations, in which Bx represents the basicity ofethyl sulphuric acid :—

Strecker, in 1848, thought he had brought the rules into amore general form when he made a statement to the followingeffect:—2T The basicity of the coupled compound is equal tothe sum of the basicities of the compounds, less one-half ofthe number of hydrogen equivalents removed,28 or the basicityis diminished by one unit for each pair of hydrogen atomsremoved. But in this way of stating the matter, the sameresult as was required by Gerhardt's rule was, necessarily,always obtained. It can only be regarded as a simplification(not as a wider generalisation) in which a single applicationwas sufficient in all cases.

Although it was afterwards shown that even this form ofthe law of basicity does not always lead to accurate conclu-sions,29 and although the exceptional position which was givento certain classes of substances in consequence of the idea ofcoupled compounds, was more recently recognised as incor-rect,80 still, it cannot be denied that the assumption of thecopula played a definite part in the historical development of

26 Gerhardt at that time employed the word equivalent in Gmelin'ssense, so that for us it often means atom, and often molecule. 27 Annalen.68, $1. * Strecker assumes the equivalent of water=9, compared withthat of hydrogen chosen = 1. ^ Compare Becketoff, Bulletin phys.-math.de l'Acad&nie de St Petersbourg, 12 (1854), 369' 80 Compare KekulAnnalen. IOJ, 130,


chemistry. In particular, these views led to new character-istics for the recognition of polybasic acids, and this was ofgreat importance at that time, when so few of these acidswere known.

The underlying idea with respect to coupled substances,that the majority of compounds could be regarded as madeup of the residues of other substances, was of very greatvalue for the progress of the science, simply because it wasopposed to the rigidity and immutability of the radicals.How fertile these ideas were is shown, for example, by thediscovery of the anilides and the anilid-acids.

According to Gerhardt the amides areto be looked upon ascompounds of the residues of ammonia and of acids ; thus hesupposes oxamide to be formed according to the equation :3 1

that is to say, by the replacement of two oxygen atoms bytwice the imid- residue NH. Regarding a similar replace-ment as also possible by means of the residue of aniline, thenature of which had been settled by Hofmann's comprehen-sive and interesting researches,32 and then trying to showthis replacement by direct experiment, he succeeds inpreparing oxanilide, the formation of which is representedby the following equation :

C2H2O4 + 2C6H7N = C2H2O2 (C6H5N)2 + 2H2O.

He carries the analogy between ammonia and aniline stillfurther by the discovery of the anilid-acids, which he regardsas analogous to the amid-acids.88 Thus he writes the formulaof sulphanilic acid, which he obtains by the action of sulphuricacid upon oxanilide, SH2O8. C6H6N, and, in its existence, findsa new proof of the dibasic character of sulphuric acid.

It may perhaps seem strange that Gerhardt introduces theresidues NH and C6H5N into these compounds, instead ofNH2 and C6H6N. In this he may have been influenced byLaurent, who had already tried, a few years previously, to

31 Comptes Rendus. 20, 1032. n Annalen. 45, 250; 47, 37. n Journ,cle Pharm. [3] 9, 40$ ; Ip, 5 j compare nnalen. 6Q, 308,


replace the amid- by the imid- group.34 Gerhardt was able toassociate himself with this conception without involving him-self in any further consequences, since his formulas did notattempt to express the arrangement of the atoms but were onlycontracted equations. They were not intended to representwhat the compounds are, but merely what their nature is andwhat becomes of them :35 they were meant to indicate themodes of formation and of decomposition of substances. Ger-hardt was the first to advance the view that we should not con-clude from the decomposition products as to the arrangementof the atoms, because the latter are set in motion by the re-action.36 According to him, several formulae were thereforepossible for the same substance, and different residues (radi-cals) might be assumed in it according to the decompositionswhich it was desired to emphasise. In this way the point ofthe controversy, which had been carried on so fiercely andso long, as to the nature of the radicals, was demolished.Afterwards, Gerhardt comes to employ empirical formulae,which had been recommended by Liebig on account of theconstantly growing divergences of opinion as to rationalconstitution.87 Conjointly with Chancel, he introduces, in1851, the synoptic formulae,38 which never met with anygeneral acceptance because they were inconvenient and noteasily understood. If the form was new^ still the idea wassimply the old one. This mode o.f writing formulae waslikewise intended only to represent the formation and decom-position of substances, and it too consisted of contractedequations. The great advantage of this way of regardingthe matter lay in the possibility of advancing several rationalformulae for one substance, whereby new analogies anddifferences made their appearance and gave rise to a largenumber of investigations.89

34 Comptes Rendus. 1, 39. 35 Gerhardt, Introduction a Yitnde dela Chimie, 1848. m Compare Baudrimont, Comptes Rendus. 1845.317 Annalen. 31, 36. 88 J. pr. Chem. 53, 257. 39 It may be remarkedhere, in passing, that Gerhardt, a few years later, again replaces imid, NH,-by amid, NH2, after Laurent (J. pr. Chem. 36, 13), in 1844, had adoptedand sought to establish Hofmann's conception of aniline as phenamM.

LFXT. x.] HISTORY OF CHEMISTRY 187

Gerhardt's activity was perhaps of still greater importancein connection with another question ; that is, with the fixingof the atomic and molecular weights. Although the movementfor the revision of these most important numbers originatedwith him alone, still he was influenced in the further elabora-tion of the work by Laurent, with whom he was at that timein very intimate communication. Indeed, I might almostsay that it was Laurent who first clearly stated40 whatGerhardt wished to advance. It is, however, extremelydifficult to separate from one another the services of thetwo chemists, because they published a great deal con-jointly, and probably discussed everything together. Iwould ask, therefore, that my statements with respect tothis matter may not be taken too literally.

Gerhardt's first paper on the subject in question dates fromthe year 1842.41 In this paper he frequently employs the wordequivalent in a sense in which it was introduced into chemistryby Wollaston and Gmelin, although this sense is one of whichwe cannot any longer approve. With Gerhardt the word isone whose signification he does not seekfor in its origin, other-wise he could not call H2SO4 and HC1 one equivalent each ;for he himself wishes to prove that sulphuric acid is dibasic, inwhich case it is not equivalent to hydrochloric acid. WhatGerhardt wishes to determine are atomic and molecularweights, which, however, he does not yet understand how todistinguish from each other, and for which he uses the word"equivalent," at the same time designating as "atomicweights" the numbers which he is attacking.

Gerhardt's equivalents, so-called, are not really equiva-lent, but merely comparable quantities; and in determiningthem, the most various points of view which can be of con-sequence in estimations of atomic and molecular weightsand of equivalents, are taken into consideration.

It must appear striking and peculiar to any unprejudicedperson, that the numbers which Gerhardt proposes as the

40 Ann. Chim. [3] 18, 266. 41 J. pr. Qh(?m. 27, 439 j also Ann.Chim. [3] 7, 129; 8, 238,


"equivalents" oftheelementarysubstances (with the exceptionof the numbers for the metals), agree almost completely withthe atomic weights of Berzelius, of the year 1826. It is alsonoteworthy that Gerhardt does not mention Berzelius, and isobviously quite unaware that he, to a large extent, adopts hisnumbers. On the other hand, the Swedish chemist does notappear to have noticed this agreement, since he violentlyattacks Gerhardt's paper.42 What I consider as most remark-able, however, is the fact that, at the time when Gerhardtmakes his proposal, very eminent chemists (I only mentionLiebig and his pupils43) are actually employing atomicweights for the most important elements, such as carbon,oxygen, hydrogen, chlorine, etc., with the ratios whichGerhardt recommends as new ; but that, a few years after-wards, the equivalents of Gmelin, against which Gerhardt'spaper was directed, are almost universally adopted.

The valuable part of Gerhardt's paper consisted, however,much less in the suggestion which he makes as to the " equiva-lents " of the elements, than in his views respecting the equiva-lents of compounds. In following up, by means of equations,the decompositions of organic substances, he arrives at thegeneral proposition that the quantities of carbonic acid, water,and ammonia produced in these decompositions are expressibleas multiples, by whole numbers, of C2O4, H2O2, and NHg.44

Hence, according to him, these quantities must represent anequal number of equivalents, whereas it was at that time as-sumed that the equivalents of carbonic acid and of waterwere only half as great as these formulae would indicate.

Guided by quite similar considerations, he fixes the equiva-lents of carbonic oxide and of sulphurous acid as C2O2 andS2O4, and, in doing so, adds, as an important support of hisassumptions, that these quantities occupy exactly the samespace in the gaseous state. He is thus able to assert that the

42 Berzelius, Jahresbericht 1844, 319. 43 Liebig (Annalen. 31, 36)points out that it will probably never be possible to ascertain the trueatomic weights, and that it is therefore better to employ the equivalents,* C 6 Q = 8 N = i4 H = T


equivalents of carbon, oxygen, and sulphur are not 6, 8, and16, as Gmelin's school assumed, but twice these numbers, thatis, 12, 16, and 32 ; and he proves, by many examples, thatthere do not exist any equivalent formulae, constructed on theprinciples advanced by himself, which contain less of the re-spective elements than these latter quantities. He also provesthat an even number of atoms of carbon, of oxygen, and of sul-phur always occurs in compounds containing these elements,if they are represented by means of Gmelin's equivalents.

Consequently Gerhardt doubles the equivalents of carbon,of oxygen, of sulphur, etc., wi th reference to those of hydrogen,of chlorine, of nitrogen, etc., whereby he obtains Berzelius7

numbers. He differs very materially from the followers ofBerzelius in the formulae which he proposes for organic com-pounds. According to him, these had been doubled, as com-pared with many inorganic substances, consequently he halvesthem ; they were, as he expresses it, referred to H = 2, or to0 = 200, whilst, for the majority of inorganic compounds, thenumber chosen for comparison was H = 1 or O — 100. Hencesubstances were divided into those like water, carbonic oxide,carbonic acid, etc., which occupy two volumes (H = 1 = 1 vol.),and those like alcohol, ethylene, chloride of ethyl, etc, (thatis, all the substances which were then called organic, andof which the vapour densities were by no means alwaysknown), which correspond to double this volume.

I may be permitted to leave off the consideration of Ger-hardt's views here, in order to glance backwards and seek forthe reasons which had caused chemists to write " four-volume " formulae for organic substances. This way of writ-ing them must appear all the more remarkable since bothBerzelius and Dumas, at first, at least, believed that theymust choose the atomic weights of compounds so that theyshould represent equal volumes in the state of vapour.

Relatively few vapour densities were known at that time,and hence the rule was broken in many cases without the factbeing known. Another very important reason lay in the widelyspread assumption that the acid was the substance united to


the base in a salt, or, as it can also be expressed, that theusually hypothetical anhydrides (instead of the hydrates) wereregarded as acids. Thus, on the basis of the atomic weights ofBerzelius, the analysis of acetate of potash led to the formulaK.C4H6O4, and from this, after deductibn of the potash, KO,the atom of acetic acid remained as C4H6O3, which did notpermit of any further division. Those who regarded atom andequivalent as identical had to find confirmation of the accuracyof this formula in the fact that this quantity of acetic acid isneutralised by one equivalent of potash; KO ; and so theformulae of all monobasic acids were necessarily doubled.The establishment of the theory of polybasic acids causedLiebig to double the formulae of several dibasic acids, as forexample, that of tartaric acid (see p. 159). This, in turn, affectedthe atomic magnitudes of neutral substances, such as alcohol,compound ethers, etc. To the first of these the formulaC2H6O, corresponding to two volumes, had at first been as-signed, and, in order to arrive at this formula, Berzeliusassumed a radical in alcohol different from that in ether.45 ButLiebig, with whom alcohol was the hydrate of ether, adoptedthe group ethyl, C4H10, as the basis of both,46 and the closerelations between alcohol and acetic acid then became promi-nent for the first time. Ethylene was now written C4H8, andchloride of ethyl C4H1OC12 ; that is to say, all the compoundsof the ethyl series contained four atoms of carbon. Quitesimilar reasons caused the doubling of the other formulae.

Gerhardt wishes, as we have stated, to halve these, and heis moved to do so from other points of view besides that of thevolume relations. According to him, the salt-forming metallicoxides do not consist, as Berzelius assumes, of one atom ofmetal and one of oxygen, but they are comparable with water(which he now writes H2O) and contain two atoms of metal ;47

whereas in the hydroxides one atom of metal and one ofhydrogen are united with one atom of oxygen.48 He is there-

45 Compare . p. 132. 46 Compare p. 133. 47 Compare also Griffin,Chemical Recreations, Seventh Edition (1834), 9**93 and 228-229.48 Ann. Chim. [3] 18, 266.


fore obliged to halve the atomic weights of the metals, andto assume K = 39, Na = 23, Ca = 2O, etc. He calls thatquantity of a monobasic acid an equivalent which yields aneutral salt by the replacement of one part of hydrogen by39 parts of potassium, whilst he assumes the equivalents ofthe dibasic acids to be twice that quantity.49 The formulaof acetic acid therefore becomes C2H4O2 while that of oxalicacid, C2H2O4, remains unchanged.

That Gerhardt, in spite of these well-considered and excel-lent observations, which are now for the most part adopted,had not reached the point of view which we take up, is shownby a part of his paper50 where he thinks he should point outthat, as a consequence of his proposals, the atomic theory, thetheory of volumes, and the equivalent theory all coincide.According to our present views this is not attainable. Thevarious conceptions were first separated from one another in1846, by Laurent,61 who thereby rendered Gerhardt's numbersadmissible. He showed that these values were not by anymeans equivalent, and consequently did not deserve the name.They express, as he points out, those quantities which enterinto reaction, and accordingly represent molecular weights.

Although Gerhardt's endeavours in the determination ofequivalents tended in the direction of employing comparablequantities only, this was first stated and elevated to a principleby Laurent. According to the latter, it is necessary to startfrom a " terme de comparatson )} and to refer the formulae of allcompounds to it. Since he is quite clear as to the fact thatthe quantities contained in equal volumes do not always pro-duce the same chemical effect, he considers the questionwhether he will compare substances in the gaseous stateaccording to the space which they occupy, or whether he willcompare their equivalents. He rejects the latter comparison,on account of the difficulty associated with the determinationof equivalents of substances which are not analogous, anddecides in favour of the first comparison ; that is to say, hechooses the formulae (and therefore the molecules) of sub-

* Ann. Chim. [3] 7, 129. m Ibid. [3] 7, 140. w Ibid. [3] 18,


stances so that they represent two volumes (H = i = i vol.) inthe state of vapour. In doing so he is obliged, however, toadmit certain exceptions to which he draws attention. Thus itwas known from Bineau's investigations52 that the formulaeNH4C1, for sal ammoniac, and SO4H2, for sulphuric acid,53

correspond to four volumes, but, in spite of this, these quan-tities are regarded by Laurent as representing their molecularweights. There were definite grounds, in these instances,which appeared to make this assumption necessary. Theisomorphism of sal ammoniac with potassium chloride ex-cluded the formula NjH2Cl5; the dibasic character of sul-phuric acid, which Laurent looked upon as proved, demandeda molecular weight at variance with Avogadro's hypothesis.Even if this hypothesis, therefore, was regarded as the chiefstandard in the fixing of formulae, still the results obtainedwere subject to modification by chemical reactions, and byphysical properties such as specific heat, specific volume,crystalline form, etc. Further, the law of the even numberof atoms, which had been outlined for special cases byGerhardt54in 1843, played an important part in these deter-minations. Laurent now states this law, to the effect that inall compounds the sum of the atoms of hydrogen, chlorine,bromine, nitrogen, etc., must always be an even number.The law becomes of increased significance from the fact thatLaurent applies it in order to prove that the molecules ofthese elements, which he calls dyads, consist of two atoms.55

Gerhardt's ideas were greatly elucidated by Laurent, whomade them more generally accessible and comprehensible bylaying greater weight upon the terms he employed, and bydefining these terms precisely. As a result of this, animportant advance, was effected, because the separation ofatom, molecule, and equivalent was. now really accomplished jand hence it again became possible to employ Avogadro'shypothesis (thirty-five years after its promulgation) as the

53 Ann. Chira. [2] 68, 416. « Compare ibid. [3] 18, 289. M Ibid.[3] 7» 129' M Ibid. [3] 18, 266 ; compare also Laurent, Mdthode deGhimie. 77 ; E. 62.


basis of a system. With Laurent, the molecule is the smallestquantity of a substance which is required in order to giverise to a compound, and which, in the form of vapour,always (or at least with few exceptions) occupies double thevolume of an atom of hydrogen. The atom is the smallestquantity of an element which o'ccurs in compounds, whilstthe equivalents represent quantities of analogous substanceswhich have the same value in reactions.56

I shall try to give an idea of the importance to chemistry ofthese definitions, by stating some of the conclusions that weredrawn from them in view of the experimental work of the period.

The consistent application of the idea of equivalentsnecessitated the assumption by Laurent and Gerhardt ofseveral equivalents for many of the metals.57 " The idea ofthe equivalent includes the notion of an identity of function ;we are aware that one and the same element can play the partof two or of several others, whence it must occur that differentweights also correspond to these different functions. On theother hand we find different weights of the same metal, suchas iron, copper, mercury, etc., replacing the hydrogen of acids,and, in doing so, forming salts which contain the same metalbut possess different properties. These metals have, conse-quently different equivalents.'7

This idea was not new,68 but as really equivalent formulaehad never been employed, it had not up to this time had anyfurther consequences. Now, when Laurent and Gerhardt in-troduce this mode of writing formulae, it acquires a certainvalue. Thus, for example, these reformers of chemistry seekto find analogies where they had hitherto remained concealed ;the formulae of the sesquioxides can be assumed to be similarto those of the normal bases, and a uniformity can thus beintroduced into the way of regarding salts which has not beenpossible hitherto. It is known that the neutral sulphate ofthe protoxide of iron (ferrous sulphate) contains, for the same

m Ann. Chim. [3] 18, 296; also Comptes rendus des travaux chimiquespar Laurent et Gerhardt, 1849, 257. OT Ibid. 1849, I, etc. m Comparep. 103.

N


quantity of sulphur, one and a half times as much iron as theneutral sulphate of the peroxide of iron (ferric sulphate).Thus we may express it that 28 parts of iron in the ferroussalt can take the place of one part of hydrogen, while thelatter can also have its place taken by i8§ parts of iron, inthe ferric salt; both quantities are, therefore, equivalent toone part of hydrogen. If we distinguish, as Laurent andGerhardt did, by the terms J'errosum (Fe = 28) and ferricum(fe = f.28) the equivalents of iron in the ferrous and in theferric salts respectively, then the formulae of ferrous sulphate(Fe9) SO4 and of ferric sulphate (fe2) SO4 become compar-able with each other. A similar thing holds for othermetals such as copper, mercury, tin, etc. ; in the salts corre-sponding to the lower and to the higher oxides of thesemetals, different equivalents, of which the one is double theother/9 must be assumed in each case.

Complete analogy is attained in the mode of writingsalts when equivalent formulae are employed for acids also.We then have :—

Ferrous sulphate Cupric chloride Mercurous chlorideSO4(Fe2) Cl2(Cu2) Cl2(hg2)

In this mode of writing formulae, the differences betweenmonobasic and polybasic acids disappear ; and it certainly isan advantage of the molecular formulae that they permitthese highly important peculiarities to become prominent.Laurent and Gerhardt recognised this very fully, and it wasthe latter, especially, who endeavoured, and with muchsuccess, to bring about the separation of these classes of sub-stances by more definitely advancing new characteristics.60

The formation of double salts with non-isomorphous basesdid not appear to Gerhardt sufficient for fixing the basicity ofan acid ; he points out that dibasic (and polybasic) acids can

59 Laurent's views with respect to the mode in which the existence ofseveral equivalents for the same element may be explained are given inhis Me'thode de Chimie, 127 ; E. 103. m Laurent and Gerhardt,Comptes rendus mensuels des trauvaux chimiques, 1851, 129 ; J. pr. Clienu53* 460.


form two (and more) ethers, of which one (or more) is acid,and one is neutral. The molecule of the latter, if it is assumedto correspond to two volumes, contains one alcohol residue inthe case of monobasic acids, and two (or several) such residuesin the case of dibasic (or polybasic) acids. Further, the amid-compounds, and also the anilid-compounds (discovered a shorttime previously01) furnish additional evidence. Thus whilstthe monobasic acids produce only one amide, one nitrile, andone anilide, the acid ammonium salts of dibasic acids, by theloss of water, give rise to the formation, besides, of an amid-acid and of an imide ; and they alone can yield anilid-acids.

Laurent had already drawn attention, a few years earlier,to another difference between these substances.02 Accordingto him, only the formulae of the dibasic and polybasic acidspermit the assumption of their containing water, whereas inone molecule of a monobasic acid the constituents of onlyhalf a molecule of water require to be present, for whichreason, the latter are not able to form anhydrides.

Thus nitric acid is :—HNO8 = (HO4) + NO2 | ; while sul-phuric acid is :—H2SO4== H2O -f SO3.

With Laurent, hypochlorous anhydride, which wasalready known at that time, is C1H0 in which one atom ofhydrogen is replaced by one atom of chlorine.63

Laurent's views as to the molecules of the elements werealso very important. It was a consequence of Avogadro'shypothesis, with which Laurent identifies himself,64 that themolecules of certain elementary substances should be regardedas composed of at least two atoms. Laurent tries to supportthis view by means of chemical reasons. According to himthe so-called u dyads,7' such as hydrogen, chlorine, bromine,

61 Compare p. 185. m Ann. Chim. [3] 18, 266. ^ These viewswere looked upon as refuted by Gerhardt's discovery of acetic anhydrideetc., but strictly speaking, they were not. It was, in fact, an extension ofthe word anhydride to apply it to the substances discovered by Gerhardt.The relation between C3H402and QjHgOgon the one hand is not quite thesame as that between C4Hf}04 and C4H4O3 on the other. U He doesnot seem to be aware, however, that it was first announced by Avogadro,

196 HISTORY OF CHEMISTRY [LEGT. X.

iodine, nitrogen, phosphorus, arsenic, and antimony, alwaysoccur in even numbers only; and this rule, if it is to holdfor the molecules of the elements as well, renders the exist-ence of single atoms in the free state impossible. Besidesthis, Laurent brings forward the well-known effects of thenascent state, and explains them by the assumption that atthe moment of the separation of the elements from their com-pounds the single atoms are isolated, and therefore combinefar more easily with others than in those cases where it is aquestion of molecules or of atomic groups which must firstbe decomposed before reaction can take place.

Lauren t and Gerhardt, with their far-reaching reforms, metwith almost no immediate recognition ; on the contrary, itseems as if the conception of the equivalent, in its first uncer-tain form, had now found more adherents than formerly, andas if Gay-Lussac?s law of volumes appeared to chemists to beless adapted than ever to constitute the basis of a system.For this reason there was not, in general, the slightest tend-ency exhibited to assume, with Laurent, the divisibility of themolecules of the elements. It is no doubt true that urgentgrounds, and especially, chemical grounds, were still wanting.It was a very happy idea that Laurent and Gerhardt had hitupon when they stated that the formulae of substances mustrepresent comparable quantities-; but the standard was stillwanting. The spaces occupied by the gases were known onlyin relatively few instances, and even amongst these there weresome cases where the molecular weights deduced were un-serviceable because they were at variance, or at least theyappeared to be at variance, with the chemical properties. A.series of facts which confirmed these ideas, and eventually pro-cured for them general recognition, was still wanting. Weare indebted for our knowledge of them to Williamson, whoshowed how to find the molecular weight by chemical methods,and thereby rendered a service to our science which cannotbe too highly estimated. Even although he did not instigatethe reform of chemistry, still it was his investigations whichfirst made its accomplishment necessary and possible.

L E C T U R E XI.

REASONS FOR THE ASSUMPTION OF THE DIVISIBILITY OF ELEMENTARYMOLECULES—FIXING OF THE MOLECULAR WEIGHTS, BY WILLIAM-SON, BY MEANS OF CHEMICAL REACTIONS — THEORY OF THEFORMATION OF ETHER—FUSION OF THE RADICAL THEORY WITHDUMAS' TYPES—SUBSTITUTED AMMONIAS—POLYATOMIC RADICALS•—GERHARDT'S THEORY OF TYPES AND SYSTEM OF CLASSIFICATION.

IT has often been stated that chemistry should advance bydevelopment from within itself alone, and that the influence ofthe other sciences is injurious if it is exerted in any other direc-tion than that in which chemical facts appear to lead us. Iunderstand perfectly well that in chemistry a theory is notchosen which is in contradiction to certain facts, in order toattain a more complete agreement with physical laws, and Iappreciate this from the didactic point of view in particular ;but I consider it just as proper and essential in our science tomodify our views so as to produce harmony with recognisednatural laws, and also with theories and hypotheses, as soon asthe facts permit of our doing so. It therefore appears to meappropriate, now when in our historical development we havearrived at the middle of a far-reaching reform, to advance someof the not exclusively chemical reasons which told in favour ofthe system of Laurent and Gerhardt. In doing this, I confinemyself to facts which appear to support the divisibility of themolecules of the elements, for the special reason that thishypothesis, even after it had been repeatedly announced, stillmet with no approval.

Amongst the results, of extreme value in chemistry, whichFavre and Silbermann announced in 1846 in their research onheats of combustion,1 there is a very remarkable fact which

1 Comptes Rendus. 2$, 200.

198 HISTORY OF CHEMISTRY [LECT. XI.

deserves to be stated here. They observed that on burningcarbon in oxygen, less heat is produced than when nitrousoxide is employed. They considered that this striking factcould only be explained on the hypothesis that in both cases,besides the formation of carbonic anhydride, a decompositiontook place ; that is to say, a change occurred involving theseparation of atoms which had previously been combined.Accordingly they thought that even in the free particles ofoxygen gas, several (two) atoms must be assumed, and thatthe quantity of heat required for their decomposition mustbe greater than that required for the separation of the oxygenfrom the nitrogen in nitrous oxide.

By the application of a hypothesis relating to chemical com-bination and decomposition, Brodie2 arrives at the divisibilityof molecules of hydrogen and of oxygen. It appears to him thatthe contrast which was drawn, in accordance with the viewsprevailing at that time, between the formation of compoundsand the separation of elements, has no natural foundation. Inhis view, every combination is merely the consequence of adecomposition, and this can only be occasioned by new com-binations. He tries to prove the accuracy of this view bymeans of various examples, and, in doing so, introduces certainsigns and expressions designed to give an idea of the contrast(more generally, of the relation) between the atoms enteringinto combination. According to Brodie, there exists betweenthese a relation (polarity) of such a kind that the one isdesignated as positive or negative as contrasted with theother. This relation, which Brodie also calls chemical differ-ence, depends upon the peculiarities of all those particles withwhich the atom is for the time being combined.

To permit of a better comprehension of this, I give heresome of the examples adduced by Brodie. Silver does notunite directly with oxygen, whereas silver chloride is decom-posed by boiling With potash, silver oxide being formed.According to Brodie, this is due to the fact that it is only

2 Phil. Trans. 1850, 759.

LECT. x i . ] H I S T O R Y O F C H E M I S T B Y 199

b y t h e i r c o m b i n a t i o n w i t h c h l o r i n e a n d p o t a s s i u m respec-

t ively t h a t t h e s i lver a n d o x y g e n acqu i r e t h e po la r i t y

necessary for t h e i r c o m b i n a t i o n . H e wr i t e s : —

+• - + -A g C l + K O = A g O + K C 1 . 3

A c c o r d i n g t o F a r a d a y , 4 pe r fec t ly d r y ca lc ium c a r b o n a t e is

n o t d e c o m p o s e d even a t t h e h i g h e s t t e m p e r a t u r e s , w h e r e a s in

t h e p resence of w a t e r t h e d e c o m p o s i t i o n beg ins a t once ; a n dsimilarly, a c c o r d i n g t o Mil lon , 5 s u l p h u r i c a n h y d r i d e can be

disti l led ove r c a r b o n a t e of p o t a s h , t h e fo rma t ion of a salt on ly

t a k i n g p lace on t h e a d d i t i o n of w a t e r . B r o d i e w r i t e s :—

K O -H H S O 4 = H O + K S O 4 .

In t h i s case , especial ly , i t can be d i s t i nc t ly recogn ised w h y

Brod ie cons ide r s t h a t c o m b i n a t i o n is a l w a y s a c c o m p a n i e d by

decompos i t ion , wh i l s t t h e first e x a m p l e s e e m s to jus t i fy t h e

converse s t a t e m e n t . B u t t h e ex i s t ence of free e l e m e n t a r y

a t o m s is i n c o m p a t i b l e w i t h th is , so t h a t B r o d i e is a t pa in s to

show t h a t t h e s e a lways a p p e a r in pa i r s , a n d t h e n u n i t e w i t ho n e a n o t h e r . T h e m o s t s t r i k i n g of t h e examples w h i c h h e

adduces is t h a t of t h e e v o l u t i o n of h y d r o g e n w h i c h t a k e splace w h e n c o p p e r h y d r i d e , d i s cove red b y W u r t z , 6 is t r e a t e d

w i t h h y d r o c h l o r i c ac id :—

That a similar evolution is not observed on treating themetal with the acid is looked upon as owing to the fact thatthe same species of polarity is always associated with thehydrogen in hydrochloric acid, whilst the affinity of copper forchloriue is not sufficient to decompose hydrochloric acid.7

3 H = i, 0 = 8, K = 39, Ag = io8, C = 6, etc. 4 The statement is nowregarded as erroneous ; compare Gay-Lussac, Ann. Chim. [2] 63, 219.0 I have been unable to find this fact, quoted by Brodie, in Millon's papers.6 Annalen. 52, 256. 7 Compare Wurtz, Lecons de philosophic chimique,64,


The formation of nitrogen by heating ammonium nitriteis also explained in the same way :—

N O 4 H 4 N = 4 H O + N N .

This mode of regarding the matter is very specially suitablefor explaining the reductions by means of hydrogen peroxide,which were at that time partially known and had been chieflystudied by Brodie himself.8 He regards the formation ofoxygen as a consequence of the different polarities which thiselement possesses in the two oxides. We have, for example :—

+ - + + + - -f- -HOO + Agh Agfi = HO + 0 0 + Ag.

The reduction of potassium permanganate and of potas-sium bichromate is supposed to proceed in a similar manner.Two atoms are always set at liberty simultaneously, andthese, in consequence of their chemical difference, thenunite with each other.9

Further, the discovery of ozone by Schonbein;10 therecognition of its nature as an isomeric modification ofoxygen ; u and in particular the proof that it is condensedoxygen (a fact which was first stated by Andrews and Tait12

on the strength of so me highly in teresting experiments, but wasespecially demonstrated by Soret13) only find an explanation inthe hypothesis of the divisibility of the elementary molecules. Ifozone, as appears from Soret ?s experiments, possesses a relativedensity, one and a half times as great as that of oxygen, thenthe smallest particles of this latter gas must contain at least twoatoms, whilst those of ozone consist of three atoms. But if thisassumption is admitted in the case of oxygen, it cannot easily

8 Phil. Trans. 1850, 759. 9 Compare also the explanation whichWurtz gives of the fact that the combination of nitrogen and oxygen takesplace much more easily in presence of hydrogen (Wurtz, Lee. on s. 65).10 Pogg. Ann. 50, 616; 59, 240 ; 63, 520; 65, 69, 161, 190, etc.; compare(< Ueber das Ozon," Basel 1844. n Archives des sciences phys. et natur.,Geneve 12, 315; 17, 61 ; 18, 153. 12 Annalen. 104, 128; 112,185,*Ann. Chim. [3] 52, 333, and 62, 101. 18 Annalen. 138, 45 ; Supplemeat-' "UKI 5, 148.

LECT. xi.] HISTORY OF CHEMISTRY 201

be avoided in the cases of the other elements. The differentvapour densities which were found in the case of sulphur u

can only be explained by the assumption that, at lowtemperatures, the molecule consists of three times as manyatoms as at a very high temperature.

The fact is certainly not without interest, that, in 1857,Clausius was led, by the mechanical theory of heat, to thedivisibility of the physical molecule.15 Since, according tothis theory, the kinetic energy of translatory motion inequal volumes, of two gases at the same pressure, is pro-portional to the absolute temperature, Clausius concludedthat the kinetic energy of translatory motion of the singlemolecules of all gases at the same temperature is the same.This assumes the fulfilment of Avogadro's hypothesis.

The number of facts drawn from different branches ofnatural science which tell in favour of this hypothesis mightbe multiplied still further ; but I confine myself to the state-ment of those that I have mentioned, and pass on to chemicalarguments which, after all, were the only ones that eventuallyled to the recognition of the hypothesis. Amongst these, theexperiments which were now carried out and led to the con-ception of the chemical molecule, distinctly take the firstplace. I will not assert that this conception was not alreadyextant; but it now appeared in a much more definite form.This latter assertion will certainly be justified when I collecthere the facts and hypotheses which existed and exercisedan influence upon the determination of molecular weightsby chemical methods, prior to Williamson.

The atomic theory gave the first clue to these magnitudes.The formula of every compound required to be expressible bymeans of multiples, by whole numbers, of the atomic weights;but this had no obligatory consequences so long as the atomicweights had not been determined with certaint}^, because, incase of necessity, even the atomic weight of a constituent couldbealtered. It was unquestionable, however, that even with only

14 See Dumas: Ann. Chim. [2] 50, 170, and Deville and Troost,Comptes Rendus. 49, 239. 15 Pogg. Ann. 100, 353-


a moderately consistent application of atomic considerations,there was a certain connection between the different formulae.A relation of this kind was attained, especially by Laurent andGerhardt, in the case of organic compounds in particular. Itappears to me to follow from the nucleus theory that Laurentassumed the number of carbon atoms in the radical to remainunchanged until carbon was separated in some form or other.10

It was'Gerhardt who first clearly stated this rule17 (which, how-ever, is not always accurate, as, for example, in the formationof polymeric substances), and it will readily be conceded thatby means of it, the molecular weights of many series of com-pounds were fixed from a knowledge of these magnitudes inthe cases of a few substances. The so-called law of the evennumber of atoms gave a further guide to the smallest formula,and, in consequence of it, Laurent and Gerhardt foundfrequent occasion to alter the formulae previously adopted.

The conception of the polybasic acids exercised a very im-portant influence in the fixing of formulae, since even Liebigfor example (who first definitely grasped this conception) wasinduced to double the formula of tartaric acid,18 in order tomake it conform with the chemicalcharacterof this substance.In the case of one special class of substances—the acids™—therecognition of a criterion of their polybasicity, for which weare indebted to Liebig, Laurent, and Gerhardt, was just asfar-reaching as the later experiments of Williamson in thecases of other groups of compounds.

The phenomena of substitution, as may readily be under-stood, contributed something towards rendering the moleculesof different compounds comparable with one another. Theformula of a substance frequently required to be multiplied bytwo or by three so that it might not be necessary to assumefractions of atoms in the products arising from the substanceby the action of chlorine, etc. It only became necessary toattend to these considerations in consequence of the unitarynotions introduced by Dumas at the same time, and of the ruleof Gerhardt mentioned above. This does not apply to the

16 Compare p. 145. w J. pr. Chem. 27, 439. w Annalen. 26, 154.

LECT. xi.] HISTORY OF CHEMISTEY 203

opponents of these views, as I shall prove by means of anexample. Kolbe and Frankland prepared methyl (ethan), in1848, by treating ethyl cyanide with potassium, and theyassigned to it the formula C2H3 [C —6].19 They subjectedthis substance to the action of chlorine in order to convert iteventually into methyl chloride. Instead of the latter theyobtained a compound having the same composition as ethylchloride, but which instead of becoming liquid at 120, remainedgaseous at - 180. This they regarded as isomeric with ethyl

T-Tchloride, and they formulated it C2H3. C2^3 that is, as acoupled compound of methyl with another atom of methyl inwhich one atom of hydrogen is replaced by chlorine. WithKolbe and Frankland, therefore, the existence of the first sub-stitution product C4H5C1 was no reason for assigning the for-mula C4H0 to the original hydrocarbon. Laurent was of adifferent opinion. Prior to the isolation of the alcohol radicals,hehadproposedforthem,in case they should be discovered, theformulae now adopted.20 Afterwards, when Kolbe had dis-covered a general method for the preparation of the alcoholradicals by the electrolysis of salts of the fatty acids,21 Laurentand Gerhardt return to this view in a detailed manner anddesignate them as homologues of marsh gas.22 A. W.Hofmann allies himself with them in so far that he leaves thepossibility of isomerism between the alcohol radicals and thehomologues of marsh gas an open question.23 It is otherwisewith Frankland, who defends the formulae corresponding to twovolumes for the alcohol radicals, as opposed to those corre-sponding to four volumes, and formulates methyl, C2H3, andethyl hydride, C4H6, afterwards just as he had done before.24

The conception of the chemical molecule was more readilyappreciable by Laurent, Gerhardt, and Hofmann. They en-

19 Jcmrn. Chem. Soc. I, 60; Annalen. 65, 279. 20 Ann. Chim. [3] 18,283. 21 Journ. Chem. Soc. 2, 157 ; A. C. R. 15, 17 ; Annalen. 69, 257.22 Comptes rendus mensuels des travaux chirniques, 1850. ** Journ.Chem. Soc. 3, 121 ; Annalen. 77, 161. u Journ. Chem. Soc. 3, 322 ;Annalen. 77, 221.


deavoured to make their ideas in this connection plain to theiropponents, but still they do not appear to have succeeded inconvincing them. Telling experiments were still wanting.Gerhardt required hundreds of examples in order to show thatH4O2, and not H2O, corresponds to the formulae N2HC andH2G2. Laurent's proof with respect to the doubling of themolecular weights of hydrogen, chlorine, etc., is also clumsy.In spite of this, it cannot be denied that these chemists atthat tirne possessed accurate fundamental ideas, and I donot doubt that they also would have been able to deducethe same conclusions, productive as they were for ourscience, from the facts which were now discovered and soexcellently turned to account by Williamson.

By the action of potassium ethylate upon ethyl iodide,Williamson had hoped to be able to effect the synthesis of analcohol; 25ethylwasexpectedtotaketheplace of the potassium,with the formation of an ethylated ethyl alcohol—an expecta-tion quite in conformity with the views of the period. A shorttime previously, Wurtz had discovered ethy la mine,26 which heregarded as a substituted ammonia—a view which was con-firmed by the interesting mode of formation of this and manyanalogous substances, discovered by Hofmanri.27 Franklandhadalready tried by means of zinc etlryl, a substance discoveredby himself,28 to introduce alcohol radicals into organic sub-stances.29 But Williamson's experiment gave unexpectedresults: instead of an alcohol, he obtained ether. He under-stands, however, how to adapt his ideas, which had beenturned in an altogether different direction, to his results, and

•he at once recognises the full importance of his experiment.He explains the formation of ether under the conditionswhich he had observed, and then the formation of ether ingeneral; and he proves the accuracy of his view by meansof a series of brilliant experiments.

25Phil. Mag. [3] 3% 350; A. C. R. 16, 7, S; Annakn. 77, 37.^Comptes Rendu<.28, 223. i/! Journ. Chem. Soc. I, i$q, etc. ; Annalen.66, 129, etc ^ Journ. Chem- Soc 2, 297; Phil. Trans. 1852, 417;Annalen. 71, 213 ; 85, 329. M Phil. Trans. 1852, 4.32.

LECT. xr.] HISTORY OF CHEMISTRY 205

Different views were at that time held as to the formulae ofalcohol and of ether. In conformity withLiebig's ethyl theory,alcohol was pretty generally written C4H12O2,and ether C4H10O[C = 12, O = 16]. Now, with the halving of the atomic weights,the formulae had, in many cases, been halved : alcohol becameC4H6O2, and ether C4H5O [C = 6, 0 = 8], whereas Gerhardtassigned to these substances the formulae C2H6O and C4H10O[C= 12, O = 16]. Further, Laurent had already drawn atten-tion to the fact, in 1846,30 that the formulae of alcohol andether, as well as those of potassium oxide and hydroxide,are derivable from that of water.31 He wrote :—

HHOWater.

EtHOAlcohol.

EtEtOEther.

KHOPotassiumHydroxide.

KKOPotassium

Oxide.

Williamson perceived that the last view alone was in agree-ment with his experiment. He formulated as follows theequation which represented the reaction he had discovered:—

In order to meet the opposing view, in accordance withwhich the equation ought to be written :—

C4H5O . KO 4- C4H5I = 2(C4H5O) + KI [C - 6]

since the assumption was that potassium alcoholate is a com-pound of potassium oxide with ether, and that the latterseparated during the decomposition, whilst a second " atom"of the same substance was simultaneously produced from theethyl iodide, Williamson carried out the reaction with methyliodide. He expected to obtain methyl ethyl ether, whereas, inaccordance with the view just referred to, a mixture of methylether and ethyl ether ought to be produced. The experimentwas, therefore, decisive and justified Williamson7s hypothesis.Both by the action of methyl iodide upon potassium ethylate

30 Ann. Cbim. [3] 18, 266. 81 Griffin claims priority with respect tothe view that the alkalies do not contain any water. See Griffin, RadicalTheory, 9.

206 HISTORY OF CHEMISTRY [user. xi.

and by that of ethyl iodide upon potassium methylate theso-called mixed methyl ethyl ether was produced :—

?«O + ICH8 = Q 2 2 5 O + IK

^ 2 5 ^ g

These experiments proved to Williamson that ether isproduced from alcohol by the replacement of an atom ofhydrogen by ethyl, and, therefore, that it contains morecarbon in its molecule than alcohol does. The formation ofthe mixed ethers furnished him with a reason for excludingevery other view.

It now became a question to explain the formation of etherunder known circumstances, especially by the treatment ofalcohol with sulphuric acid ; and the solution of this problemwas also furnished by Williamson, after chemists had beenengaged upon it for decades. At first the process of etherifica-tion hadbeen explained by the dehydrating action of sulphuricacid ,32 This view agreed very well with D umas' etherin theory.Hennell,although an adherent of the etherin theory,consideredthis view irreconcilable with the formation of sulphovinic acidobserved by him;33 and it was also in contradiction with thefact that water distils over at the same time as the ether. Itwas Liebig, especially, who established by means of numerousexperiments a new theory of etherification.84 He ascertainedthat the formation of ethyl sulphuric acid precedes that ofether, and, according to him, the sulphuric acid does notwithdraw water from the alcohol, but ether, which latterunites with the sulphuric acid. Indeed ethyl sulphuricacid was at that time looked upon as a compound of thesetwo substances, that is, as an acid salt of ethyl oxide.

Thus ethyl sulphuric acid = C4H10O + 2SO3 + H2O

32 Compare especially Fourcroy and Vauquelin, Scherer's Journal 6,436 ; also Gay-Lussac, Ann. Chim. 95, 311, and [2] 2, 98. ** Phil. Trans.1828, 369. M Annalen. 9, 31 ; 13, 27 ; 23, 31.


Ethyl sulphuric acid, according to the experiments ofLiebig, breaks up between 1270 and 1400 into ether and sul-phuric acid. The remarkable phenomenon that a substance isproduced and undergoes decomposition in tbesameoperation,was explained by Liebig on the assumption that the forma-tion only occurred at those places where the alcohol droppedin, and where, therefore, the temperature was lowered to theboiling point of this liquid. The process of etherificationtherefore consisted, according to Liebig, in the combination ofthe sulphuric acid with the ether of the alcohol, and the decom-position into its constituents of the compound so formed, atparts of the liquid where the temperature was higher. Etherthen distils over, and, along with it, the water which wasseparated on the formation of the ethyl sulphuric acid.

In opposition to this theory Berzelius85 advanced anotherview, which was supported and developed by Mitscherlich inparticular.86 According to this view, the sulphuric acid actsby contact, taking no part in the reaction, and simply decom-posing the alcohol into ether and water by catalytic action.This was a mode of expressing the facts by means of speciallychosen words, but it can scarcely be called an explanation.Liebig's hypothesis really embraced an explanation whichwas pretty generally accepted. It was only called in questionafter Graham had shown, in i8$o,8r that alcohol and ethylsulphuric acid are both necessary for the formation of ether;and that the latter of the two does not yield ether evenwhen heated alone to 1430, but decomposes, in presence ofwater, into alcohol and sulphuric acid.

Williamson clearly understands how to apply these facts.The formation of ether is explained by the following equation:

Ethyl c 1 u -sulphuric Alcohol S u ^ n C Ether.

acid.88 Berzelius, J ahresbericht, 1836, 241. m Pogg. Ann. 53, 9$ ; 55,

209, *? Journ. Chern. Sac. 3, 24; Anrmlen. 75, 108 ; compare Journ,de pharm. [3] 18, 30,

208 HISTORY OF CHEMISTRY [LECT. XT.

whilst the formation of the ethyl sulphuric acid is representedby this one":—

| s o 4 + c f f i o = g o + c | f *so4.

If the latter equation is read from right to left, it explainsGraham's experiment of the decomposition of ethyl sulphuricacid into alcohol and sulphuric acid. As soon as the doubledformula for ether was admitted, it was comprehensible whyether is not produced by heating ethyl sulphuric acid.

Williamson, however, not contented with having shown theaccuracy of his opinions by their being in harmony with theknown facts, devises new experiments by which he can testthem.38 The method he adopts is the same as previously. Hechooses the two substances which act upon one another fromgroups containing different numbers of carbon atoms. He nowcauses ethyl sulphuric acid and amyl alcohol to interact, andthis reaction gives rise to the expected ethyl amyl ether :—

5SO4 + C^O « ^SO 4 + §H1X

He studies, besides, the action of sulphuric acid upon mix-tures of ethyl and amyl alcohols, and is able to show theformation, in this case, of three ethers—ethyl ether, amyl ether,and ethyl amyl ether. He finds u in these reactions the bestevidence of the nature of the action of sulphuric acid in form-ing common ether, or in accelerating the formation of the so-called compound ethers; for acetic ether is formed from aceticacid, just as ethylic ether from alcohol, by the replacementof hydrogen by ethyle. And if the circumstance of contain-ing hydrogen, which is replaceable by other metals or radicals,be the definition of an acid, we must consider alcohol asacting the part of an acid in these reactions/'

A further consequence of Williamson's experi ments was thefixing of the molecular weight of acetic acid. According toWilliamson, this acid is formed from alcohol by the replace-ment of two hydrogen atoms of the ethyl group by an atom of

88 Journ. Chem. Soc 4, 229; A. C. R. 16, 24; Annalen. 81, 73-

LECT. xi.] HISTOEY OF CHEMISTRY 209

oxygen, the radical C2H6, ethyl, being converted by oxidationinto C2H3O, othyl. Acetic acid is now regarded as water inwhich one atom of hydrogen is replaced by othyl. Theformula C6H12O3 fixed upon for acetone by Kane,89 did notappear to be in harmony with this ; it had, however, alreadybeen halved, and Williamson endeavoured to explain theformation of the substance during the distillation of theacetates, by means of the following equation :—

C0H3On C2H3On COKO n ^ C2H8O2 H 3 O n COKO n ^ C2H8K K u = = K U + CH3

According to him, the potassium peroxide is replaced duringthe reaction by methyl derived from the othyl. Here, also,he checks his opinion by applying the method already men-tioned. He distils mixtures of acetates and valerianates andobtains mixed ketones:—

C2H3OO + C5H9OO _ COKOO + C g O_

Williamson concludes this paper, which did a great dealto advance chemistry, with the words: " The method hereemployed of stating the rational constitution of bodies bycomparison with water, seems to me to be susceptible ofgreat extension ; and I have no hesitation in saying that itsintroduction will be of service in simplifying our ideas, byestablishing a uniform standard of comparison by whichbodies may be judged of."

The u terme de comparaison " which Laurent had alreadysought for in vain, had now been found. Substances were tobe conceived as formed from water in accordance with the pro-posal, which Williamson makes in 1851 in his famous paperupon salts,40 to regard this substance as the type for all com-pounds. His method of fixing molecular weights is a purelychemical one ; he regards compounds as formed from water bythe replacement of one or of two atoms of hydrogen. Inas-much as he tests all his views by means of facts already known,as well as by means of new experiments, these views obtain, I

* Annalen. 22, 378. m Journ. Chem. Soc. 4, 350 j A. C. R. 16, 41,O

210 HISTOHY OF CHEMISTRY [LECT. XI.

might almost say, absolute confirmation. His experiments,moreover, are not chosen at random, but are always sug-gested by the same kind of logical deductions. Williamsonfurnished thinking chemists with the means of determiningmolecular weights by chemical methods. The very generalapplicability of the method which he discovered, is demon-strated by the tine experiments of Gerhard t, who was led by itto the preparation of the mixed anhydrides,41 and of Wurtz,who employed it with similar success in fixing the formulae ofthe alcohol radicals.42 The mode, also, in which Friedel andCrafts43 determined the molecular weight of silicic etherdepends upon a train of ideas that only became clear tochemists after the publication of Williamson's investigations.

The fact must not remain unmentioned here that, only afew months after Williamson's publication, Chancel, on 7thOctober 1850, published a paper44 in which, by means ofsimilar experiments, he arrived at the same results as William-son. Chancel distils potassium ethyl sulphate with potassiumethylate, and with potassium methylate, and thus obtains ethylether and ethyl methyl ether. His mode of fixing the mole-cular weight of dibasic acids is peculiar to himself, althoughit coincides, in principle, with Williamson's method. By dis-tilling potassium ethyl sulphate with potassium methyl car-bonate and with potassium methyl oxalate, Chancel preparesethyl methyl carbonate and ethyl methyl oxalate. The reac-tions are represented by the following equations :—

c A ( c H 2 ) K + S 0 < c f H 4 ) K = c A ( c H 2 ) ( c y O

The experiments of Williamson and of Chancel were ofthe greatest importance in the development of the science, forour present views are founded upon the conception of thechemical molecule. Unfortunately it is not always possible

41 Ann. Chim. [3] 37, 332 ; Annalen.82,127 ; 83, 112 ; 87, 57 and 149.<* Ibid. 96, 364, ** Ann. Chim. [4] 9, 5. u Comptes Rendus. 31, 531.

LECT. xr.] HISTORY OF CHEMISTRY 211

to determine this with the same exactness as in the cases con-sidered above. It became manifest, however, that with fewexceptions, to which we shall afterwards return, the chemicalmolecule agrees with the molecular weight deduced from theobserved volume, on the basis of Avogadro's hypothesis.This led to the assumption of the general identity of thephysical and the chemical molecule, whereby a new means,and an almost always sufficient one, is furnished for ascer-taining the highly important molecular weight.

But Williamson's experiments and opinions also exertedan influence in another direction ; that is, with respect tothe views concerning the constitution of compounds. Theway was now prepared for a fusion of the newer radicaltheory or theory of residues with Dumas' theory of types,from which Gerhardt's theory of types arose. In this develop-ment the labours of other chemists were, however, of at leastas great importance, especially as these had already beenpartially carried out. Accordingly, we shall now direct ourattention more particularly to the latter.

In 1849, Wurtz, by treating cyanic ether, cyanuric ether,and the substituted ureas prepared from these by himself, withpotash, obtained bases extremely like ammonia, which hecompared with the latter inasmuch as he regarded them asammonia in which an atom of hydrogen is replaced by aradical such as methyl, ethyl, amyl, etc.45 This way ofregarding these substances involved an important step inadvance, since it was the first successful attempt to introduceradicals into the types.46 The fact that Liebig, as early as1839, expressed a similar view concerning these substances,which were then only hypothetical,47 satisfactorily proves theclear perception of this gifted scientist, but cannot detractfrom the merit of Wurtz,

The views of Wurtz respecting the constitution of these

m Comptes Rendus. 28, 224, 323; 29, 169, 186; Annalen. 71, 330.46 Compare, however, Laurent, Ann. Chira. [3] 18, 266. 47 Hand-worterbuch &er Chemie, by Liebig, PoggendorfF, and Wtfhler, 1, 698.

212 HISTORY OF CHEMISTRY [LECT. XT.

artificial bases, received important support from Hofmann'smethod of preparing them.48 Hofmann succeeded, by treatingthe alkyl iodides with ammonia, in introducing the radicalsinto the latter, and his experiments possess all the greaterimportance from the fact that he also showed how to preparesecondary and tertiary compounds, as well as substances cor-responding to ammonium chloride and ammonium hydroxide.

Thus:

+ I C H =

H C2H6

NC2H5

C2H5 ( 2 6

N C H 8 -rIC2Hfi = N C H 3 ICBHU C5Hn

(.(-'o 5/2 rn- TT \N C H3 1+ AgHO = Agl + Ntes2**O.

° « H " C5HU

I shall not leave the fact unmentioned that Paul Thenardhad discovered the organic phosphorus compounds ia 1845,49

but that these only received their correct explanation now,50

Of other investigations, carried out at the beginning ofthe fifties, which contributed to the establishment of the newtheory of types, I mention the discovery of the acichloridesby Cahours ;51 that of the anhydrides of monobasic acids byGerhardt; Williamson's researches on dibasic acids; and,finally, the preparation of the acid amides of Gerhardt andChiozza.52

48 Annalen. 6<5, 129 ; (fj, 6r, 129; 70, 129; 73, 180; 74, I, 33, 1x7 ;75> 356; 7^ 2S3; 79. «• 49 Cornptes Rendus. 21, 144; 25, 892.50 Compare Fran Wand, Annalen. 71, 215. a Ibid 70, 39. Liebjg andWShler had prepared benzoyl chloride and benzoyl-amide twenty yearsbefore-; see Annalen. 3, 249. w Cornptes Rendus, 37, 86.


Gerhardt had at once grasped the bearings of Williamson ?sinvestigations, and was only able to perceive in them confirma-tion of the views already upheld, at an earlier date, by himselfand Laurent but never stated with the same precision.53 Heperceived that Williamson's reaction for theformation of ethermight also be applied to the monobasic acids, and that theoxides or anhydrides of the latter should be obtained in thisway.54 The experiment succeeded, and thus it was reservedfor Gerhardt, who, as well as Laurent, had denied the existenceof anhydrides of monobasic acids, to disprove this view by hisown experiments. He had, it is true, previously only statedthe impossibility of withdrawing a molecule of water from onemolecule of acid, and this statement still held ; for he showedthat two molecules of a monobasic acid are always concernedin the formation of the anhydride, and the proof was furnishedby W illiamson's method. By treatment of potassium acetatewith acetyl chloride, Gerhardt obtained acetic anhydride :—

~ K J V J + C 2 H 8 OC1

and by employing benzoyl chloride he obtained the inter-mediate anhydride of benzoic and of acetic acids :—

C2H3O1O + C7H6OC1 = Q2H*Q }O + KCL

Before turning to the researches carried out jointly byGerhardt and Chiozza on the anhydrides and amides of dibasicacids, I must give an account of the conception which William-son introduced concerning these acids, whereby the stimuluswas given that led to these researches. The extension whichWilliamson gave in i85r (that is, a year after his first investi-gation on etherification) to the views already arrived at, wasan extremely important one. Even if it may perhaps be saidthat in his previous publications he leaned towards the views ofLaurent and Gerhardt, and merely confirmed these by meansof new, although certainly most decisive, experiments, he nowappears in a perfectly independent and original manner.

03 See Ms claim : Annalen. 91, 198. M Ann. Chira. [3] 37, 332.


Williamson shows how the existence of dibasic acidsdepends upon the presence of radicals with basicity greaterthan one.55 The formation of the substituted ammonias, inthe reaction discovered by Wurtz, which he formulates :—

gives him occasion to express himself as follows :—" One atomof carbonic oxide is here equivalent to 2 atoms of hydrogen,and by replacing them, holds together the 2 atoms of hydratein which they were contained, thus necessarily forming a

bibasic compound, v ' O2, carbonate of potash."

By further assuming that carbonic oxide can double itsatomic weight, without alteration of its basicity (or equiva-lence), he obtains in C202the radical of oxalic acid, and is able torepresent the formation of oxamide by means of the equation:—

The conception of sulphuric acid as a dibasic hydrate ofthe radical SO2 is highly important, and the experimentswhich Williamson carries out in support of this view are mostinteresting. Besides the known chloride SO2C!2, whichRegnault had prepared from sulphurous anhydride andchlorine,56 he succeeds in isolating also chlorosulphonic acid,by treating sulphuric acid with phosphorus pentachloride.57

Thus:

SO2 + PC15 - ^ + poc i a + HCL

H H

By means of this experiment he disproves the view of Ger-hardt, in accordance with which theformation of theanhydrideis always supposed to precede that of the chloride in the case

55 Journ. Chem. Socr 4, $5<x m Ann. Chim. [2] 69, 170; 71, 445.57 Prpc. Roy. Soc. 7, 11; Annalen. 92, 242.


of dibasic acids.58 Thus Gerhardt, conjointly with Chiozza,had published^ in June 1853 (and thus half a year before thislast paper of Williamson's appeared), investigations on thederivatives of dibasic acids, and especially on their anhydridesand chlorides, in which he thought it was shown, amongstother things, that the first action of phosphorus pentachlorideconsists in the removal of water, and that it is only in the secondstage of the reaction that a substance containing chlorine isproduced. Gerhardt and Chiozza arrived at this time, however,at very important results; they regarded thedibasicanhydrides,for the first time, as water in which both the hydrogen atomsare replaced by a single radical, and they also showed how toprepare succinyl chloride and similar chlorides. In two sub-sequent papers59 they deal with the investigation of the amidescorresponding to the polybasic acids. They show that theseare either derived from two molecules of ammonia which areheld together by the replacement, by a dibasic radical, of oneatom of hydrogen in each, or that they may be derived fromone molecule of ammonia. The amid-acids correspond tothe mixed type NH3 + H2O, which can only be produced bythe polyatomic acid radical entering the molecule ; and theearlier statement of Gerhardt that only dibasic acids couldgive rise to the formation of amid-acids is thus explained.

By these and similar experiments, but especially byadopting the ingenious conclusions that Williamson haddrawn from his investigations, Gerhardt is able to establisha complete classification of organic compounds according toa new principle,00 and he expounds this classification in thefourth volume of his excellent Hand-book.

An important point in Gerhardt's system consists in hisshowing the connection between substances of oppositecharacter by means of intermediate substances.

Unlike the dualists, he does not contrast such substancesas potash and sulphuric acid as absolutely opposite in character,

m Comptes Rendus. 36, 1050. m Ibid. 37, 86; 38, 457. °° Gerhardt,Traite de Chimie organique, 4, quatrikme partie.

216 HISTOET OF CHEMISTEY [LECT. XI.

but he connects them by means of transition compounds, andthus obtainsseries in which hearranges substances. In arrang-ing these series he makes use of two generalisations, of which,however, one does not originate with himself. In 1842,Schiel had pointed out61 that the alcohol radicals form a serieswhose separate members differ by n. CH2) and that the corre-sponding alcohols show a difference in boiling point of 18* foreach CH2, as Kopp had already proved in the case of ethyland methyl compounds.02 In 1843, Dumas showed08 that thefatty acids also possess, amongst themselves, the same differ-ence in composition. Gerhardt now employs this very strikingregularity, which, as is well known, occurs amongst very manyorganic substances, and calls the compounds which differ byn. CH2, homologous. It had been found that such compoundspossess great similarities to one another, and that theirphysical properties slowly and progressively change. Thishad especially appeared from Kopp's detailed and excellentinvestigations.64 Gerhardt establishes, further, the idea ofisologous compounds : these substances are also chemicallysimilar but their difference in composition is not n.CH2.Acetic and benzoic acids are well-known examples belongingto this class of substances.

The homologous and isologous series constitute the onepart of Gerhardt's classification ; the other part is representedby the heterologous series. All substances are referred to thelatter which can be obtained from one another by means ofsimple reactions (by double exchange); these substances areallied in their mode of formation but they are chemicallydifferen t. Gerhardt very appropriately regards this arrange-ment of the compounds as similar to a game of cards which isbased upon the colour as well as upon the value of the separatecards. Just as in the latter every card which is wanting ischaracterised by its vacant place> as of a certain value and of acertain colour, so the chief properties, the formation, and the

61 Annalen,43, 107. ffl Ibid. 41, 79. m Ibid. 45, 330. w Ibid.4*i l69 5 50, 7i; 55, 166; 64, 212 ; 92, I ; 94, 257 ; 95, 121, 307; 96,1, 153, 303; 98, 367 ; 100, 19, etc.


decomposition of the terms which are wanting in the chemicalclassification can be stated beforehand.

Gerhardt compares the members of one and the sameheterologous series (representative, therefore, of the varioushomologous and isologous series) with four very minutelystudied inorganic substances, as prototypes, viz., water, hydro-chloric acid, hydrogen, and ammonia—all compounds ofhydrogen. A substance which was to be regarded as belongingto one of these types, was necessarily capable of being con-ceived as derived from it by the replacement of hydrogenatoms by radicals. Thus Gerhardt refers alcohols, ethers,acids, anhydrides, salts^aldehydes, ketones, etc., to the watertype, and to this type there also belong the rnercaptans, sul-phides, etc. The latter really correspond to the type ofsulphuretted hydrogen, but this is merely a subdivision of thewater type. Chlorides, bromides, iodides, and cyanides arereferred to hydrochloric acid. Ammonia was the prototype ofthe amines, amides, imides, and nitriles, as well as of thecorresponding phosphorus compounds. Finally, the hydro-carbons, the alcohol radicals and the radicals containingmetals were referred to the hydrogen type, H2.

The great step had, accordingly, now been taken ; radicalshad been introduced into the mechanical types of Regnaultand of Dumas. If we look back and inquire to whom we arechiefly indebted for this excellent extension of the earlier theoryof types, the names of Laurent and of Wurtz especially deserveto be mentioned. As early as 1846, the former had referredalcohol and ether to water; three years later, Wurtz discoveredethylamine which he regarded as a substituted ammonia. Thisview met with acceptance all the more rapidly that the simi-larity between the two substances is so startling.05 I shall notomit to observe here again, that the conception of the radicalwas now adopted in the sense in which Gerhardt had defined

65 With respect to the share which Hunt took in the development ofthe theory of types, compare Silliman's Journal [2] 5, 265 ; 6, 173 ; 8, 92 j9, 65 ; also Phil. Mag. [4] 3, 392. His claim is printed in Camples Rendu®.52, 247, and the reply of Wurtz in Rupert, de Chimie pure. 3, 418.


it in 1839. Radicals were residues of compounds, i.e., theywere atomic groups which, in certain reactions, could be trans-ferred, undecomposed, from one substance to another ; theydid not, however, on this account at all require to exist in-dependently, and they were only intended to express therelation in which elements or atomic groups are replaced.66

The formulae which are thus obtained for compounds, donot indicate the arrangement of the atoms ; they are merelyreaction formulae, which recall a series of analogies. It canthus be understood how Gerhardt could imagine severalradicals and several rational formulae in the case of the samesubstance. The determination of the true constitution ofsubstances appeared to him a task not capable of accomplish-ment, since modes of formation and of decomposition canalone lead to a judgment respecting it, and the multiplicityof these does not permit of any conclusion as to the arrange-ment of the atoms. Thus, for example, barium'sulphate isformed from sulphuric acid and baryta, from sulphurous acidand barium peroxide, and besides, from barium sulphide andoxygen. The constitution of the salt might, therefore, berepresented symbolically by means of the three formulae :—

( , 3 , 5)By means of this single example Gerhardt considered himselfable to prove that all endeavours directed towards therepresentation of the arrangement of the atoms by means ofsymbols, must lead to nothing.

With Gerhardt, reactions are double decompositions ; andhere the contrast is seen between his system and the dualisticsystem, in which all compounds are conceived as formed byadditions. Gerhardt goes so far as even to assume a doubledecomposition, or as he calls it, a typical reaction, when twomolecules unite to form a single one. Thus ethylene chloride,according to him, is produced from olefiant gas in consequenceof the substituting action of chlorine. The chloride C2HBC1

66 Gerhardt,.Traits de Chimie organique. 4, 569. ® Compare p, 177.


is formed, and this remains united with the hydrochloricacid which is produced simultaneously.68

Thegeneral arrangement and thecomprehensive characterof Gerhardt's system leave nothing to be desired. Evenalthough our views have been considerably changed andcleared up since that time, and although we are compelled,from our present standpoint, to look upon the types asinsufficient, still Gerhardt's services to chemistry can neverbe questioned. Unfortunately he was not long able to con-gratulate himself on the acceptance that his admirableHand-book met with, as he died shortly after its completion.

08 Compare p. 149.

L E C T U R E XII .

MIXED TYPES—RELATION BETWEEN KOLBE'S VIEWS AND THE COPUL/EOF BERZELIUS — RADICALS CONTAINING METALS —CONJUGATEDRADICALS—KOLBE AND FRANKLAND AND THE VIEWS REGARDINGTVPES—POLYBASIC1TY AS AN EVIDENCE FOR THE ACCURACY OFTHE NEW ATOMIC WEIGHTS — DISCOVERV OF THE POLYATOMICALCOHOLS AND AMMONIAS.

I AGAIN desire to direct attention to the theory of types inthe form in which it had been established by Gerhardt. Thelatter had divided organic substances into natural families, ifI may so express myself, represented by the four types, water,hydrochloric acid, ammonia, and hydrogen, which were alsocalled by him types of double decomposition. In this con-nection it must be pointed out that Gerhardt assumes theexistence of conjugated radicals, so as to be able to includesubstitution products also in the types, and "to connect withone another several systems of double decomposition of asubstance."1 For this purpose he employed, in part at least,the same mode of regarding substances as Kolbe, an ex-position of which I have still to give.

It must now be pointed out that the conjugated compoundsare no longer considered in the sense previously stated byGerhardt; and not only has the name of the " corps copules "been changed into " corps conjuges," but the signification ofthe thing itself has been altered. The law of basicity, alreadydiscussed in detail, no longer finds any application ;2 mono-basic acids can now give rise to conjugated compounds byinteracting with neutral substances ; and to this class of con-jugated compounds there are now reckoned all the substancesproduced by substitution (acids especially), and consequently

1 Gerhardt, Traits 4, 604. 2 Compare p. 183.

LECT. XII.] HISTORY OF CHEMISTRY 221

the substances obtained by the action of chlorine, bromine,iodine, nitric acid, sulphuric acid, etc., upon organic materials.Accordingly, the conjugated radicals were, as we should nowsay, substituted radicals, and they embraced, besides, theatomic groups containing a metal, such as cacodyl, etc.

c c\ en

Whilst Gerhardt placed chloracetic acid 3fjf0, picric

acid CoH2(N02)3 j O j sulphobenzoic acid C7H4(SO2)O J Q ^ e tc>

in this class of substances, other chemists, Mendius for example,3

only cared to call substances of the latter kind conjugated ;whereas sothers still, such as Limpricht and Uslar,4 wouldhave wished to see almost all organic compounds placed inthis category. A discussion took place in connection with thissubject which ended with the introduction of the mixed typesand the abandonment of the conjugated compounds.

Gerhardt had already referred the amid-acids, which heplaces in his text-book amongst the u acides conjuges," to thetype ammonia + water 5 in 1853. Returningto this idea, but,at the same time giving it an important extension, Kekuleshows, in 1857, upon the assumption of mixed types, how adistinction between conjugated and other compounds becomesquite unnecessary.0 The possibility of this hypothesis restedupon the conception of the polybasic radicals introduced in1851 by Williamson, and by means of this conception it be-came comprehensible how two molecules, previously separate,may be united into a single one. Williamson had explained thejoining together of two molecules of water as depending uponthe nature of the radical SO2, and in this way the condensedtypes arose. , Kekule employs this view in establishing themixed types. With respect to this he expresses himself asfollows:—" A union of several molecules of the types can onlyoccur when, by the entrance of a polyatomic radical in place

8 Annalen. 103, 39. 4 Ibid. 102, 239. 8 Compare p. 215.6 Annalen. 104, 129.

222 HISTORY OF CHEMISTRY [LECT. XII.

of two or three atoms of hydrogen, a cause is furnished for theholding of these molecules together." Since an unlimitednumber of heterogeneous molecules may unite in this manner,even the most complicated compounds could be referred totypes. There was thus no longer any necessity to have re-course to conjugated compounds, and Kekule further pointsthis out:—u The so-called conjugated compounds are notcomposed in any manner different from other compounds;they can be referred in the same manner to types in whichhydrogen is replaced by radicals ; they follow the same lawswith respect to their formation and saturating capabilitiesas hold for all chemical compounds."

With a view to facilitating a better comprehension ofKekule's ideas, I give below a few of the formulae proposedby him:—

C H H C H \ HI H Q / Hofx 5 referred/H e n 5 } referred/HJ SOA referred \ H^ 2 O to X&\o c H ^ t 0 "^H"l S 0 2 { ° t 0 / H

H } 0 \ H j 0

Benzene sulphonic acid. Sulfobenzide. Nordhausen sulphuric acid.

H

HIsethionic acid. Carbyl-sulphate. Sulfobenzoic acid.

The way in which Kekule employs the reaction withpentachloride of phosphorus, in order to distinguish fromeach other the types H2 and H2O, is interesting, and I shallmention it here in passing. Kekule points out how theoxygen in water is replaced, by means of this reagent, bytwo atoms of chlorine, whereby the molecules corresponding

LECT. xn.] HISTORY OF CHEMISTRY 223

to this type are broken up, whilst those deducible from thehydrogen type are preserved :—

C 2 H f l 0 C2H5C1Ethyl sulphuric acid SO2< gives SOoClt>

C6H5 ^while benzene sulphonic acid S O 2 \ ^ gives

It was by means of the theory of mixed types (the lastconsequence of this mode of regarding substances) that Ger-hardt's system first attained that uniform character in which itdominated organic chemistry for several years. But after theidea of the types had been recognised,7 they themselves becameunnecessary. The theory of types was only a formal con-ception, which lost its importance as soon as its real teachinghad been grasped. It had been necessary, however, for theorigination of the views as to atomicity then in process ofdevelopment. Particularly active in this connection wereWilliamson, Wurtz, Odling, and especially Kekule-—that is tosay, the chemists who had already taken an important part inestablishing the theory of types. Simultaneously, however,such important services were rendered, from an altogetherdifferent side (that is, from the opponents of Gerhardt andsuccessors of Berzelius), both by means of theoretical specula-tions and of experimental investigation, that before we turnto the theory of atomicity and the views arising from it asto the mutual relations of the atoms, we shall look moreclosely at the labours of that school which had sprung upfrom the ruins of the system of Berzelius.

In doing so, I may be permitted to go a long way back andstate the facts which, in my opinion, led from the copuke ofBerzelius to the important views of Kolbe.

Magnus has shown, at the beginning of the fourth decadeof the nineteenth century,8 that the salts of ethyl sulphuric

7 Ann. Chim. [3] 44, 304 ; compare also Hunt, loc. citAnn. 27, 267.

224 HISTORY OF CHEMISTRY [LECT. xn.

acid, dried in vacuo over sulphuric acid, correspond to the oldSO MO H O [ C S

, 4 s 3O = 16]. Liebig confirmed this,10 and was thereby led to pro-nounce ethyl sulphuric acid isomeric with isetbionic acid.11

He found very essential differences, however, in the behaviourof the two acids towards potassium hydroxide. Whilst theformer acid was converted, on simply boiling with this reagent,into alcohol and potassium sulphate, the latter acid was onlydecomposed on fusion with it and gave rise to the formationof a sulphate and a sulphite. This reaction induced Liebigto assume the existence of dithionic acid in isethionic acid.Berzelius, who adopted Liebig's view, employed it inarranging into two classes the substances produced by theaction of sulphuric acid upon organic compounds.12

Kolbe, in 1844, tried to bring into harmony with theopinion of Berzelius,13 the ingenious views of Mitscherlich14

in accordance with which (following the analogy of theordinary acids) the sulpho-derivatives of the first class wereregarded as compounds of sulphuric acid, and those of thesecond class as compounds of carbonic acid. He was at thattime engaged upon an examination of the substance dis-covered by Berzelius and Marcet15 in acting with chlorineupon carbon bisulphide. He fixes its formula as CC12SO2

[C = 6, 0 = 8, S = i 6 ] and calls it sulphite of perchlorideof carbon. By treatment with potash he converts thissubstance into Chlorkohlenunterschwcfehilure (trichlor-methyl-sulphonic acid), which, in turn, is converted bymeans of the reaction of Melsens16 (that is, by the actionof nascent hydrogen) into Chlorfomiylunterschwefehiture(dichlormethyl-sulphonic acid), Chlorelay hmterschwefe failure(chlormethyl-sulphonic acid), and Mcthyluntenchwefeh&ure(methyl-sulphonic acid). Kolbe regards these compounds

9 Ann. Chim. [2] 39, I S3 J 42, 222 ; Pogg. Ann. 15, 20. w Annalen.13, 28. 1] Compare p. 131. 12 Annalen. 28, I. ls Ibid. 54, 145.14 Ibid. 9, 39; Pogg. Ann. 31, 283 ; compare also Mitscherlicb, Lehrbuch,Third Edition, 1, 107 and 586. 15 Gilb. Ann. 48, 161. m Comparep. 171.


as hyposulphuric (dithionic) acid coupled with differentradicals, and writes their formulae :—

C2C13 + S2O5 + HO Chlorkohlenunterschwefelsaure.C2HC12 + S2O5 + HO Chlorformylunterschwefelsaure.C2H2C1 + S2O5 4- HO Chlorelaylunterschwefelsaure.C2H3 + S2O5 + HO Methylunterschwefelsaure.

Kolbe succeeds in effecting the synthesis of trichloraceticacid in a similar manner, that is, by treating chloride of carbonwith chlorine in sunlight, in presence of water. In this hefinds a ground for the assumption by Berzelius of the presenceof chloride of carbon in trichloracetic acid, and thereby securesan important footing for Berzelius' whole mode of regardingthese compounds. At the same time the analogy of thesubstance discovered by Dumas, with the compounds con-taining sulphur, prepared by Kolbe, is now furnished, sincetrichloracetic acid was written, after the style of Berzelius,C2Cl8 + C2O3-i-HO. It was thus a conjugated oxalic acid,whilst the others were conjugated hyposulphuric acids.

Koibe admits, as Berzelius had done previously, the replace-ment of hydrogen by chlorine in the copula. That a substitu-tion of this kind should be possible without essential altera-tion of the properties, depended upon the assumption thatthe nature of the copula exercised only a subordinate influ-ence upon the character of the compound. Kolbe, no doubt,perceives (what Berzelius never admitted) that he therebyadopts an essential point in the theory of substitution.

It appears to me necessary to state distinctly that Kolbe,and also Frankland (who, at that time, agreed completelywith Kolbe's views), adopt the conception of a radical in itsearlier sense. They believe in the existence in compounds ofcertain atomic groups, and are, therefore, far from admitting,with Gerhardt, that different radicals may be assumed to bepresent in a substance. Both Kolbe and Frankland attack theproblem of ascertaining the constitution of compounds, and,by doing so, they essentially distinguish themselves from

P


the adherents of the theory of types who, with the excep-tion of Williamson,17 write reaction or decompositionformulae only.

With the assumption of distinct atomic groups in complexsubstances the idea of the possibility of their isolation was alsocombined, and thus we find Kolbe and Frankland, in 1848,engaged-in experiments which have for their aim the separationof radicals ;18 in particular, it appeared to Kolbe extremelydesirable to decompose acetic acid into methyl and oxalicacid, of which it was the conjugated compound. He succeedsin isolating one, at least, of the radicals19 by aid of theelectrical current. Under the influence of this agency, aceticacid splits up into methyl and carbonic acid. According toKolbe, the reaction took place in such a manner that theconjugated groups first separated from one another, and thatthe oxalic acid was then converted into carbonic acid at theexpense of the oxygen of the water ; and the simultaneousevolution of hydrogen appeared to confirm this view.

The preparation of methyl cyanide by heating ammoniumacetate with phosphoric anhydride,20 discovered a short timepreviously by Dumas, told in favour of the views of Kolbeand Frankland, and so did the conversion of the nitrilesinto the corresponding acids, which was carried out by thelatter chemists themselves.21

Upon his isolation of ethyl from ethyl iodide by means ofzinc,22 it appeared to Frankland that he had removed everydoubt as to the accuracy of his and Kolbe's mode of regardingcompounds. The ethyl theory was now to resume its oldplace, in the form stated by Liebig in 1835. According toFrankiand : " The isolation of four of the compound radicalsbelonging to the alcohol series, now excludes every doubt oftheir actual existence, and furnishes a complete and satis-

17 Journ. Chem. Soc. 4, 350; A. C. R. 16, 41. 1S Journ. Chem. Soc. 1,60;Annalen. 65, 269. 19 Journ. Chem. Soc 2, 157 ; "A. C. R. 15, 17 ; Annalen:69,257. 2° Comptes Rendus. 25, 383 and 473 ; Annalen. 64,332. * Annalen.65, 288. s2 Journ. Chem. Soc. 2, 263 ; Annalen. 71, 171. ,

LECT. XIL] HISTORY OF CHEMISTRY 227

factory proof of the correctness of the theory propounded byKane, Berzelius, and Liebig fifteen years ago." 23

This series of investigations, carried out between 1844 and1850, rehabilitated the theory of copulae. Even if it onlyappeared to be justified by the reactions in the case of asmall class of substances, still it was justified for the mostimportant compounds, and for those, in particular, which hadinfluenced Berzelius in setting up his views. Experimenthad shown that the assumption of methyl in acetic acid, ofchloride of carbon in trichloracetic acid, of ethyl in alcohol,etc., had a real foundation, and it soon appeared to be clearthat the way opened up by Kolbe and Frankland mustfurther lead to many brilliant discoveries.

While occupied with the isolation of ethylfrom ethyl iodide,Frankland discovered zinc ethyl,24 a substance which com-manded thegreatest interest on account not only of its physicalbut also of its chemical properties. After the discovery of thiscompound, the efforts of no small number of chemists weredirected towards making it available for synthetical purposes ;2G

and even although all the hopes which were based upon it werenot realised, still there are few compounds which have beenemployed in so many ways in investigations in organic chem-istry. Intimately connected with the discovery of zinc ethylis the preparation of the other organo-metallic compounds.We are indebted to Wohler2(J for the discovery of telluriumethyl; the antimony compounds were prepared by Lowig andSchweizer,27 and the tin compounds simultaneously by Frank-land28 and by Lowig;20 mercury ethyl was prepared by Frank-land,80 and aluminium ethyl was prepared by CahoursSl but

. 'a Journ. Chem. Soc. 3, 46; Annalen. 74, 63. ** Journ. Chcrn. Soc.2, 297; Annalen. 71, 213. *M Pebal and Freund, ibid. 118, I ; Wurtz,Comptes Rendus. 54, 387 ; Annalen. 123, 202 ; Rieth and Beilstein, ibid.124, 242; 126, 241 ; Alexeyeff and Beilstein, Cornptes Rendue. 58, 171 ;Rutlerow, Zeitschrift fiir Chemie. 7, 385 and 702 ; Friedel and Ladenburg,Annalen. 142, 310; Lieben, ibid. 146, 180, etc. m Ibid. 35, 111 ; 84,69. w Ibid. 75, 315. ^ Phil. Trans, 1852, 418; Annalen. 85, 332.89-Ibid. 84, 308, m Journ. Chem. Soc. 3, 324; Annalen. 77, 274.81 Ibid. 114, 227 and 354


first studied by Buckton and Odling.32 Highly importantwas the discovery of potassium and of sodium ethyl whichwas made by Wanklyn,33 whilst Friedel and Crafts34 showedhow to obtain silicon ethyl, etc.

I have intentionally referred to these compounds herebecause they exercised a distinct influence upon the furtherdevelopment of the theory of conjugated radicals. Kolbewas the first to explain correctly the nature of cacodyl; hecalls it methyl coupled with arsenic, As CH8 [C = 6] ;3r> andeven if we do not now employ the word " coupled," still wehave in other respects retained this view with regard to thesubstance, and our conception of the relation of the metalto the radical has not become much clearer.

Kolbe has a similar way of looking at other organic com-pounds ; all of them contain conjugated radicals, most ofthem with carbon as the copula. Thus, in acetic acid andthe allied compounds, he assumes the radical C2 C2H8 which,following the example of Liebig, he calls acetyl;m and hewrites:—

(C2H3fc20, HO Aldehyde,(C2H8fC2O8, HO Acetic acid,(CgH3)

/C2Cl8 Regnault's chloride of acetyl,87

| § Acetamide [C - 6, O - 8].

Although this formula for acetic acid does not differ essen-tially from that of Berzelius, still there was much that was newand valuable in the considerations underlying these symbols.For example, Kolbe now draws attention to the fact that thefour carbon equivalents of acetic acid (equivalents in Grnelin'ssense) do not really possess the same function, but that two ofthem are contained in it in the form of methyl, while the othertwo serve as a place for engaging the affinity of the oxygen.

The formulae of the other fatty acids are obtained from that32 Proc. Roy. Soc. 14, 19 ; Annalen. Supplementband 4, 109. u Proa

Roy. Soc. 9, 341; Annalen. 108, 67. 34 Ibid. 127, 31. « Ibid. 75,2ii, and 76, 1. x Compare p. 137. w Annalen. 33, 319.

LJECT. XII.] HISTORY OF CHEMISTRY 229

of acetic acid by replacing the methyl by ethyl, propyl, amyl,etc., while in benzoic acid the radical phenyl occupies theplace of the methyl. In general, Kolbe employs the so-called homologous and isologous radicals as equivalent toone another, just as Gerhardt had done.

The radical ethyl is assumed in alcohol, which Kolbewrites (C4H5)O, HO, as Liebig also did, only with differentatomic weights ; on oxidation it splits into C2H3 and C2H2,and the latter is then converted further into C2O2. Thisexplanation is complicated in comparison with the onegiven by Williamson, but still it afterwards led to importantconclusions (see p. 237).

Kolbe formulated Leblanc's monochloracetic acid,88 andDumas7 trichloracetic acid—

HO(C 2 {%})c 2 , O3 and HO(C2C13)C2, O,.

The formulse become much more complicated in the case ofthe products obtained by the action of sulphuric acid uponorganic acids, where the mode of writing them approximatesto that proposed by Dumas and Piria for the " acidesconjuge"s.>; 89 Sulphacetic acid, for example, becomes

2 H O { C 2 ] S 6 2 [ C 2 , o 8 }

Kolbe is still undecided at this time as to whether heshould admit the existence of dibasic acids, and he, therefore,retains the old formulae. Accordingly, oxalic acid is HO,C2O3, and succinic acid is HO, (C2H2)C2O3.

It is also worthy of mention that Kolbe assumes radicalscontaining oxygen in anisic and in salicylic acids, and that he,therefore, no longer agrees with Berzelius upon this point.

In addition to the conjugated metallic radicals andcarbon radicals, Kolbe also recognises radicals containingsulphur, and thus the analogy, already noticed, between the

38 Ann. China. [3] 10, 212, m Annalen. 44, 66.


ordinary and the sulpho-acids, is preserved. Thus, wehave, for example,

HO(C2C135S2, O5 HO(C2C13)C2, O3.Chlorkohlenimterschwefelsaure Chlorkohlenoxalsaure

(Trichlormethyl-sulphonic acid). (Trichloracetic acid).The paper of Kolbe referred to here forms the complete

foundation of a chemical system, from which I have only beenable to select the most important parts. In it the attempt ismade to maintain the radical theory, but the fundamentalconception of this theory has undergone important changes.Thus the capability of radicals to undergo substitution wasnow necessarily admitted, and, with this admission, the radicalsceased to occupy an exceptional position. Besides this, theconjugated radicals had been tacked on to the theory, andthese had not been by any means sharply defined.

Kolbe tries to rescue the electro-chemical theory, but he isobliged to make very important admissions to the opponentsof Berzelius. Opposite electrical conditions are still supposedto exist between the constituents of a compound ; but whichis the positive and which the negative constituent remainsundecided, simply because Kolbe assumes that the sameelement may possess different electro-chemical properties—anassumption for which justification is found in the existence ofelements in allotropic conditions. But the very admissionwhich Kolbe makes, becomes the central point of the con-troversy ; and it is only demonstrated anew that the theoryof Berzelius, in the old form, is no longer tenable.

In addition to Frankland, Kolbe had only a few specialadherents, and when the former made important changes inthe notions with respect to coupling, in 1852, Kolbe, havingregard to the facts, was obliged to modify his views. Thenew hypotheses which he advances, now approach much morenearly to the notion of types, even although his mode ofnaming and of formulating substances is peculiar to himself.As regards its fundamental principles, the system of Kolberanks below that proposed by Gerhardt, particularly becauseit does not contain any distinction between molecule, atom,


and equivalent; but still it also possesses over the lattersystem, certain advantages which are to be found especiallyin the greater importance that is attached to the formulae,and in the breaking up of the radicals containing carboninto simpler ones.

I have just pointed out that, in Kolbe's opinion, the radical(or element) with which a substance is conjugated has onlya subordinate influence upon the nature of the compound;Frankland attacks this doctrine in i852,40and he succeeds inconvincing Kolbe that it cannot be maintained.

Frankland justifies his views by reference, especially, tothe radicals containing- rnetals. In the coupling of arsenicwith methyl, the former, according to Frankland, changes itssaturating capacity. Whilst it possesses in the free state, thecapacity for uniting with five atoms of oxygen, the higheststage of oxidation of cacodyl contains only three atoms of thiselement. The remaining organo-metallic compounds giveoccasion to similar considerations, and in consequence Frank-land is led to make the following important observations:

" When the formulae of inorganic chemical compounds'areconsidered, even a superficial observer is struck with thegeneral symmetry of their construction ; the compounds ofnitrogen, phosphorus, antimony and arseaic especially exhibitthe tendency of these elements to form compounds contain-ing three or five equivs. of other elements, and it is in theseproportions that their affinities are best satisfied ; thus inthe ternal group we have NO8, NH8, Nig, NS3, PO3, PH3,PC18> SbO8, SbH3J SbCl3, AsO3, AsH8, AsCls, etc.; and inthe five-atom group, NO5, NH40, N H J , PO6, PH4I, etc.Without offering any hypothesis regarding the cause of thissymmetrical grouping of atoms, it is sufficiently evident, fromthe examples just given, that such a tendency or law prevails,and that, no matter what the character of the uniting atomsmay be, the combining power of the attracting element, if Imay be allowed the term, is always satisfied "by the samenumber of these atoms. It was probably a glimpse of the

40 Phil. Trans. 1852, 417 ; Annalen. 85, 329.


operation of this law amongst the more complex organicgroups, which led Laurent and Dumas to the enunciation ofthe theory of types ; and had not those distinguished chemistsextended their views beyond the point to which they werewell supported by then existing facts—had they not assumed,that the properties of an organic compound are dependentupon the position and not upon the nature of its single atoms,that theory would undoubtedly have contributed to t h edevelopment of the science to a still greater extent t han ithas already done ; such an assumption could only have beenmade at a time when the data upon which it was foundedwere few and imperfect; and, as the study of the phenomenaof substitution progressed, it gradually became untenable, andthefundamental principles of the electro-chemical theory againassumed their sway. The formation and examination of t h eorgano-metallic bodies promise to assist in effecting a fusionpfthe two theories which have so long divided the opinions ofchemists, and which have too hastily been considered i r re -concilable ; for, whilst it is evident that certain types of seriesof compounds exist, it is equally clear that the nature of t h ebody derived from the original type is essentially dependentupon the electro-chemical character of its single atoms, a n dnot merely upon the relative position of those atoms." 41 I t isthen pointed out, in conclusion, how " Stibethin furnishes us ,therefore, with a remarkable example of the operation of t h elaw of symmetrical combination above alluded to, and showsthat the formation of a five-atom group from one containingthree atoms, can be effected by the assimilation of two atoms,either of the same, or of opposite electro-chemical character." 42

Frankland, consequently, gives up the idea of coupling,and now regards cacodyl as sulphide of arsenic in which b o t hsulphur atoms are replaced by methyl. He had now adopted,although in a somewhat different form, the theory of types,and if he considers that he really differed from the decidedadherents of this theory, inasmuch as he did not assume, wi th

41 Phil. Trans. 1852, 441; Annalen. 85, 368. « Phil. Trans. 1852,442; Annalen. 85, 371.

LECT. xn.] HISTORY OF CHEMISTRY 233

them, " that the properties of an organic compound are de-pendent upon the position and not upon the nature of itssingle atoms," I cannot altogether agree with him in this.Since the investigations of Hofmann 43 on substituted bases,the idea of substitution was no longer held, even by Laurent,in the absolute sense in which the latter had at one timestated it.44 The chlorinated ethers previously prepared byMalaguti45 could not by any means be brought into harmonywith complete invariability of the type ; and when William-son referred ether, alcohol, and acetic acid to the water type,it was plain that he used the word type more in the senseof the mechanical than of the chemical types.

By this paper of Frankland's the first step was taken inthe approach towards one another of the heretofore separatedschools, and the way to a mutual understanding was provided.It was destined to lead to a fusion of the different opinions,out of which the theory of valency then arose. The changeof opinion on the part of Frankland was again to the supportersof the theory of types, since he brought with him novel ideaswhich were capable of being turned to excellent account. Ido not assert that they might not themselves have been ableindependently to make the last great advance—that is, thestep to the classification of the atoms according to their valence.In the way in which the development actually took place, theinfluence of Kolbe, and more particularly that of Frankland,upon the supporters of the Gerhardt-Williamson school(Wurtz, Kekule, and Odling) can hardly fail to be recognised.Both schools were required, in order to raise the significanceof the formulae to what it subsequently became ; especiallyas Williamson, the only one who desired, even at that time,to write anything more than decomposition formulae, with-drew from the further development of chemistry.

It may now be expedient to explain at once the transition,for which the way had been prepared by Frankland, from thetheory of copulae to that of types; and then, when I enter

40 Annalen. 53, I. 44 Comptes rendus par Laurent et Gerhardt, 1845.45 Annalen. 24, 40; 56, 268.

234 HISTORY OF CHEMISTRY [LECT. xit.

upon t!ic consideration of atomicity and of structural formulae,I shall only need to refer to Kolbe's views, and shall be thebetter able to point out the influence which he exercised.

It was not an easy matter for Kolbe to follow Frankland inhis most recent developments ; to assume that the affinity ofthe elements is always satisfied by the same number of atomswithout regard to their chemical character, amounted to givingup the electro-chemical theory altogether, and to admittingthat the electro-chemical nature of the elements was withoutinfluence upon the formation of compounds. Kolbe was not,at first, able to reconcile himself to this.40 In his Text-book,while recognising the premises of Frankland's arguments, heendeavours to combine these with the electro-chemical prin-ciples by means of new hypotheses ;47 and it is only in 1857

4f> Kolbe, Lehrbuch der Chemie 1854, I, 20 et seq.47 As'evidence for this statement, which Kolbe attacked as erroneous

(J. pr. Chem. [2] 23, 365), 1 quote the following passage from Kolbe'sLehrbuch. I, 23 :—" Frankland felt himself justified in concluding fromthis that in cacodyl, stibmethyl, stannethyl, etc., a real replacement ofdifferent oxygen atoms by the same number of atoms of methyl or ethyltake-; place ; in other words, that cacodylic acid is arsenic acid which con-tains two atoms of methyl in place of two atoms of oxygen, and that oxideof stannethyl must be regarded as composed according to the rational formula

i\ 0, in which the substitution of one atom of oxygen by one atom of

ethyl is evident. However little it is possible to agree with this opinion,there can still be no doubt that a regularity does prevail here. The circum-stance is perhaps deserving of attention, that, as is well known, those veryelements which stand next after potassium in the electro-chemical series—that is, the metals of the alkalies and of the alkaline earths—unite withoxygen in but few proportions ; whereas those upon the other side, such aschlorine, sulphur, nitrogen, phosphorus, etc., take up oxygen, on the con-trary, in very numerous proportions. Accordingly, when one of theseelements by virtue of its coupling with hydrogen or with ether radicals,approaches more closely to potassium in respect to its electro-chemicalcharacter and its affinities, its capacity of now uniting with fewer atoms ofoxygen than previously, in consequence of this change of position in theelectro-chemical series, may probably be found less surprising ; although itmay not by any means be explained how it comes that the number ofatoms of the copula and of oxygen is regularly increased up to a definitenumber."


that he adopts Frankland's views,48 which he further developsand turns to account, especially in organic chemistry. Heonly publishes the detailed statement of the opinions so arrivedat, in 1859, in a paper " On the natural relationship of organicto inorganic compounds,"49 which contains many new ideas.

Frankland had compared the radicals which contain metalswith the corresponding oxides. Kolbe now says that " Thechemical organic substances are wholly derivatives of inorganiccompounds, and are formed from these, directly to some extent,by extremely simple substitution processes." Carrying out asuggestion made by Liebig,50 he derives the compounds ofcarbon from carbonic acid, and those of sulphur from sulphuricacid. The experimental bases of these opinions are the workpartly of Mitscherlich/51 partly of himself (compare p. 224 etseg.), and partly also of Wanklyn,62 who had succeeded in pre-paring propionic acid from sodium ethyl and carbonic acid.

Kolbe employed at this time, as well as long afterwards,the atomic and equivalent weights of Gmelin, adopting at thesame time molecular weights for the majority of compoundsagreeable to the determinations of Gerhardt, Laurent, andWilliamson. Accordingly he writes carbonic acid C2O4, andfrom this anhydride he apparently derives the organic com-pounds such as acids, aldehydes, ketones, alcohols, etc. I say" apparently," for I shall afterwards show that it is not reallyso ; but I shall, in the first place, state Kolbe's system in theform in which he applied it.

In carbonic acid, oxygen atoms are distinguished from oneanother according to whether they are within, or outside of theradical. The formula is therefore written (C2O2)O2, carbonicoxide being regarded as the radical of carbonic acid. Whenan atom of oxygen outside the radical is replaced by hydrogenor an alcohol radical, the series of the fatty acids is obtained :—

HO, H (C2O2) O Formic acid,HO, C2H3 (C2O2) O Acetic acid, etc.

When the second oxygen atom also is replaced by an alcohol48 Annalen. IOI, 257. ^ Ibid. 113, 293. m Ibid. 58, 337- B1 Ibid.

9, 39« S2 Journ. Chem. Soc. II, 103 ; Annalen. 107, 125.


radical, a ketone is formed, and when it is replaced byhydrogen an aldehyde is formed :—

H } 2 ° 2 A l d e h Y d e ; C2

By the replacement of three atoms of oxygen by three ofhydrogen, or by two of hydrogen and a radical, the alcoholsare obtained :—

HO H3 C2, 0 Methyl alcohol.

HO c H 2 } C 2> ° Ethyl alcohol, etc.

If this mode of deriving substances is more minutelyexamined, we recognise that Kolbe's procedure is not alto-gether justified. By the replacement of an atom of oxygenin carbonic acid by hydrogen, HC2O3 is obtained, and notformic acid. Kolbe simply adopts from the dualists the errorof sometimes adding HO to the formula and sometimes omit-ting it. It is true that he states a reason for his doing so,inasmuch as the basicity of a compound (and therefore alsothe number of HO groups) is determined, according to him,by the number of oxygen atoms outside the radical. Thusnitric acid is monobasic, since it is (NO4) O ; sulphuric acidis dibasic because it contains two oxygen atoms outside theradical (S2O4) O2 ; and phosphoric acid is tribasic (PO2)O8.

Since, in accordance with this mode of regarding thematter, every atom of oxygen outside the radical carries withit one OH group, when Kolbe speaks of the replacement ofsuch an atom of oxygen, this means that O + OH = O2H issubstituted ; and, taken in this sense, the view can be sustainedin a strictly logical manner. It is necessary, however, to startfrom the hypothetical hydrated carbonic acid 2HO, (C2O2) O2.

By the replacement ofO2H by H we get HO,H(C2O2)O Formic acid,O2H „ C2H8 „ HO,(C2H3)(C2O2)O Acetic „

2O2H „ H2 „ §}C2O2 Methyl aldehyde,

2O2H „ (C2H8)2 „ c ,H 38}CA Acetone,

LKCT. xn.] HISTORY OF CHEMISTRY 237

O2H by H and of O2 by H2 HO,H3C2)O Methyl alcohol,

O2H ,,.C2H8 „ O2 „ H2 HO, r H 2 ) C 2 P Ethyl alcohol.

On the conversion of the alcohols into the correspondingacids, the two hydrogen atoms are replaced again by oxygenequivalents. Kolbe's view is now more definite than William-son's. Whereas the latter assumes the conversion of the radical

C2H5 into C2H3O [C=i2], according to Kolbe, C2£?2H is

2 3f H

p r o d u c e d from C J H . T h e d i f f e r e n c e i s i m p o r t a n t , a n d

l C 2 H s

i t l e a d s K o l b e t o f o r e s e e t h e e x i s t e n c e o f a n e w c l a s s o f

a l c o h o l s , w h i c h h e a n n o u n c e s a s f o l l o w s : — 5 3

" I f t h e u n d e r n o t e d f o r m u l a e , b y m e a n s o f w h i c h I p r e -

v i o u s l y r e p r e s e n t e d t h e r a t i o n a l c o m p o s i t i o n o f a c e t i c a c i d , a n d

o f t h e c o r r e s p o n d i n g a l d e h y d e a n d a l c o h o l , a r e i n s p e c t e d —

H O . ( C 2 H 3 ) [ C 2 O 2 ] O A c e t i c a c i d ,

C ^ | 2 O 2 ] A l d e h y d e ,

HO. { C 2 H B } C2>° Alcohol,

it will be understood, at the first glance, how it comes thatof the five hydrogen atoms in the ethyl oxide of the alcohol,only two are substituted in the oxidation of the latter andonly one in that of aldehyde. It is those atoms of hydrogenin alcohol and in aldehyde which stand by themselves thatare subject to the oxidising influences, and that presentthemselves to the oxygen as points of attack far more easilyaccessible than the other hydrogen atoms which are morefirmly held in the methyl radical.

((The above conceptions of the chemical constitution ofthe alcohols reveal to us the prospect of the discovery of newalcohols as yet unknown, as well as of a new class of substanceswhich, while closely related to the alcohols in respect to theircomposition, will also probably share many properties with

53 Annalen. 113, 305-306.


them, but must also behave differently from them in manyessential points."

These new substances can also be obtained from carbonicacid or the fatty acids by substitution.

We have :—2HO (C2O2), O2 Carbonic acid,HO C2H3 (C2O2) O Acetic „rrr\ H \ r n Alcohol with one hydrogen atom substi-

(C2H3)2J U2' U tuted by methyl (dimethyl-carbinol),

HO(C2H3)3 C2, O Alcohol with two h}'-drogen atoms substi-tuted by methyl (trimethyl-carbinol).

Kolbe goes so far as to prophesy the chemical character ofthese hypothetical substances. Thus, accordi ng to him, alcoholwith one hydrogen atom substituted by methyl must yieldacetone on oxidation, by virtue ofa reaction which is analogousto the conversion of the normal alcohols into aldehydes :—

HO C 2 H 3 } C 2 , 0 gives C2^[3}c2O2 Aldehyde,

HO C^Hg tc2 , O gives p 2 S 3 ) c 2 0 2 Acetone.

All these conjectures have been justified in the mostbrilliant manner, and consequently they have exercised aguiding influence upon the development of the considera-tions regarding constitution. For this reason, I mustreturn to them in the next lecture.

Kolbe now admits the existence of dibasic acids. Theseacids are produced, according to him, from two "atoms" ofcarbonic acid by the replacement of two oxygen atomsoutside the radical (arid therefore of twice O2H) by bivalentradicals such as ethylene, phenylene, etc.54 thus:—

Succinic acid 2HO (C4 H4) |£2§2]-02

Phthalic „ 2HO (C 1 2 H 4 ) | ^g 2 | o 2 .

54 The idea of polyatomic alcohol radicals is not due to Kolbe, but toWilliamson and Wurtz, as the accompanying development of the subjectshows.


Tribasic acids are derived in the same way, from three atomsof carbonic acid, by the replacement of three oxygen atomsby trivalent radicals.

Of the other highly interesting matters discussed in thepaper, I mention only the mode of regarding the sulpho-acids in which the analogy with the carbon acids, mentionedalready, again makes its appearance. Just as the latter maybe derived from carbonic acid, so the former may be derivedfrom sulphuric acid. We have—

2HO (S2 O4) O2 Sulphuric acid,HO (C2 H8) (S2O4) O Methylsulphonic acid,HO (C12H5) (S2O4) O Phenylsulphonic „

The dibasic sulpho-acids are produced from two atoms ofsulphuric acid :—xsr\ / / ^ u ^ /SOOA ^ Disulphometholic acid (methylene

2HO(C2H2) ^ 0 * j ° 2 disulphonic acid),•U/-1 /^ u \ /S,OA rs Disulphobenzolic acid (phenylene

2HO (C12H4) Qsjol) ° 2 disulphonic acid).

Besides these, Kolbe is acquainted with intermediate acidswhich are derived from an atom of carbonic acid and an atomof sulphuric acid ; amongst them are sulphacetic and sulpho-benzoic acids:—

2HO (C2H2) ( § § 2 ) ° 2 S u l P h a c e t i c acid>

2HO (C12H4) ( s 2 ^ 2 ) °2 Sulphobenzoic acid.

This mode of regarding them furnishes a simple explanationof the conversion, observed by Buckton and Hofmann, ofsulphacetic acid (really of acetonitrile) into disulphometholicacid by treatment with sulphuric acid.55 In this operationC2O2 is replaced by S2O4.

I cannot enter here upon the other points in this extremelyimportant paper, but must advise that a study be made of it, asit is full of clever ideas. It is true that there are views advancedin it with which I cannot agree. Thus, for example, Kolbe did

88 Annalen. 100, 1:9.

240 HISTORY OF CHEMISTRY [LKCT. xn.

not regard the conception of polyatomicity in the way in whichit had been advanced by Williamson, otherwise he could nothave inquired why amid-acids with a monobasic radical do notalso exist, if these are referred, as was done by Gerhardt andKekule, to the type NH3 -h H2O.56 There can, consequently,be no discussion as to whether Kolbe had not recognisedthe quadrivalence of carbon before Kekule. Even althoughthe former rendered unquestionable services with respect t othe origination of constitutional or, as Butlerow calls them,structural formulae, still his participation in the develop-ment of the notions as to the atomicity (valency) of theelements and radicals is not of importance, because, as Ibelieve, he did not distinguish between molecule, atom, andequivalent, and also, as follows from the foregoing, becausehe had not then grasped the idea of the part played by thepolyatomic radicals in holding the molecule together.

The doctrine of valency was possible, and was bound toensue as soon as atom and equivalent were separated fromeach other. If the atoms were not equivalent, the question asto the valency of one when referred to another most necessarilyarise. Consequently, those who first distinguished the twoideas from each other, took the first step towards considera-tions of atomicity, and Dumas, Liebig, and Laurent, mustbe mentioned in this connection. Whilst the differentvalency of the elements was recognised by means of t hephenomena of substitution, the theory of polybasic acids ledto the conception of the polyatomic radicals. Both viewsremain side by side for a long time without exercising anyimportant influence upon each other, until a fusion of the t^rotook place at the hands of Kekule ; that is, until the valencyof the radicals was explained by that of the elements.

We have already seen in the preceding lecture67 howWilliamson was led to advance the conception of the poly-atomic radicals. He employed it to explain the formation ofchemical compounds, inasmuch as the polybasic radicals

m Annalen-113, 324. ^ Compare p. 214.

LECT. xii.] HISTORY OF CHEMISTRY 24i

possess the power of holding together several atomic groups.There were but few who then understood the meaning whichlay in Williamson's words, and likewise few who recognisedthe extension that might be given to them. Amongstthese, it was Kekule in particular whose sagacity led himto perceive at once the bearing of Williamson's ideas, andwho made use of these ideas to elucidate the relations ofthioacetic acid which he had discovered in 1853.58

Kekule compares the reactions of phosphorus penta-chloride and of phosphorus pentasulphide upon acetic acid,writing :—

SC2H3O

[C=i2, O = i 6 , S = 32, etc.]

Referring to this he remarks :—u The above diagram . . .exhibits . . . the relations between the reactions obtainedwith the chlorine and with the sulphur compounds of phos-phorus. In fact, it is perceived that the decomposition is-, in itsessential features, the same; only, when the chlorides of phos-phorus are employed, the product breaks up into chlorothyl[C2H3OC1] and hydrochloric acid, . . . whereas when thesulphur compounds of phosphorus are employed, both groupsremain united, because the quantity of sulphur which isequivalent to the two atoms of chlorine is not divisible.'7

These points of evidence lead Kekule to declare in favourof the accuracy of the u new atomic weights " (Gerhardt's equi-valents). These, according to Kekule, are a better expressionof the facts than the mode of representing them previouslyin use. Even if the new formulae are adopted, and the oldequivalents retained, it cannot be perceived why phosphorussulphide produces mercaptan from alcohol whilst phosphoruschloride forms ethyl chloride and hydrochloric acid (C4H5C1and HC1) ; why the latter do not remain combined just as

68 Annalen. 90, 309.Q

242 HISTOEY OF CHEMISTRY [LECT. XI

C4H5S and HS do, etc. It is not merely a difference in thmode of representing them, but it is a real fact that an atoiof water contains two atoms of hydrogen, and only one <oxygen ; also that the quantity of chlorine equivalent tone indivisible atom of oxygen is divisible by two^ whereathe sulphur, like oxygen, is dibasic, so that one atom iequivalent to two atoms of chlorine.59

The investigations by Frankland, of the radicals containing metals, and his views as to saturating capacity (companp. 231) had an important influence upon the development othe theory of polyatomic radicals, as also had the interestin|paper on salts by Odling, and the important researches o:Eerthelot on glycerine and of Wurtz on glycols. We shalconsider these more minutely.

Odling60 makes a distinct advance by applying the ideaof polybasicity to the metals also, and by reintroducingmolecular formulae for all salts, even for those of the sesqui-oxides, for which Gerhardt had written equivalent formulae.He not only refers the polyatomic acids to condensed types,as Williamson had done before him, but he is also acquaintedwith polyatomic bases, which can be regarded in a similarway. Thus, for example, he writes :—

Oxide of Bismuth | Q 3 O ; Nitrate of Bismuth ^fysOJ1

Those metals which possess, according to Gerhardt, variousequivalent weights, now have several atomicities assigned tothem, Odling knows, for example, rnonatomic and triatomiciron, and monatomic and diatomic tin, whence he obtainsthe following formulae :—

^xide Fe2/"! °s corresponding to citric acid C *5

Ferrous Fe'l ^ . . vrnoxide F e ' j 0 » m t n c » M69 ^nnalen. 90, 314. so j o u r n > Cliem> So(^ ^ % 61 Odling indi-

cates the atomicity of the elements by means of the dashes to the right oftheir symbols.

2

LECT. xir.] HISTORY OF CHEMISTRY 243

His conception of the acids of phosphorus is likewise interest-ing. Odling writes:—

Ordinary phosphoric acid n- iO3

Pyrophosphoric acid h \o,

Metaphosphoric acid Hf^

Phosphorous acid A [O5

pjTT mHypophosphorous acid r | 0.2.

According to Odling, therefore, phosphorous acid standsto pyrophosphoric acid in the same relation as hypophos-phorous acid to metaphosphoric acid. A similar relationexists between dithionic and sulphuric acid on the one hand,and oxalic acid and carbonic acid on the other :—

CO1

Carbonic Oxalic Sulphuric Dithionicacid. acid. acid. acid.

Kay, a pupil of Williamson's,62 almost simultaneouslypublished a research (obviously suggested by his teacher)which deserves our notice. By the action of sodium ethylateupon chloroform, he had obtained an ether which he calledtribasic formic ether, and which had been produced accord-ing to the following equation:—

Williamson specially draws attention to the fact that theresidues of three molecules of alcohol are held together inthe new substance by the trivalent radical CH. This wasthe first example of a polyatomic hydrocarbon radical, andit was soon to be shown how valuable was this mode ofregarding the substance. Berthelot, occupied at the timewith the investigation of glycerine (which was completed in

62 Proc. Roy. Soc. 7, 135.

244 HISTORY OF CHEMISTRY [LECT. xri.

1854 so far as its most important parts were concerned),03

found the very important result that glycerine can unitewith acids in three different proportions. Thus :Monostearine = 1 Glycerine + 1 Stearic acid - 2 Water

= CCH8OG + 2 C30H3GO4 - 2 HO[C=6, 0 = 8]

Distearine = 1 Glycerine -I- 2 Stearic acid - 4 Water= CGH8O0 +2C36H30O4 - 4 HO(In the paper 2 HO is given.)

Tristearine =1 Glycerine+-3 Stearic acid - 6 Water= CGH8OG +3C36H36O4 - 6 HO

Monochlorhydrine = 1 Glycerine 4-1 Hydrochlo-ric acid - 2 Water

= C6H8O6 +HC1 - 2 HODichlorhydrine = 1 Glycerine + 2 Hydrochlo-

ric acid - 4 Water= CGH8OG +2HCI - 4 HO

Berthelot interprets these facts in the following manner:—*' These facts show us that glycerine exhibits the same rela-

tion to alcohol that phosphoric acid does to nitric acid. Infact, whilst nitric acid does not produce more than one seriesof neutral salts, phosphoric acid gives rise to three distinctseries of neutral salts, the ordinary phosphates, the pyro-phosphates, and the rrietaphosphates. These three series ofsalts, when decomposed by powerful acids in presence ofwater, reproduce one and the same phosphoric acid.

u Likewise, whilst alcohol only produces one series ofneutral ethers, glycerine gives rise to three distinct series ofneutral compounds. These three series, on complete de-composition in presence of water, reproduce one and thesame substance, glycerine."

This comparison between glycerine and phosphoric acid onthe one hand, and alcohol and nitric acid on the other, is ofgreat importance, even although it is unfortunately to some

68 Comptes Rendus. 38, 668; Ann. Chim. [3] 41,216; Annalen. 88,304; 92, 301.

LECT. xii.] HISTORY OF CHEMISTRY 245

extent impaired by taking account of pyrophosphoricand meta-phosphoric acids. Odling'sorthophosphoricacid^hasneitherthe same composition nor the same basicity as these two latteracids, whilst it is always the same substance, glycerine, which iscontained in the ethers prepared by Berthelot. Wurtz washappier in his view of these remarkable facts, looking, as hedid, upon glycerine as a triatomic alcohol, and representing it as

CTJ5 [Ou.G0 The compounds examined by Berthelot are

produced, according to Wurtz, by the replacement of one, two,and three hydrogen atoms by acid radicals. In this connec-tion he points out how the monatomic group C6Hr passesinto the trivalent residue C0H5 by loss of H2. [C — 6, O = 8.]

It could not escape the notice of a clever investigator likeWurtz that the existence of monatomic and of triatomicalcohols necessarily involves that-of diatomic alcohols, and heat once institutes experiments which have for their aim thepreparation of such substances. From his way of looking atthe subject he necessarily expected a diatomic radical in thestill hypothetical alcohol ; the univalent group CGH7 and thetrivalent group C0H5 rendered possible the formation of themonatomic and of the triatomic alcohols ; the homologues ofC6H6 must correspond to the diatomic alcohols.06 Thechlorides and bromides which were already well known (inpart, at least) favoured the accuracy of this view, and it wasnow simply a question of converting them into the corre-sponding hydroxy- com pounds and the object was attained.The action of the basic hydroxides did not realise the hopeswhich Wurtz had entertained, but earlier experiences nowcame to his aid. Four years previously, he had discovered areaction whereby a similar conversion could be carried*out ;67

and this reaction had subsequently proved serviceable inseveral cases.08 It was now to appear that it deserved to be

84 Phil. Mag. [4] 18, 368. eB Ann. Chim. [3] 43> 492. «6 CompareOdling, Journ. Roy. lust. 2, 62. 07 Comptes Rendus. 35, 310 ; 39, 335 ;Ann. Chim. [3] 42, 229. m Zinin, Annaten. 96, 361 ; Cahours andHofmann, ibid, 100, 356,

246 HISTORY OP CHEMISTRY [LECT. XII-

called a general method. Thus, by heating ethylene iodidewith silver acetate, Wurtz obtained an acetic ether, which,on decomposition with potash, yielded the desired alcohol:—

C.HA + * C «^f }OS-%![;0 }O4 + * Agl

In this way Wurtz succeeded in preparing glycol, the firstof the diatomic alcohols.69 He was well rewarded for thedifficulties of the investigation, for it is seldom that the dis-covery of.a single substance has exercised such an influenceupon the development of chemistry, and seldom that a singlecompound has giveri rise to such a series of elegant and usefulinvestigations as this glycol did. I may be permitted t o justifythis assertion by making some observations with respect tothe compounds which stand in the closest relation to glycol.

By the oxidation of glycol, Wurtz obtained glycollic acidand oxalic acid.70 The former was identical with the sub-stance that Horsford had prepared from glycocoU ten yearspreviously,71 the nature of which had been announced byStrecker.72 In exactly the same way lactic acid is produced

C H O Ifrom propylene glycol.78 Wurtz proposed °rj 2 \O4 as the

formula of the former, regarding it, and also glycollic acid, asdibasic acids.74 The discovery of ethylene oxide and of thepolyethylene alcohols was also of great importance. By the

H |treatment of glycol-chlorhydrine C4H4 [O2,

75 (obtained fromClJ

glycol by the action of hydrochloric acid) with potash solution,Wurtz obtained the ether of the diatomic alcohol, which stands

69 Comptes Rendus. 43, 199, 1856; compare also 43, 478 ; 45, 306 ;46, 244 ; 47, 346. 7a Ibid. 44, 1306. 71 Annalen. 60, 1. n Ibid,68,55; compare SocolofFand Strecker. 80, 38. n Comptes Rtndus. 46,1228. 74 Strecker (Annalen. 81, 247) adopted a formula for lactic acidwhich was 1 double the above, 78 Comptes Rendus. 8, 101 ; 49, 813; £0,

« 5 7

LECT. xii.J HISTORY OF CHEMISTRY 247

in the same relation to the latter as the anhydride ofsulphuric acid does to the acid :—

Ethylene oxide. Sulphuric anhydride.

Glycol. Sulphuric acid.By heating ethylene oxide with glycol, Wurtz next

prepared the polyethylene alcohols 76 which Lourenco hadobtained, a short time previously, from ethylene bromideand glycol.77 The importance of these substances wasenhanced by the acids obtained from them by oxidation ;78

they furnished excellent examples of the formation of sub-stances according to the condensed types such as Wurtzafterwards so well understood how to employ in explainingthe silicates.79 Finally, by the action of ammonia and itsanalogues, Wurtz obtained from ethylene oxide bases con-taining oxygen.80 These substances afterwards became ofgreater interest in consequence of the synthesis of neurinefrom glycol-chlorhydrine and trimethylamine.81

The adoption of ethylene as a diatomic radical furnishedHofmann 82 with a means of correctly regarding the bases pre-pared by Cloez in 1853.83 These now appeared as substancesderivable from two molecules of ammonia, and bearing thesame relation to glycol that ethylamine does to alcohol. Bythe further study of these substances,84 Hofmann succeededin adducing new proofs of the accuracy of the theory ofpolyatomic radicals, and in preparing the way to a clear con-ception of the complicated metal-ammonium compounds.

It would be unjust if I were to close these observationswithout alluding to H. L. Buffs claim with respect to the

76 Comptes Rendus. 49, 813. 77 Ibid. 49, 619. 78 Ann. Chim. [3]69, 317. 79 Rupert, de Chimie pure. 2,449; compare also Lecons dephilosophic chimique, Paris, 1864, 181. 80 Comptes Rendus. 49, 898;53, 338. 81 Ibid. 65, 1015. ® Ibid. 46, 255. ^ L'lnstitut. 1853,213; Jahresbericht. 1853, 468. 84 Proc. Roy. Soc. 10, 224 and 594;Comptes Rendus. 49, 781 ; 51, 234; Proc. Roy. Soc. II, 278; ConrptesPendus. 53, 18, etc,

248 HISTORY OF CHEMISTRY [LECT. xa.

recognition of. the bivalence of ethylene. Buff, in a pre-liminary communication, several months prior to the firstpublication by Wurtz on the subject of glycol, and in a paperlaid before the Royal Society one month prior to thispublication, had endeavoured to prove the diatomic natureof the hydrocarbons CnHn [C = 6].85 By treating ethylenechloride with potassium thiocyanate, he had obtained asubstance of the formula C4H4Cy2S4, and this, on oxidationwith nitric acid, yielded a compound identical with Bucktonand Hofmann's disulphetholic acid8C for which Buff proposesthe name ethylene sulphurous acid, representing it by

H rs2o0

the formula CJXA The formation of ethylene thio-H lS2OG.

cyanate is represented by the equation :—

r Cy [S2

C4H4C19 + 2 ~y 1S2 - C4 H J + 2 KCLj Cy"ls2

It appears from the whole paper that Buff had recognisedthe diatomic nature of ethylene, and that, in this respect,he can lay claim to priority over Wurtz. As regards theexperimental proof of this view, the two investigationsscarcely admit of comparison. The examination of theglycols by Wurtz is amongst the most brilliant achievementsof the period; and by its means the hypotheses as to thedifferent valencies of the radicals were provided with sobroad a basis that nothing more could be desired in thisrespect. Buffs experiments conformed to the same theo-retical conceptions, but they never could have led to theconclusions which were drawn from the labours of Wurtz.

85 Annalen. 96, 302 ; Proc. Roy. Soc. 8, 188, m Annalcn. 100, 129.

L E C T U R E XIII.

IDEAS REGARDING THE TYPES—ELUCIDATION OF THE NATURE OF THERADICALS BY THE VALENCY OF THE ELEMENTS—QUADRIVALENCEOF CARBON—SPECIFIC VOLUME—CONSTITUTIONAL FORMULAE—SEPARATION OF THE IDEAS OF ATOMICITY AND BASICITY—ISO-MERISM AMONGST ALCOHOLS AND ACIDS—PHYSICAL ISOMERISM—UNSATURATED SUBSTANCES.

IN the preceding lecture I showed how the views respectingthe different valencies of the radicals and elements attaineda great importance through the labours of Williamson,Frankland, Kekule", Odling, Berthelot, Buff, and Wurtz. Ishall begin this lecture by showing how the types can beexplained by means of these views.1

At the period in question Kolbe had attacked Gerhardt'smode of regarding substances as an arbitrary one.2 Wurtzendeavours to show that this is not so, and that Gerhardt's fourtypes, which, in his opinion, can be reduced to three, representdifferent states of condensation of matter. Besides the hydro-gen type H2, Wurtz also assumes the types H2H2 and H3H3.Water, H2O, formed by the replacement of H2 by O, corre-sponds to the former, while ammonia represents triplecondensed hydrogen, in which one half of this element isreplaced by trivalent nitrogen. Since all of these formulaecorrespond to the same volume of the substances in thegaseous state, the view of Wurtz is fully justified. One atomof hydrogen corresponds to one volume, to one half of avolume, or to one third of a volume, according to whether itis present in compounds which belong to the type H2, H4,or H6. Wurtz looks upon the existence of still more highly

1 Ann. Chim. [3] 44, 3°6, a Lehrbuch 4er Qhewie. j , 50,249

250 HISTORY OF CHEMISTRY [LKCT. XIII.

condensed states of matter as possible, but he does notproceed to introduce types corresponding to these states.

It was in 1857, on the occasion of the discussions respect-ing the constitution of mercury fulminate, that Kekule 3 firststated that the constitution of this substance, as well as thatof the other compounds of the methyl series, may be referredto the marsh gas type, C2H4; and to the latter type mercuryfulminate is, according to Kekule's experiments, to bereckoned as belonging.4

He therefore writes :—

C2H4 C2H3C1 C2HC13 C2(NO4)C18

Marsh gas. Methyl chloride. Chloroform. Chlorpicrin.C2(NO4)2C12 C2H3(C2N) C2(C2N)(NO4)Hg2

Marignac's Oil.6 Acetonitrile. Mercury fulminate.A beginning had thus been mad e, but still the type C2H4 was

only of very little use. So long as it could not be extended toall carbon compounds, there could be no question of making itthe basis of a system of organic chemistry such as it afterwardsactually became. The idea which rendered this possible wasstill wanting. Kekule had perhaps already conceived it at thetime, and merely did not dare to publish i t ; or it may be thatthe hypothesis of the linking of carbon atoms had not yetoccurred to him. Tn any case, the views which he then-heldmust have approximated very closely to those which he pub-lished in 1858 concerning the nature of carbon, for, at the endof 1857, when he refers the types to the different valenciesof the elements,6 he definitely mentions the quadrivalence of

3 Annalen. 101, 200. 4 [C = 6, 0 = 8.] Kekule* here employs theseatomic weights again, whereas he had discarded them as inaccurate fouryears previously, Kolbe (J. pr. Chem. [2] 23, 374) regards as importantthe fact that Kekul i then pointed out that he did not employ the wordtype " in the sense of Gerhardt's unitary theory, but in the sense in whichit was first employed by Dumas upon the occasion of his fruitful investiga-tions on the types." I consider this unimportant, more especially as Kekule"proceeds as follows:—"I shall indicate the actual relations in which thesubstances mentioned stand to one another, by saying that, under theinfluence of suitable agencies, the one can be produced from, or convertedjflto, t^e o her." 5 Annalen. 38, 16. 6 I id. IOA, 129.

LECT. xin.] HISTORY OF CHEMISTRY 251

carbon j but this is only referred to incidentally, and doesnot lead him to any further conclusions.

At length, in the spring of 1858, the paper appears whichhas become of such fundamental importance in chemistry.7

In this paper Kekule begins to direct attention to the necessityof studying the nature of the elements. This alone, in hisopinion, can lead to an explanation of the valency of theradicals. As regards organic chemistry, the chief part in suchconsiderations is played by carbon ; and, consequently, theproperties of this element are subjected by Kekule to a veryminute examination. u When the simplest compounds of thiselement are considered (marsh gas, methyl chloride, chlorideof carbon, chloroform, carbonic acid, phosgene gas, sulphideof carbon, hydrocyanic acid, etc.) it is perceived that thatquantity of carbon which chemists have recognised as thesmallest possible, that is, as an atom, always unites with fouratoms of a monatomic or with two atoms of a diatomicelement; that, in general, the sum of the chemical units ofthe elements united with one atom of carbon is four. Thisleads us to the opinion that carbon is tetratomic (or tetra-basic)." The hypothesis of the linking of carbon atoms alsoappears now, and is dealt with in a very detailed manner."In the cases of substances which contain several atoms ofcarbon, it must be assumed that at least some of the atomsare in the same way held in the compound by the affinity ofcarbon, and that the carbon atoms attach themselves to oneanother, whereby a part of the affinity of the one is naturallyengaged with an equal part of the affinity of the other.

" The simplest, and, consequently, the most probable caseof such an attachment of two carbon atoms is that in whichone unit of affinity of the one atom is united with one of theother. Of the 2 x 4 affinities of the two carbon atoms, twoare therefore employed in holding the two atoms themselvestogether ; six thus remain over which can be held by atomsof other elements.'1

7 Annaien. 196, 129.

252 HISTORY OF CHEMISTRY [LECT. XIII.

On the assumption which is here made, the number ofvalencies of other elements that can unite with n carbonatoms which are united to one another may be expressed bythe equation :—

71 ( 4 - 2 ) + 2 = 211 + 2.

It is true that this species of mutual union of carbon atomsdoes not pass with Kekule as the only one ; he draws atten-tion to the fact that in benzene and its homologues a closerunion, "the next simplest," may be assumed.

In order to indicate the standpoint which Kekule adoptedat that time, I may add that, so far as the value of formulaeis concerned, he is a follower of Gerhardt, and does not con-ceive these as representing the arrangement of the atoms,but merely as reaction formulae. Accordingly he retainsGerhardt's mode of writing them, and assumes, as the latterhad done, that several rational formulae are possible for onesubstance. Kekule is well aware that, starting from thehypothesis of quadrivalent carbon, formulae may appear in anew guise, but he avoids entering more particularly into thematter. This is comprehensible when we recollect that Kekuleonly attaches such a limited significance to formulae ; since hebelieves that the physical properties of substances can alonelead to the establishment of hypotheses regarding the arrange-ment of the atoms. These views possess all the more interestfrom the fact that it had already been stated in a very in-fluential quarter that two formulae cannot be assigned to onesubstance, and that one substance cannot be referred to dif-ferent types. In collating the results of his excellent investi-gation on the specific volumes of liquids, Kopp8 showed thatthese specific volumes can be calculated from the compositionof the substances if a certain specific volume is assigned toeach element. This volume is not the same in all cases, how-ever, but is dependent upon the part which the element playsin the compound. Thus, for example, according to Kopp,

8 Annalen. 92, 1 ; 95, 121 ; 96, I, 153, 303; 97, 374; apd especially100, I}. "

LECT. XIII.] HISTORY OF CHEMISTRY 253

two different specific volumes may be assigned to oxygen,dependent upon whether it exists within or outside theradical. Hence it would not by any means be a matter ofindifference, in calculating the specific volumes of aldehydesand of ketones, whether they were referred to the hydrogenor to the water type; whereas Gerhardt had declared bothto be admissible.9 Kopp's rule agreed with the first alter-native only. Kopp draws attention to this, and points outthat it is exactly in this way that propyl aldehyde is dis-tinguished from the isomeric allyl alcohol:—

C8H6O| C 8 H 6 | O

Propyl aldehyde. Allyl alcohol.

It may certainly be looked upon as a very important signof the times that the chemists of Gerhardt's school are nowforced, upon physical grounds, to attach greater value to theirformulas and speculations than had hitherto appeared to themto be justified. It is true that no special stimulus was nowrequired. Indeed it would appear that Couper had alreadytried to write real constitutional formulae. Assuming thequadrivalence of carbon (quite independently of Kekule)Couper had pointed out how the existence of a great numberof organic compounds might be explained. I should liketo compare this paper of Couper's10 with that of Kekule,published a short time previously, in order to show how thesetwo investigators, starting from different points of view, arriveat very similar results. Kekule, in recognising and explainingthe real essence of the types, hit upon the quadrivalence ofcarbon and the mutual union of the atoms. Couper, on theother hand, rejects the types because they do not appear tohim to satisfy the philosophical requirements which areessential to a theory. According to him, Gerhardt's systemrests upon general statements from which individual cases arededuced ; whereas he declares the opposite method to be the

9 Gerhardt, Traitd 4, 632 and 805. 10 Comptes Rendus. 46, 1157;Ann. Chim. [3] 53, 4 9-

254 HISTORY OF CHEMISTRY [LECT. XHI.

only correct one. Couper considers it necessary to study firstthe properties of the elements; and these he regards as:

1. Elective affinity,2. Degree affinity.

The latter governs the limits of combining capacity, andcoincides approximately with what we now call valency oratomicity. In his further consideration of the subject Couperconfines himself to the determination of the degree affinityof carbon, and believes that, by means of it, he can elucidateorganic compounds. There are two essential properties ofthis element which serve to characterise it : r, It combinesonly with an even number of hydrogen atoms; and 2, Itcombines with itself. The latter assertion is justified byreference to compounds containing carbon. Hydrogen,oxygen, etc., can be removed from these compounds and theirplaces can be taken by chlorine without interfering with thelinking, consequently the cause of this linking cannot besought for in the atoms capable of substitution. The maxi-mum number of atoms united to one carbon atom is four,and from this Couper obtains for what we should now callsaturated organic compounds the expression :—

?zCM4 - mM2

These observations are sufficient to enable us to under-stand Couper's formulae, of which I shall quote a few examples[0 = 1 2 , 0 - 8 ] I—I1

CH3 CH8 CH8 H8C

CO—OH CO—O °Alcohol. Acetic acid. Ether.

11 Couper makes curious hypotheses with respect to the properties of theoxygen atom, obviously in order that he may not be obliged to assume inthe formation of salts the replacement of the hydrogen by metal (and there-fore a reduction of the oxide). According to birn 0 = 8 is bivalent, but oneValence must always be satisfied by oxygen. The limit of the valency ofnitrogen is adopted as 5. (In the paper in Ann. Chim., the formulae arewritten on the basis of C=6.)


Ô—OH Ô—OH

Ô—OHGlycol. Oxalic acid. Hydrocyanic acid. Cyanic acid.

We here meet, for the first time, with constitutional for-mulae in the present sense of the term—with symbols calledforth in consequence of the recognition of the atomicity ofthe elements. In connection with these formulae it must beremarked that the views as to alcohol and acetic acid repre-sented by them, coincide with those of Kolbe,12 and that theonly differences are in the manner of writing them.

These two papers of Kekule and of Couper constitute thefoundations of our views respecting the structure of com-pounds. As a consequence of them, organic chemistry tookan altogether new direction, and they may be regarded asthe most important advances of our science, on the specula-tive side, in recent times. Adopting the doctrine of thequadrivalence of carbon, the endeavours put forth since thattime have been largely directed towards arriving at con-ceptions regarding the mutual relations of the combiningatoms; and I have now to deal with this portion of thehistorical development of chemistry.

In these considerations respecting constitution, besidesthe hypothesis as to the nature of carbon, numerous series ofexperimental data were required ; and in view of the fact thatthe accumulation of the latter was only very incomplete, thelabours of years were frequently necessary in order to renderpossible any application of the principles stated above, to thedetermination of rational formulae in the cases of certainclasses of substances. Even up to the present we have notsucceeded in completely solving this problem, as there aremany compounds still which we cannot arrange in the system.What is most important, however, has been accomplished.We are satisfied that the doctrine of atomicity can be employed

13 Compare pp. 235-236.

256 HISTORY" OF CHEMISTRY [LECT. XUL

as the foundation of an edifice, and in this connection, ourthanks are due to Kekule, who, in his excellent text-book,has furnished us with the proofs. Although Kekule has beenreproached from many sides, that in carrying out the principleshe had advanced, he does not always adhere to them faithfully—an accusation which is not altogether groundless—still Iwish to point out that any such want of adherence only tookplace in cases where the facts were insufficient at the time fora final decision, and that a quite consistent observance of theprinciples was, therefore, scarcely possible. It maybe pointedout here, however, that at the very time when many difficultiesattended the employment of structural formulae, and whenambiguities frequently arose, Butlerow13 and Erlenmeyer14

always advocated with much zeal the consistent maintenanceof the principle.

It is not the business of a historical account to followin detail the establishment of general principles- Such anaccount must be confined, rather, to developing the historyof the rise and decline of prominent ideas; whereas theenumeration of the facts, and their arrangement from onecommon view-point, constitute the sum and substance ofthe science itself, and must, consequently, be dealt with intext-books. I shall therefore content myself by adducinghere what was actually of service in strengthening thesystem, what led to new conceptions or opinions, whatappears to be irreconcilable with the principles and leads usto expect an expansion or alteration of the present theories.

I shall begin with a description of the discussion regardingthe constitution of lactic acid, which took place within theperiod 1858-60, and led to the distinction between atomicityand basicity in the case of acids. Following the lead ofGerhardt, who regarded lactic acid as a dibasic acid,16 manychemists doubled the formula for this acid and wrote it^12^12^12 [C^k, 0 = 8], whereas the interesting synthesisof alanine, and the ponversion of this compound into lactic

Trait6. I, 689.


acid by Strecker,10 made the halved formula more probable.Wurtz, in the oxidation of propylene glycol, brought forwarda decisive reason for the latter view.17 At the same timethe dibasic character of the acid also seemed to be con-firmed, so that Wurtz wrote :—

Glycol. Glycollic acid [C = 6, O = 8].

Propylene glycol. Lactic acid.The reaction with phosphorus pentachloride, which yieldedthe chloride C0H4O2Cl2 (a substance that was converted by

Q 4 ^ Oalcohol into chlorlactic ether, CJELJ 2J corresponding to

Clglycol-chlorhydrine), was a new argument in favour of thisview, whilst the vapour density of the chlorlactic etherjustified the molecular weight adopted.18

Kolbe regards lactic acid as monobasic, and calls it oxy-propionic acid, assuming the same relation between it andpropionic acid as that between oxybenzoic acid and benzoicacid.19 In the same way that Gerland was able to. convertamido-benzoic acid into oxybenzoic acid (by means of nitrousacid20), lactic acid can also be obtained from alanine. Thelatter and glycocoll were to be regarded as amido-acids—aview which found new support upon the conversion ofbromacetic acid into glycocoll by Perkin and Duppa21—sothat Kolbe was able to write :—

HO, (QH6)C2O2,0 H O . ^ g k j C A , O .

Propionic acid. Alanine.H O - ( C * { O 2

4 H ) C A . O

Lactic acid.16 Annalen. 75, 27. 17 Comptes Rendus. 45, 306. 18 Ibid. 46,

1228. 1B Annalen. 109, 257. 20 Ibid. 91, 185. 21 Ibid. 108, 106.R

258 HISTORY OF CHEMISTRY [LECT. xin.

Kolbe tries to bring the substances prepared by Wurtz intoharmony with his views, contending that lactyl chloride ischlorpropionyl chloride, which passes, by the action of alcohol,into chlorpropionic ether ; as indeed Ulrich obtains propionicether from it by means of nascent hydrogen.22 Kolbe couldalso have adduced, in favour of his ideas, the preparation ofglycollic acid from monoch lor acetic acid which Kekule hadsucceeded in effecting,23 whereas Kekule discovers in it theconversion of a monobasic into a dibasic acid.24

Wurtz now brings forward new proofs in support of theaccuracy of his view,25 finding these in the existence of thedibasic lactates which had been described by Engelhard andMadrell26and by Briining.27 Further, he succeeds in pre-paring dibasic lactic ether (by treating chlorpropionic etherwith sodium ethylate), and also lactamethan and butyro-lactic ether. The reduction of lactic acid to propionic acidby means of hydriodic acid—a reaction discovered byLautemann28—and the conversion of chlorpropionic etherinto alanine29 furnished Kolbe, on the other hand, with newgrounds for the assumption that lactic acid is a monobasicoxy-acid. Acids of this kind he defines as monobasic acidsin which a hydrogen atom within the radical Is replacedby HO2, hydrogen peroxide.30 The analogy between thecarbonic and the sulphonic acids, already alluded to severaltimes, is employed in support of the views he is now defend-ing, lactic acid being compared with isethionic acid.

Thus :—

HO(C 4 H 5 )C 2 O 2 ,O ^ J

Propionic acid. Lactic acid.

H0(C4H5)S2O4,0 ( { ^ )

Ethylsulphonic acid. Isethionic add.

22 Annalen. 109, 268. » Ibid. 105, 286; compare also RL. Hoffmann,ibid. 102, 1. * Compare also Heidelberger Jahrbudher 185B, 339,25 Comptes Rendus. 4a, 1092. * Annalen. 63, 93. » Ibid. 104, 191.28 Ibid. 113, 217. 2* Kolbe, ibid. 113, 220. 30 Ibid. 1X2, 241.

LECT. xiii.] HISTORY OF CHEMISTRY 259

In this discussion, so far as we have considered it as yet(up to 1859), Kolbe's way of regarding the matter was betteradapted to explain the facts than that of Wurtz. In particular,it was possible to explain, in a very satisfactory manner, therelations between the fatty acids and the lactic acids, as wellas the phenomena of isomerism amongst the ethers of thelatter acids, which Wurtz discovered in the following year.31

What Kolbe misunderstands, are the relations, pointed outby Wurtz, between the glycols and these acids ;32 and evenin i860, when he returns to the constitution of lactic acid,he still adopts the same standpoint.38 He emphasises thedifference between the two hydrogen atoms, replaceable byradicals, in lactic and glycollic acids ; but he does not admitthat the hydrogen peroxide groups, which they contain,also occur in the glycols.

Wurtz, meanwhile, has gone a step further. He introducesa distinction between atomicity and basicity in the case ofacids.34 Whilst the former of these is determined by thevalency of the radical present, the latter is regulated by thenumber of hydrogen atoms replaceable by metals. Accord-ing to Wurtz, " the capacity of saturation of an acid towardsbasic oxides depends not only upon the number of equivalentsof typical hydrogen which it contains, but also upon theelectro-negative nature of the oxygenated radical. In pro-portion as the oxygen increases in this radical, the typicalhydrogen becomes more and more basic hydrogen."

This is illustrated by the following example:—

Glycol. Glycollic acid. Oxalic acid. Glycerine. Glyceric acid.2 typical 2 typical 2 typical 3 typical 3 typicalH atoms. H atoms. H atoms. H atoms. H atoms.

I basic. Both basic. I

Glyceric acid is triatomic but only monobasic; phosphorousand cyanuric acids are triatomic and dibasic. Further, Wurtz

31 Ann. China. [3] 59, 161. n Compare especially Annalen. 109, 262,etc. » Ibid. 113, 306. u Bull. Soc. Chim. 1, 33 J Ann. Chim. [3] 56, 342.

260 HISTORY OF CHEMISTRY [LECT. XHI.

regards lactic acid as dibasic, and as different in constitutionfrom glycollic acid. He is forced to this by the existence ofthe lactates described by Bruning and others.

The first part of Kekule's text-book appeared in thesame year, and it was possible to see from it how easily thenature of the lactic acids might be explained by reverting,as Kekule did, to the elements themselves. For althoughhe too employs the mode of representation according totypes, still this is elucidated by means of the so-calledgraphic formulae which are intended to express the relation-ships of the atoms. These formulae constituted a new modeby which to represent the constitution of compounds. T h e yremained in use for some time, but they were again replaced,at a later date, by written formulae which approach to thoseintroduced by Couper.

The following are examples of these formulae :—

CH3 CHS CH2OH COOH

:H20H COOH CH2OH COOHAlcohol. Acetic acid. Gly col. Oxalic acid.

The relations of the atoms in glycollic acid were furnishedto Kekule by the method for its formation from chloraceticacid, which was discovered by himself. These relations can be

CH2OHrepresented by the formula I . Both glycollic acid and

COOHlactic acid contain two typical hydrogen atoms ; that is, asKekule now explains,36 two atoms of hydrogen united to thecarbon by means of the oxygen. These two atoms differ intheir properties, inasmuch as the one behaves like the typicalhydrogen of acetic acid, being influenced by two oxygen atoms,whilst the other plays a part resembling that of the typicalhydrogen in alcohol. The solution of the difficulty was now-supplied, since this view explained to the adherents of thedoctrineof atomicity all thechemical reactions of glycollic acid,

35 Kekute, Lehrbuch der Chemie. I, 130 and 174.


as well as its relation to glycol and also to acetic acid. A fewyears later, Kekule proved, by the action of hydrobromic acidon these acids,36 that they are converted by this reagent into thecorresponding bromides just as readily as the alcohols are; andthus the views which he had previously stated regarding theexistence of " alcoholic hydrogen" in these compounds receivednew support. Perkin37 had already tried to confirm thealcoholic nature of glycollic and lactic acids, from the fact thatsodium acts upon lactic ether with the evolution of hydrogen,and from the formation of ethereal compounds and the evolu-tion of hydrochloric acid when the acids are treated withacetyl chloride or succinyl chloride. These investigations ofglycollic and of lactic acids are also highly important, becauseit was in connection with them that proof was furnished of thespecially noteworthy circumstance that a two-fold functionmay be ascribed to one and the same substance, the twosets of properties in such a case being simply superadded.

Kekule was also able to explain the fact that carbonic acid,which is homologous with glycollic acid, is a dibasic acid andforms salts with two atoms of metal. The formula of the

OHhypothetical hydrate became CO ; both hydrogen atoms

OHwere equally influenced by the oxygen, and there was no reasonfor any difference between them.38

It must be specially pointed out here that Kolbe's formulafor glycollic acid possesses a great similarity to the one thathas just been given, when the signification is alone taken intoaccount and not the form ; it was due, no doubt, to his some-what more complicated mode of writing the formula thatKolbe did not deduce from it all the consequences which itinvolved. On the whole, the advantages of Kolbe's way ofregarding compounds were now about to become moremanifest. In 1862, by the addition of hydrogen to acetone,

m Annalen. 130, ir. :!7 Zeitschrift fur Chemie. 4, i6r. ™ Kekule,Lehrbuch. i, 739.

HISTORY OF CHEMISTRY [LECT. XIII,

Friedel obtained a propyl alcohol39 identical with that pre-pared by Berthelot from propylene.40 Kolbe41 at oncerecognised this as the first representative of the group ofisomeric alcohols whose existence he had foreseen.42 He

p TT a s s i g n e d t o i t t h e f o r m u l a p 2 T j 3 f C 2 O , H O , a n d m a i n t a i n e d

t h a t t h e d i s s i m i l a r i t y b e t w e e n i t a n d C h a n c e l ' s f e r m e n t a t i o n

p r o p y l a l c o h o l 4 3 w o u l d b e d e c i d e d b y a n e x p e r i m e n t i n v o l v -

i n g i t s o x i d a t i o n , s i n c e t h e n e w a l c o h o l s h o u l d t h e r e b y y i e l d

a c e t o n e . F r i e d e l a c t u a l l y p r o v e d t h a t i t d i d s o . 4 4

K o l b e r e t u r n s t o t h i s a l c o h o l t w o y e a r s l a t e r . 4 5 B y a c o m -

p a r i s o n o f t h e a m m o n i a b a s e s w i t h t h e a l c o h o l s , h e a r r i v e s a t

t h e c o n c l u s i o n t h a t c a s e s o f i s o m e r i s m m u s t o c c u r a m o n g s t

t h e l a t t e r , q u i t e s i m i l a r t o t h o s e a m o n g s t t h e f o r m e r : —

C 2 H 3 | C 2 H 3 ] C 4 H 6 1 C J H 5 " |

H V N H k j w 0 , H 0 H VN H C 2 I O , H O

H J H J - H J H J

M e t h y l a m i n e . M e t h y l c a r b i n o l . E t h y l a m i n e . E t h y l c a rb ino l .

C 2 H 3 V N C 2 H 8 V C 2 , O , H O C 2 H 3 V I S f C 2 H 3 i C 2 , O , H O

D i m e t h y l a m i n e . D i m e t h y l c a r b i n o l . T r i m e t h y l a m i n e . T r i m e t h y l ca rb ino l .

K o l b e n o w e x t e n d s h i s o b s e r v a t i o n s t o t h e a c i d s , a n d t h i n k s

t h a t h e c a n f o r e s e e t h e o c c u r r e n c e o f i s o m e r i s m i n t h i s c l a s s

o f s u b s t a n c e s a l s o . F r a n k l a n d , a t a s o m e w h a t e a r l i e r d a t e ,

p r e p a r e d l e u c i c a c i d b y t r e a t i n g o x a l i c e t h e r w i t h z i n c e t h y l , 4 6

a n d t h i s i n t e r e s t i n g s y n t h e s i s s u g g e s t e d t h e n e w i d e a s t o K o l b e .

H e r e g a r d s F r a n k l a n d ' s a c i d ( a s F r a n k l a n d h i m s e l f h a d d o n e )

a s d i e t h y l - o x y a c e t i c a c i d , a n d r e p r e s e n t s i t b y t h e f o r m u l a

C J C 4 H 5 V C 2 O 2 , O , H O . I t c o r r e s p o n d s t o d i e t h y l - a c e t i c a c i d .

39 Comptes Rendus. 55, 53 ; Annalen. 124, 324. ^ Cotnptes Rendus.44, 1350. 41 Zeitschrift fiir Chemie. 5, 687. 42 Compare p. 237.48 Annalen. 87, 127. ** Repert. de Chimie pure. 5, 247. ^ Zeitschriftfiir Chemie. 7, 30; Annalen. 132, 102. 46 Proc. Roy. Soc 12, 396 ;Annalen. 126, 109.


Kolbe is also acquainted with a dimethyl-acetic acid, whichhe calls isobutyric acid, since it is different, according tohim, from ordinary butyric acid:—

C6H7(C2O3)O,HO

Butyric acid. Isobutyric acid.

Kolbe assumes three isomeric substances having the formulaof valerianic acid : trimethyl-acetic acid, methyl-ethyl-aceticacid, and propyl-acetic acid. Isomeric derivatives corre-spond to these compounds, such, for example, as oxy-acids,amongst which Kolbe classes Stadeler's acetonic acid.47

These views were completely confirmed, and Kolbe's cleverprediction thereby achieved a great triumph. Friedel's acetonealcohol was the first compound representative of this class ofsubstances that was prepared, and it, therefore, is of greatimportance. As the constitution of acetone had been settledby Freund's synthesis, there could scarcely be any doubt as tothe formula of the new propyl alcohol ; and this formula wasemployed by Erlenmeyer 48 (after he had found that thealcohol was identical with the one prepared from glycerine49)in explaining the constitution of the triatomic alcohol.

The discovery of the " hydrates" by Wurtz50 followedimmediately after the discovery of isopropyl alcohol. Wurtzobtained these substances by treating the hydrocarbons of theethylene series with hydriodic acid and silver oxide ; and hestudied their properties particularly in the case of amylenehydrate, where he was able to recognise its difference from amylalcohol. At first he looked upon this substance as a compoundof the hydrocarbon with water, and represented it by theformula C5H10, H2O, a view which appeared to be warrantedby its ready decomposition into these substances. At a laterperiod he believed that the divergences from the normalalcohols could be explained on the assumption that the union

47 Annalen. in , 320. ^ Ibid. 139, m . 49 Zeitschrift fur Chemie. 7,642. m Comptes Rendus. 55, 370; 5^ 7*5, 793 ; 57> 479-


of the hydrogen atom in the hydrates is different from (z'.e.,less intimate than) that in the substances isomeric with them.51

Kolbe looked on these substances as likewise belonging tothe group of secondary alcohols, whose existence he had fore-seen,62 and he tries to prove this by means of an oxidationexperiment, which, however, does not furnish any decisiveresult. Wurtz,63 who also carries out this experiment, obtainsacetone as well as acetic acid. The question thus remainedundecided54 until, at a much later date (in 1878), Wischne-gradzky was able to prove that amylene hydrate belongs tothe group of tertiary alcohols.55 These had, however, beendiscovered much earlier (in 1863) by Butlerow56 as the pro-ducts of a very remarkable, complex reaction ; but a largenumber of investigations by Butlerow and his pupils wererequired in order to establish conclusively the nature ofthese substances, and their relations to the other alcohols.

With respect to isomerism in the fatty series, Erlenmeyerhad obtained isobutyric ether in 1864, by themethod proposedby Kolbe,57 but had not been able to discover any decideddifference between it and ordinary butyric ether. Morkowni-koff, however, established the difference later by a carefulstudy of the salts ;58 and proved, besides, that acetonic acid isidentical with oxy-isobutryic acid.59 The neat syntheses byFrankland and Duppa are of considerable importance in con-nection with this question. These chemists succeeded inpassing from oxalic acid to substances of the lactic acid series,and then they further converted these into the correspondingmembers of the acrylic acid series.60 Further, by means ofaceto-acetic ether, which had been discovered by Geuther,61

they were able to introduce alcohol radicals into acetic acid,

51 Zeitschrift fur Chemie. 7, 419. m Annalen. 132, 102. m ComptesRendus. 58, 971. u Ibid. 66, 1179. m Annalen. 190, 328. 58 Rupert,de Chimie pure. 5, 582 ; Bull. Soc. Chim. [2] 2, 106 ; Annalen. 144, 1.m Zeitschrift ftir Chemie. 7, 642. M Annalen. 138, 361. 59 Zeitschriftfiir Chemie. 10, 434. m Journ. Chem. Soc. 18, 133 5 22, 28 ; Phil. Trans.1866, 309; Annalen. 133, 80 ; 135, 25 ; 136, I ; 142, 1. w Jahresbericht1863, 323.

LRCT. XIII.] HISTORY OF CHEMISTRY 265

and thus to obtain homologues of the latter.62 Wislicenusafterwards followed up these reactions, and elucidated themmore fully. Numerous syntheses were carried out byWislicenus and his pupils according to this method, and thusour knowledge of the constitution of acids containing anumber of carbon atoms was very considerably advanced.

The fact established by Schorlemmer,63 that dimethyl isidentical with ethyl hydride, had a distinct value in all con-siderations with respect to constitution ; and so also hadthe recognition of the identity of carbonic ethers containingtwo different alcohol radicalsQi (a point that was doubted atfirst65). It was only after these matters had been settled thatthe similarity of the four valencies of carbon—the first thingnecessary in order to inspire confidence in the " structuralformulae " now so commonly employed—could be assumed.

It will be apparent from the researches already mentionedthat it is really a part of the business of scientific chemistry toexplain the phenomena of isomerism. Cases of isomerismoccur so frequently that even the clearest head would not bein a position to survey the facts, if these were simply enume-rated without any theoretical assumptions. Experience hasshown, however, that graphic or structural formulae are ex-tremely valuable in explaining known cases of isomerism andin helping us to foresee new ones ; and it can thus be under-stood why the efforts of chemists were more and more directedtowards establishing such formulae. It is obvious that I cannothere take notice of all these efforts, but I must give an exposi-tion of the principle that is adopted in drawing conclusionsfrom the reactions of substances with respect to their consti-tutions. This principle states that the mutual relations ofthe atoms remain unchanged during transformations, withthe exception of those which are severed, and that withrespect to the latter the atoms or groups that enter into thenew combinations re-establish similar relations. I do not

62 Annalen. 135, 217 ; 138, 204 and 328. lu Ibid. 131, 76; comparealso Carius, ibid. 131, 173; and Schoyen, ibid. 130, 233. M Rose,Annalen. 205, 227. or) J. pr. Chem. [2] 22, 3S3-


think I am mistaken in regarding this fundamental principleas a new form of Laurent's law of substitution.66 It is ageneralising of this law, but the law has acquired, at thesame time, another meaning, in so far that chemists now nolonger desire to determine the arrangement of the atoms inspace but only their relations to one another. Unfortunately,no general proof of the principle has been furnished, andexperiments have not even been carried out which aim atthis. Its accuracy, which is certainly not beyond doubt, ismerely assumed because the conclusions drawn from it haverepeatedly yielded concordant results ; that is, because theseconclusions led to identical formulae for the same substancewhichever mode of formation was considered.

Nevertheless, this concordance is not always met with.There are many cases known where the constitution deducedfrom one mode of formation does not correspond to theformula which may be derived by starting from another modeof formation, or from the decomposition products.67 In suchcases we are obliged to assume the conversion of the substanceinto an isomeric modification, in one of the reactions thathave taken place ; that is, we must hold that the principlestated above does not here apply, and that the atoms remainingin the molecule have changed their mutual relations duringone of the reactions. Cases of this kind deserve attention.They are well calculated to shake our faith in the accuracy ofthe fundamental principle, even although an attempt has beenmade to regard them as two-fold reactions, and in agreementwith this principle. A special interest attaches to thoseinvestigations that define the conditions under which suchisomeric changes—the so-called migration or wandering ofthe atoms—take place within the molecule, and to thoseactually designed to elucidate this particular matter. Asexamples of these, Hofmann's investigation of the conversion

m Compare pp. 143-144. 67 Compare, for example, Carius, Annalen,131, 172 ; Tollens, ibid. 137, 311 ; Friedel and Ladenburg, ibid. 145, 190;Linnemann and Siersch, ibid. 144, 137 ; Butlerow and Ossokin. ibid. 145,257 ; Simpson, ibid. 145. 373 ; Erlenmeyer, ibid. 145, 365, etc

user, xin.] HISTORY OF CHEMISTRY 267

of the methyl anilines into homologues of aniline68 andDemole's examination of the spontaneous oxidation ofethylene derivatives,69 may be mentioned here.

Those phenomena of isomerism, however, which cannotbe expressed or represented by means of the ordinary formulas,are of still greater importance. Examples of this nature havelong been known, and some of them were minutely studied atan early period. More recently (after the recognition of theimportance of the matter) an attempt has been made tointroduce a special method of explaining them, which is, nodoubt, connected with the theory of valency, but is also afurther development and an extension of that theory. Wemust here enter upon a more particular account of this matter.

The discovery of racemic acid, isomeric with tartaric acid,has already been mentioned in Lecture VIL (p. 118). Therecognition of the relations between these two acids forms thesubject of an investigation by Pasteur which is of fundamentalimportance for the subject now to be considered.70 Pasteurshowed that there are four isomeric tartaric acids, viz.:racemic acid, inactive tartaric acid, and right and leftrotating tartaric acids. He showed, moreover, that the twolatter acids crystallise in similar, but in oppositely built-up (enantiomorph) forms ; that they both rotate a ray ofpolarised light through equal angles, but in opposite senses ;and that when mixed in equal quantities they yield opticallyinactive racemic acid. Further, he succeeded in decompos-ing racemic acid again into the two optically active tartaricacids, by three different methods:—

1. By preparing and crystallising the sodium-ammoniumsalt; when two enantiomorph varieties were obtained. These,after having been separated and decomposed, yielded the twotartaric acids.

2. By preparing the cinchonicine and quinicine salts. In

«« Berichte.4, 742 ; 5, 704, etc. 69 Ibid. 11, 3I5» 1302, and 1307.70 Ann. Chim. [3] 24, 442; 28, 56; 38, 437; compare also Pasteur,Recherches sur la dissyme'trie moteculaire des produits organiques natu-rels. Lecons de chitnie, Paris 1861 ; Alembic Club Reprints, No. 14.

-268 HISTORY OF CHEMISTRY [LKCT. XIII.

the case of the former the salt of left tartaric acid crystallisedfirst; in that of the latter, the salt of right tartaric acidcrystallised first.

3. By treating a solution of acid ammonium racematewith spores of Penicilliiim Glaucum, by which means thesalt of left tartaric acid alone remains in the solution afterthe development of the fungus.

Pasteur prepared inactive tartaric acid by heating cin-chonine tartrate; and Dessaignes was able to show that thisacid was partially reconverted into racemic acid by heatingit to 2000.71

Facts of a similar kind have been observed in the casesof various other substances, such, for example, as theglucoses, the terpenes, amyl alcohol, aspartic acid, etc.Exactly the same relations that are observed in the case oftartaric acid occur also in that of mandelic acid, accordingto experiments by Lewkowitsch.72

In all these cases of isomerism it is in their physicalproperties that the corresponding substances differ fromeach other; and on this account Carius73 introduced forsuch cases the designation physical isomerism.

In 1874 Le Bel74 and, shortly after him, van \ Hoff7f) en-deavoured to explain these facts also upon the theory ofatomicity, having first adopted the view that a substance onlypossesses optical activity when its molecule contains anasymmetric carbon atom ; that is, when a carbon atom ispresent in it which has its four valencies satisfied by union withfour atoms or groups all different from one another. This viewis warranted by the facts, in so far that all optically activesubstances known up to the present contain at least oneasymmetric carbon atom. It must be stated, however, that byno means all substances which contain asymmetric carbonatoms possess rotating power, and that the view stated above

71 Annalen. 136, 212. 72 Berichte. 16, 1565 and 2721. ra Antialen.126, 214; 133, 130. 74 Bull. Soc. Chim. [2] 22, 337. 7S Ibid.E2] 23, 295 ; compare also La chimie dans l'espace, Rotterdam, iSfS JChemistry in Space, translated and edited by Margb, Oxford, 1891,

LFXT. xni.] HISTORY OF CHEMISTRY 269

cannot, therefore, be turned about and generalised as if thiswere so. Van Jt Hoff has endeavoured to render the matterclear by means of a geometrical conception as to the arrange-ment of the atoms in space. This cannot, however, be morefully entered into here.

A considerable time ago, Rochleder76 pointed out thatsubstances belonging to one particular class, very easilyundergo isomeric changes ; and he called these substances de-fective because they are produced by the separation of certainatoms from saturated substances. They.are now commonlycalled unsaturated compounds, and we shall consider themparticularly, as their study is a subject of great interest.

In his paper on the theory of organic compounds, Couper77

ascribed to carbon the capacity of bringing sometimes two,and sometimes all four of its units of affinity into play ; henceit was not difficult for him to explain the existence of suchcompounds as carbonic oxide, ethylene, etc. Amongst others,Wurtz78 and Kolbe79 fell in with this view. The latter derivesthe unsaturated hydrocarbons from the type of carbonic oxide,assuming in all of these substances one or several carbonatoms which are active with two affinities. He writes C2O2

"O" XJ XTCarbonic oxide, C2r.TT Ethylene, C2r n Propylene, C 2 r Hor C2 . C2H2 Acetylene.

Kekule at first tried to explain the unsaturated substancesby the assumption of a denser arrangement of the carbonatoms,80 but afterwards, in his admirable and important investi-gations of organic acids81 he appears to have been of opinionthat, in these substances, the affinities of the carbon are notfully satisfied, and that they contain free affinities or blanks.This assumption became more probable both from Kekule'sown experiments and from those of Carius,82 in accordance

76 Ber. Wien. Akad. 11, 852 ; 12, 727 ; also ibid. 49 (second part), 115.77 Ann. China. [3] 53, 469; compare p. 254. 78 Lemons de philosophicchimique. 136. 79 Kolbe, Lehrbuch der organischen Chemie. i, 738 ;2, 576. 80 Kekule", Lehrbuch der organischen Chemie. 1, 166.81 Annalen. 117, 120; Supplementband I, 129, 338; Supplementband 2,85 ; 130, I. 82 Ibid. 124, 265 ; 126, 195 ; 129, 167.


with which the substances can combine with hydrogen,chlorine, hypochlorous acid, etc. The capacity for enteringinto direct additions thus became a characteristic of the group;but it cannot be said to be really distinctive, since somesubstances which are classed as saturated also possess thiscapacity. As examples of the latter substances the aldehydesand ketones in particular may be instanced, and these aresubstances which contain oxygen wholly united to carbon. Inexplanation of the facts, the assumption is made regardingthese compounds that, by addition, the group (C = O)" passesinto (C—0)"" ; that is to say, a diatomic radical becomes atetratomic one. Later experiments of a very detailed char-acter on the unsaturated acids, by Fittig, have led to theconfirmation of the view mentioned above ;83 that is, they haveshown that the facts are best accounted for when blanks, orbivalent carbon atoms, are assumed in some compounds atleast. That it is not possible to avoid some assumption of thiskind is shown by carbonic oxide and by the group of isonitrilesor carbylamines, discovered almost simultaneously by Hof-mann 84 and by Gautier.86 The latter interesting substancesare obtained by treatment of the amines with chloroform andby the action of the alkyl iodides on silver cyanide. They areisomeric with the nitriles, and their constitution cannot be

p , first pro-posed by Gautier,86 in which R stands for a monatomic alcoholradical. If the nitrogen is assumed to be trivalent,87 thecarbon then appears as bivalent or unsaturated.

There is, besides, a class of unsaturated substances, inwhich, following Kekule's lead, a more intimate union ofthe carbon atoms is quite generally assumed. I refer to thearomatic compounds. Under this heading a number of sub-stances which stand in a close chemical relationship to certainstrongly smelling oils were formerly grouped together.

83 Aimalen. 188, 95. ** Ibid. 144, 114; 146, 107. M ComptesRendus. 65, 468; Annalen. 146, 119. M Comptes Rendus. 65, 901.87 Quinquivalent nitrogen is referred to in Lecture 1$.

LECT. XIII.] HISTORY OF CHEMISTKY 271

Kekule showed that all these substances may be regardedas derivatives of benzene, and that their chemical nature isdominated by the constitution of this hydrocarbon.88 A largenumber of earlier observations told in favour of this view, but.some synthetical investigations which had been carried out, ashort time previously by Fittig in conjunction with Tollens w

and others90 were also of importance. These chemists em-ployed a method which originated with Wurtz;111 that is,they treated mixtures of the alkyl iodides and the brominesubstitution products of aromatic hydrocarbons with sodium,whereby they succeeded in preparing homologues of thehydrocarbons in question. They were thus able to show thatmethyl-benzene, obtained from bromo-benzene and methyliodide, is identical with toluene, but that ethyl-benzene isdifferent from xylene, which, however, approaches very closelyin its character to methyl-toluene or dimethyl-benzene. 1 donot need to enter more fully here into the further results ofthese interesting researches, as they were only obtained sub-sequent to the publication of Kekule's paper, and in this paperthey were partially foreseen. On the other hand, some <»f theresults of Beilstein's researches were of fundamental import-ance with respect to the theoretical investigations now to bediscussed. Of this character was the proof, carried out Inconjunction with Reichenbach,02 that the so-called nalylic acidwhich was regarded as a benzene-carbonic acid,, isomertc withbenzoic acid,03 was simply impure benzoic acid ; and so w;wthe fact that the chloro-benzoic acids prepared up to thatperiod could be reduced in number to three.w

Benzene, as the fundamental substance in the aromatic:group, attains to quite a special significance in consequenceofkthe views of Kekule, and the latter therefore make* aspecial study of its constitution. 1 shall deal with thismatter in the next lecture.

m Annalen. 137, 129. m Ibid. 131, 303. m Ibid 136, 303, etc«Ann. Chim. [3] 44, *75- "Annalen. l^f 309. wKolt* «n*lLautemann, ibid. il$f 183 ; Kekul , Ibid. 117, i$8 ; Grless, ihki, ttj,34; Cannizzaro, ibid. Supplementband 1, 274. M BelktHn and Scfihitt,ibid. 133, 239.

L E C T U R E XIV.

THEORY OF THE AROMATIC COMPOUNDS—DETERMINATION OF POSITIONOF SUBSTITUTED ATOMS OR GROUPS—QUINONES — ARTIFICIALDYES—RING COMPOUNDS — CONSTITUTION OF THE ALKALOIDS-SYNTHESES—CONDENSATION PROCESSES.

TAKING the quadrivalence of carbon as his starting-point,Kekule points out that in the fatty compounds the carbonatoms are linked together by one valency of each.1 In thecase of benzene, the next simplest assumption is made, inaccordance with which the carbon atoms are linked togetherby one and by two valencies alternately, so as to form aclosed chain or ring. Of the twenty-four affinities of thesix carbon atoms, eighteen are employed in linking carbonto carbon, thus:—

6 6 Q—.4+ —.2 = 182 ^ 2

S i x v a l e n c i e s t h e n r e m a i n w h i c h a r e s a t i s f i e d b y t h e s i x

h y d r o g e n a t o m s o f t h e b e n z e n e . H e n c e , a c c o r d i n g t o

K e k u l e , b e n z e n e m a y b e r e p r e s e n t e d b y m e a n s o f a r e g u l a r

h e x a g o n w h o s e s i d e s a r e c o m p o s e d o f s i n g l e a n d o f d o u b l e

l i n e s a l t e r n a t e l y , t h e C H g r o u p s o c c u p y i n g t h e c o r n e r s .

T h i s c o n c e p t i o n i s d e s i g n e d t o i l l u s t r a t e , i n t h e first p l a c e ,

t h e r e l a t i v e l y g r e a t s t a b i l i t y o f b e n z e n e a s c o m p a r e d w i t h t h e

h y d r o c a r b o n s of t h e f a t t y s e r i e s , w h i c h c o n s i s t o f o p e n c a r b o n

c h a i n s w i t h , f o r t h e m o s t p a r t , s i n g l y l i n k e d c a r b o n a t o m s .

I t f u r t h e r i l l u s t r a t e s t h e f a c t , w h i c h i s o f s u c h g r e a t i m p o r t -

a n c e w i t h r e s p e c t t o t h e a r o m a t i c c o m p o u n d s , t h a t t h e s i x

h y d r o g e n a t o m s o f b e n z e n e a r e s y m m e t r i c a l l y d i s p o s e d i n

t h e m o l e c u l e — t h a t i s , t h a t t h e y a r e i d e n t i c a l i n f u n c t i o n .

1 Bull. Soc. Chim. [ 2 ] 3, 104; Annalen. 137, 1 2 9 ; Lehrbuch derorganischen Chemie, 2, 493.

272

. xiv.] HISTORY OF CHEMISTRY 273

The aromatic compounds are obtained by the replacementof these hydrogen atoms in benzene. But it follows fromthe equivalence of all six hydrogen atoms that when onlyone of them is replaced, it must be a matter of indifferencewhich of them it is j or, in other words, only one variety ofany of the mono-substitution products of benzene can exist.A view of this kind was only possible after it had beenshown that methyl-benzene is identical with toluene, andbenzoic acid with salylic acid (compare p. 271).

When two or more hydrogen atoms in benzene arereplaced, Kekule's hypothesis predicts the existence ofnumerous isomers, occasioned by differences in the relativepositions of the atoms or groups that enter the molecule ;and the number of these isomers can be determined. Thusthere are three isomers possible when two hydrogen atomsin benzene are replaced by other atoms or by radicals ; andit is immaterial whether the atoms or radicals which enterare identical or different. Of tri-substitution derivatives ofbenzene there are three possible isomeric forms when thethree substituting atoms or groups are the same, but sixwhen two of them are different from the third. Further, thehypothesis foretells the existence of three isomeric tetra-substituted benzene derivatives but only one penta- and onehexa-substituted derivative when all the substituting atomsor groups, in each case, are the same. In accordance withthe hypothesis, by the replacement of one or more of thehydrogen atoms in benzene by any given element or groupof atoms, twelve substances can be obtained, and this has beenactually accomplished in at least one case. Thus Beilsteinwas able to show that exactly twelve chlorinated benzenesexist,2 after it had been proved that the alleged existence oftwo isomeric pentachlorobenzenes 3 was a mistake.

It follows, moreover, from the constitution of benzene,that ethylbenzene must be different from the three possibledirnethylbenzenes; and, further, that by the action of chlorine

a Beilstein anJ Kurbatow, Annalen. 192, 228. 8 Ladenburg,ibid. 172, .331.

274 HISTORY OP CHEMISTRY [LECT. XIV.

or of bromine two different classes of substitution productsshould be .obtainable from toluene, these classes being char-acterised by the fact that in the one the halogen replaces ahydrogen atom of the benzene (nucleus), and in the other ahydrogen atom of the methyl (side-chain). Differences of thiskind were actually observed,4 and Beilstein 6 showed that sub-stances belonging either to the one class or to the other areproduced according to whether chlorine acts in the cold orat the boiling temperature. The members of the first ofthese groups of chlorine compounds (of which, as di-substi-tution products of benzene, three isomers exist) do notpermit any exchange of their chlorine for iodine, for cyano-gen, or for hydroxyl or other groups containing oxygen ;whereas the chlorine derivative of the other group (which,as a mono-substitution product is the only chlorine repre-sentative of the group) behaves like the chloride of analcohol radical, and can be converted, just as easily aschlorides of this kind can, into an alcohol, an ether, etc.

The two formulae—

C6H4C1(CH8) C6H6CH2C1Chlorotoluenes. Benzyl chloride.

indicate these differences, which arise, according to Kekul6,from the fact that the chlorine atom of the chlorotoluenesstands in intimate relation to the carbon (being almost entirelysurrounded by it), whereas in benzyl chloride it is combined ina manner similar to the halogen of the alkyl chlorides. Anexplanation of an exactly similar kind is now furnished forthe essentially different behaviour of the phenols and of thearomatic alcohols. Whereas in the former the hydroxylgroup replaces a hydrogen atom of benzene, in benzyl alcoholthe replaced hydrogen belongs to the methyl group :—

C6H4(OH)CH8 C6H6CH2OHCresols. Benzyl alcohol.

On oxidation, the latter alone behaves as a primary alcohol

* Fittig1, Annaleo. 136, 301; Kekule*, ibid. 137, 192. 8 Beilstein andGeitner, ibid. 139, 331.

LECT. xiv.] HISTORY OF CHEMISTRY 275

and yields an aldehyde and an acid, whilst the ethers of theformer are converted into alkyl-oxybenzoic acids C0H4(OR)CO2H.6

Kekule's views concerning the oxidation of aromatichydrocarbons into acids are very important. " I t may besaid in general that the alcohol residues (methyl, ethyl, etc.)attached as side-chains to the nucleus Co are converted, bysufficiently vigorous oxidation, into the group CO2H. Theoxidation products always contain, therefore, just as manyside-chains as the substances from which they have been pro-duced. . . . When the reactions are more moderate it ispossible, in the case of those derivatives of benzene whichcontain two or more alcohol radicals, to restrict the actionto the formation of intermediate products ; thus, one alcoholradical only is oxidised in the first place, while the otherremains unchanged. Dimethylbenzene (xylene) in this wayyields toluylic acid. . . . On more vigorous oxidation thetoluylic acid is then converted into terephthalic acid."

p TJ CH3 p TJ CHg p TT COgH^ W U J r l ^ n

8 2 2Xylene. Toluylic acid. Terephthalic acid.

It is worth while pointing out, lastly, that Kekule in thefurther elaboration of his views, cleared up the constitutionof the azo-compounds7 discovered by Mitscherlich, and,more particularly, that of the diazo-compounds discoveredand minutely investigated by Griess ;8 besides showing theconnection existing between these groups.9

These researches upon the aromatic compounds exercisedan immense influence upon chemistry. The investigation ofthese substances, which, up to that period, had been ratherneglected, was by many chemists almost exclusively worked atduring the succeeding ten years. The countless examples ofisomerism which previously rendered this branch so difficult

8 Ktirner, Zeitschrift fur Chemie. II, 326. 7 Annalen. 12, 311*B Ibid. 106, 123 ; 109, 286; 113, 334; 117, 1 ; Supplementband 1, 100;Z2X, 257, etc.; compare also Phil. Trans* 1864, 667, etc. 0 Lehrbuch*2, 703*

276 HISTORY OF CHEMISTRY [LECT\ XIV.

to investigate (since it was only possible for a few persons toobtain a real grasp of the facts) increased the attractivenessof the investigations, now that a simple explanation of thesephenomena was forthcoming. And, what is of the greatestimportance, Kekule's views were confirmed by them in themost complete manner, and did not require alteration in anyessential particulars—isolated statements at variance withthem always proving capable of very early refutation asincorrect. Moreover, these hypotheses were widened to aconsiderable extent and perfected, as a consequence of theimmense number of facts afterwards discovered.

The problem of determining the positions of the substitut-ing atoms and groups deserves to be mentioned here first.Merely referred to by Kekule,10 it was fully solved afterwards.

By determining the position in the aromatic series, weunderstand the ascertaining of the relations to one anotherand to the carbon nucleus of the atoms or groups which re-place the hydrogen in benzene. Obviously the questioncan, at the earliest, only possess any significance in the caseof the di-substitution products. The three isomers herepossible, according to Kekule, have had the distinguishingprefixes, ortho-, meta- and para- attached to their names,and the question at once arises how these are to be conceivedas regards their constitution. The first step in this connec-tion was taken by Baeyer,11 after it had been proved byFittig12 that mesitylene is a trimethylbenzene. From themode of its formation, Baeyer draws the conclusion thatthe three methyl groups are symmetrically arranged withrespect to the benzene nucleus; that is to say, that mesity-lene and isophthalic acid are meta-compounds. This hypo-thesis was afterwards proved13 by an accurate investigationof the substitution products of mesitylene, Grabe was thenable to show by detailed discussions and experiments as tothe nature of naphthalene14 that this substance and, conse-quently, phthalic acid also, must be regarded as ortho-com-

10 Annalen. 137, 174, u Hid. 140, 306. 13 Zeitbchrift fur Chemic.9, 518. u Ladenburg, Annalen. 179, 163. u Ibid. 149, 22.


pounds. Finally, it was pointed out by Ladenburg,15 takinginto account the experiments of Hiibner and Petermann,16

that terephthalic acid and para-oxybenzoic acid belong to thepara-series. A very neat and original idea with respect to thesolution of this problem originated with Korner,17 who showedthat, by the introduction of a third atomic group into thedi-substitution products containing two similar substitutingatoms or groups, three isomeric tri-substitution products arepossible when the original substance belongs to the meta-series, or two when it is an ortho-compound, while in thecase of a para-derivative, only a single tri-substitution pro-duct is possible. By employing this method he determinedthe constitution of the dibromo-benzenes, and Griess18

determined that of the phenylene diamines.After the constitution had thus been determined in some

compounds, it was still necessary to establish the relationsbetween these and other compounds by means of simple reac-tions, so as to have the problem solved in the cases of all thedoubly substituted benzenes. Not only has this been quitepossible, but the position of the substituting groups has alsobeen ascertained in the higher substitution products. In thewhole of these often very extensive investigations, which wereonly practicable by the co-operation of many hands, the reac-tions of Griess (see above) rendered very important services.

It is likewise of considerable importance for the theory ofaromatic compounds that, starting from the quadrivalence ofcarbon and a series of accurately determined facts, it hasproved possible to establish the two fundamental principlesas to the constitution of benzene, as follow:—i. Theequivalence of the hydrogen atoms of benzene ; and 2. Thesymmetry of two pairs of hydrogen atoms in benzene, withrespect to the third pair of hydrogen atoms.19

It must further be pointed out here that a prolonged con-10 Berichte. 2, 140. 10 Annalen. 149, 129. 17 Gazzetta Chimica

Italiana. 4, 30$; journ. Chem. Soc. 29, 204. 18 Berichte. % 1226.19 Ladenburg, Theorie der aromatischen Verbindungen, Braunschweig1876 j Berichte. 10, T224; Wrobjewsky, Annalen. 192, 196,

278 HISTORY OF CHEMISTRY [LECT. XIV.

troversy arose concerning the formula of benzene (that is, asto the mutual linkings of the carbon atoms which it contains)after attention had been drawn to the fact that Kekule's for-mula does not altogether take account of the requirementsinvolved in the two principles stated above.20 From this con-troversy it appeared that only the so-called prism formula cangive a clear idea of the bearings of isomerism in the aromaticseries, since it furnishes likewise an accurate expression forthe thermal relations, according to Thomsen, and for themolecular volume of benzene and its derivatives, accordingto R. Schiff.21 Nevertheless Kekule's hexagon formula hasbeen generally retained, because it is superior to the otherformula in many respects.

Amongst the notable researches which were instigated byKekule's investigations, I only enter into detail here respectinga single one which may probably be looked upon as the mostimportant amongst them. I refer to Grabe's examination ofthe quinones.

Kekule propounded a peculiar view respecting quinone,22

a substance which had been discovered by Woskresensky.23

This substance was supposed to consist of an open chain ofsix carbon atoms, which were joined to one another by singleand double linking alternately. In opposition to this viewGrabe24 advanced another, in accordance with which quinoneis a benzene derivative in which two hydrogen atoms are re-placed by two oxygen atoms ; and these latter are furtherunited to each other. He bases this view especially uponthe already well-known relations of quinone to hydroquinone,and upon the conversion of chloranil into hexachlorobenzeneby means of phosphorus pentachloride. These grounds wereso convincing that Grabe's view was generally adopted, even

20 Ladenburg, Berichte. 2, 140. 21 Ibid. 13, 1808 ; see also Thomsen,Thermochemische Untersuchungen. 4; also Schiff, Annalen. 220, 303.According to SchrGder (Wiedem. Ann. 15, 667) this also holds for themolecular refraction ; whereas, according to Bruhl (Annalen. 200, 229),the opposite is the case. ' Annalen. 137, 134. (® Ibid. 27, 268,* Ibid. 146, 1.

LECT. xiv.] HISTORY OF CHEMTSTRY 279

although it appeared soon after that quinone did not belongto the ortho-compounds, as Grabe supposed, but to thepara-compounds.25 Grabe afterwards studied other quinonesalso, and so arrived at the investigation of alizarine, thenature of which as a quinone he desired to establish. Inconjunction with Liebermann, and by making use of a methoddiscovered by Baeyer,26 he showed that alizarine was not, aswas then supposed, a naphthalene derivative, but that it wasderived from anthracene ;27 that it was a quinone ; and that,in particular, it was a dioxy-anthraquinone. These chemistsafterwards accomplished the synthesis of this valuable colour-ing matter,28 which was at once prepared technically accord-ing to a method elaborated by Grabe, Liebermann, andCaro ;29 thus leading to one of the most extensive industriesof the present time.

It may be stated generally that the theory of the aromaticcompounds had a great influence in technology and especiallyin that of dyes. Although the aniline colour industry wascalled into existence quite independently of these investiga-tions (especially by Hofmann's comprehensive researches onaniline, and the bases homologous with it), and although thefirst aniline colours had been discovered and turned to accountlong prior to the publication of Kekule's celebrated paper—mauveine by Perkin30 as early as 1S56, and fuchsine byVerguin31 in 1859, after it had been previously observed byNatanson,82 Hofmann,83 and others-—still its further develop-ment is intimately connected with the more accurate insightinto the constitution of the aromatic compounds. Withrespect to this, it is only necessary to recall the discovery oforthotoluidine by Rosenstiehl,34 and the explanation of the

<2S Petersen, Berichte. 6, 368 and 400. <26 Annalen. 140, 295.27 Berichte. 1, 49. m Annalen. Supplementband 7, 257; Berichte.2, 14. <29 Ibid. 3, 359. *° Perkin, Zeitschrift fur Chernie. 4, 700;Annalen. 131, 2or. B1 Repert. de Chimie applique'e. 2, 114, 299;compare also Dingl. Polyt. Journ. 154, 235, 397. M Annalen. 98,297. *» Jahresbericht 1858, 351. M Zeitschrift fiir Chernie. 11, 557 ;12, 189-190.


chemical nature of rosaniline, which was eventually furnishedby E. and O. Fischer,35 the way having been prepared byHofmann.86

The manufacture of other classes of dyes has also arisenindependently of this theory, although no doubt advancedby means of it. Examples of these substances are thephenol dyes, of which the first representative is rosolic acid,discovered by Kolbeand Schmitt37 and simultaneously by J.Persoz; s s and this group was greatly enlarged by thephthaleines, discovered and studied by Baeyer.39 Otherexamples are the azo-dyes, which are, almost without excep-tion, connected with the important researches of Griess.

The influence which Kekule's conception of the aromaticcompounds exercised upon the views concerning the morecomplicated hydrocarbons is much more direct.

Erlenmeyer,40 in an interesting paper on aromatic acids,which contains a criticism of Kekule's views, assigns tonaphthalene, C10H8, the formula:—

H H H H

C = C—C = C—C-CL | | J

H — C C — HII II

H — C — C — H

I n a c c o r d a n c e wi th th i s formula , n a p h t h a l e n e could be con-

ce ived as composed of t w o b e n z e n e h e x a g o n s w i t h t w o ca rbon

a t o m s c o m m o n to b o t h of t h e m . G r a b e r e n d e r e d th i s concep-

t i o n v e r y p robab le b y m e a n s of e x p e r i m e n t a l inves t iga t ions a n d

t h e o r e t i c a l cons idera t ions . 4 1 A r o n h e i m's syn thes i s of n a p h t h a -

l e n e from p h e n y l b u t y l e n e 4 2 also tells in s u p p o r t of i t , a n d so,espec ia l ly , does F i t t i g ' s syn thes i s of ^ - n a p h t h o l 4 3 ( the h e x a g o n

35 Annalen. 194, 242. m J. pr. Chem. 87, 226; Jahresbericht 1863,417; 1864, 819; Annalem 132, 160 and 289. ^ Ibid. 119, 169.88 French Patent, 21st July 1862. m Annalen. 183, I j 202, 36.40 Ibid. 137, 327. 41 Ibid. 149, 1. 42 Ibid. 171, 233. « Fittig andErdmann, Berichte. 16, 43 ; Annalen. 227, 242.


formula for benzene being assumed). This view concerningnaphthalene leads to the assumption of two isomeric mono-substitution products. Faraday had, in fact, prepared twonaphthalene mono-sulphonic acids,44 and many similar caseshave been observed since then. It has even proved possiblein the cases of naphthalene derivatives to carry out deter-minations of the positions of the atoms with a very highdegree of probability ; and here, also, after a detailed studyof the naphthalene series, a marked agreement between factand theory is observed.45

Anthracene, the starting-point in the preparation of so manyinteresting compounds and valuable dyes, was early recognisedas a closed carbon chain, and as " a nucleus derivable frombenzene." Grabe and Liebermann,4C in their first communica-tion on the connection between alizarine and anthracene, pro-posed a formula for the latter, in accordance with which it isrepresented as tribenzene ; that is, as made up from threemolecules of benzene and as having four of its carbon atomscommon to two different hexagons. Besides this, in theirdetailed paper, they afterwards advanced another similarformula for anthracene, which, however, seemed to them tobe less probable. After the discovery of phenanthrene (whichis isomeric with anthracene) and the special study of it, forwhich we are indebted to the almost simultaneous investiga-tions of Grabe and Glaser 47 and of Fittig and Ostermeyer,48

the first anthracene formula was recognised as representingphenanthrene and the second one was retained for anthracene.The latter formula meets all requirements, and this is amatter which is really astonishing, in view of the numerousisomerisms in the anthracene group. It is also capable ofgiving a clear representation of the neat syntheses of anthra-quinone, alizarine, quinizarine, and purpurine, effected by

44 Phil Trans. 1826, 140; Ann. Chim. [2] 34, 164. 45 Compareespecially Reverdin and Nolting, Ucber die Constitution des Naphtalins,Genf 1880; also Liebermann and Dittler, Annalen. 183, 228. #> Bericbte.1, 49. 47 Ibid. 5, 861 and 968 ; Annalen. 167, 131. 4fJ Berichte.5, 933 ; Annalen. 166, 36r • compare also Hayduck, ibid. 167, 177*


Kekule and Franchimont,49 by Baeyer and Caro,50 and byPiccard,51 since it brings out plainly the relations betweenphthalic anhydride and anthraquinone.

Further, the constitutions of fluorene, fluoranthene, chry-sene, and retene, as well as their relations to benzene, are nowcleared up. Fluorene, C13Hao, discovered by Berthelot,52 isobtained by Fittig53by the distillation of diphenylketonewithzinc dust, and is thus recognised as diphenylene methane,

C•<SH4/)CH2. Fluoranthene (idryl), ClnH10, isolated by

Goldschmiedt54 from a semi-solid by-product obtained duringthe distillation of the ores of mercury at Idria, and also occur-ring in coal tar,55 is probably represented by the formula—

CGH4—CH

/ CH/ I'

gHg—Crl

Chrysene, C18H1Q> is recognised, from a synthesis of it effec-C0H4 —CH

ted by Grabe,f)(5 as a naphthalene-phenanthrene | IIC1OHC—CH

that is to say, as phenanthrene in which one of the phenylenegroups is replaced by a naphthalene group. Finally, retene,C18H18, according to the investigations of Bamberger andHooker/'7 is a methyl-propyl-phenanthrene—

CH-CCH4

I

Hence, it can scarcely be doubted that the other hydro-

49 Berichte. 5, 908. M Ibid. 7, 972 ; 8, 152. 51 Ibid. 7, 1785.m Annalen. Supplementband 5, 371. w Berichte. 6, 187; Annalen.193) X34« M Berichte. 10, 2022. 55 Fittig and Gebhard, ibid.10, 2143; Annalen. 193, 142; Fittig and Liepmann, ibid. 200, 1.W Berichte, 12, 1078. 57 Ibid. i§, 1024 and 1750 ; Annalen. 229, 102,


carbons wi th h i g h molecular weights, which are not so fullyexamined as ye t (such as pyrene, picene, etc.), may bederived from, benzene in an analogous manner.

Some o t h e r investigations in which a relationship can berecognised be tween certain compounds containing nitrogen—especially t h e alkaloids—and benzene, appear to be probablystill more i m p o r t a n t than the foregoing. This branch of thesubject, only opened up a few years ago, already presents somany remarkab le results that it cannot be omitted here.

T h e analogy of t he formulae of benzene, C6H6, and naph-thalene C 1 0H 8 , on the one hand, with those of pyridine,C5H5N, and quinol ine, C 9H rN, on the other, admitted of thehypothesis t h a t the latter compounds might be derived fromthe former b y the replacement in each of a CH group by N,and consequently the following formulae were advanced forpyridine arid quinoline.

CH

CH ¥Pyridine. Quinoline.

This v i ew was made known by means of private com-munications by Korner, and is usually known as Korner'shypothesis. I t was first published by Dewar.58

A large n u m b e r of facts can now be adduced in supportof the view, and the more important of these may bementioned h e r e .

Anderson , the discoverer of pyridine, had already found inanimal oil, besides pyridine, a number of homologous bases.59

T h e further examination of bone tar has yielded, as yet, onlymethyl pyridines,6 0 jus t as methyl benzenes only are con-

m Chem. News. 23, 38 ; Zeitschrift fur Chemie. 14, 117, 59 Annalen.60, S6; 70, 32 ; 75, 80; 80, 44 ; 94, 358 ; see also Unverdorben, Pogg.Ann. 11, 59. wWeidel, Berichte, 13, 19895 Ladenburg and Roth,ibid. 18, 47 and 913.


tained in coal tar. Ethyl and propyl pyridines are alreadyknown, however.61 On oxidation, these bases behave ex-actly like the alkyl derivatives of benzene ; that is to say,every side-chain, by sufficiently energetic oxidation, yields aCO2H group ; so that, in this case also, conclusions may bedrawn as to the number of side-chains in the base oxidised,from the basicity of the acid produced.

The isomerisms amongst the derivatives of pyridine arefar more complicated that in those of benzene, since thehydrogen atoms are not similarly related to the pyridinenucleus and three different mono-substitution productsmust exist, as was probably first pointed out by Weidel.62

This conclusion is likewise confirmed by experiment, as threemono-carbonic acids,03 three methyl-pyridines 64 and threeethyl-pyridines05 are known. Determinations of the positionsof the substituting atoms or groups in the pyridine serieshave been apcomplished with tolerable certainty by Skraup.66

As additional supports for the pyridine formula, theremay also be adduced the synthesis of pyridine by Ramsay 67

(which is upon the same lines as the famous synthesis ofbenzene from acetylene by Berthelot68) as well as thesyntheses of pyridine derivatives.69 Finally the conversionof pyridine derivatives into benzene- derivativesT0 is ofimportance in this connection.

The formula for quinoline is also based upon numeroussyntheses, of which that by Konigs71 may be mentioned hereas the first. This was followed by that of Baeyer 72 and then

61 Williams, Jahresbericht 1855, 549 ; 1864, 437 ; Cahours and Etard,Comptes Rendus. 92, 1079 • Ladenburg, Berichte, 16, 2059 ; 17, 772 and1121 ; 18, 1587 ; Hofmann, ibid. 17, 825. & Ibid. 12, 2012. ^Huber,Annalen. 141, 271; Berichte. 3, 849 ; Weidel, ibid. 12, 1989 ; Skraup, ibid.12,2331. M Weidel, ibid. 12, 1989; Behrmann and Hofmann, ibid.17, 2681. m Wischnegradsky, ibid. 12, 1480 ; Ladenburg, ibid. 16,2059. m Skraup and Cobenzl, Monatshefte. 4, 450 ; compare alsoLadenburg, Berichte, 18, 2967. ^ Ibid. 10, 736. ^ Ann. Chim. [4]9, 469. m Hantzsch, Annalen. 215, I ; Pechmann and Welsh, Berichte.17, 2384; Behrmann and Hofmann, ibid. 17, 2681. 70 Ladenburg, ibid,|$, 2059. ™ Ibid. 12, 453- n Ibid. J3, 460,


by the one of Skraup '3 which has become so important forthe whole group. This latter synthesis depends upon thecarrying out of one of the ideas indicated by Grabe.74

Quinoline, also, is the starting-point for a large numberof compounds, which are formed from it and can be con-verted into it just as benzene passes into aromatic com-pounds and can be obtained from them.

The relations between pyridine and quinoline, which arequite analogous to those between benzene and naphthalene,are also worthy of mention. In the same way that thelatter is converted by oxidation into benzene-ortho-di-carbonic acid (phthalic acid75), so quinoline, according toHoogewerff and Van Dorp,70 is converted by oxidation intoan ortho- (a/?-) pyridine-dicarbonic acid.

But what is of the greatest significance is the fact that themost important alkaloids are derivatives of pyridine and ofquinoline (or of their hydrogenised derivatives) in the sameway that the aromatic oils are derivates of benzene. Thefirst fact bearing upon this relationship was found out byGerhardt in 1842, when he discovered quinoline as a productof the decomposition of quinine, of cinchonine, and ofstrychnine.77 Huber obtained in 1867, by the oxidation ofnicotine,78 an acid C6H5NO2 which he recognised, threeyears later, as pyridine-carbonic acid79—a fact which was atfirst disputed and then confirmed.80 Piperidine, which wasdiscovered by Wertheim and Rochleder by the decomposi-tion of piperine,81 and the correct formula of which wasestablished by Cahours 82 and by Anderson,83 was regardedby Hofmann as a hydrogen addition product of pyridine,84

a view which was proved to be correct by Konigs andothers.85

7:} Monatshefte. I, 317 ; 2, 141. 74 Annalen. 201, 333- ,75 Laurent,ibid. 19, 38 *, 41, 98. 76 Berichte. 12, 747. 77 Annalen. 42, 310 ;44, 279. ro Ibid. 141, 271. 79 Berichte. 3, 849. 80 Weidel, Annalen.145, 328 ; and Laiblin, ibid. 196, 129. 81 Ibid. 54, 254 ; 70, 58. »Ibid.84, 342. m Ibid. 84, 345. m Berichte. 12, 984. & Ibid. 12, 2341 ;Schotten, ibid. 15, 421 ; Hofmann, ibid. 16, 586 ; Ladenburg, ibid. 17,l$6> 388 ; Ladenburg and Roth. 17, 513.


These and other facts, which seemed to place beyonddoubt the relations of several natural bases to pyridine,86 ledWischnegradsky to the opinion stated above regarding theconstitution of the alkaloids ;8r and this was more fully dis-cussed and established, a year later, by Konigs.88 Sincethen, this view has gained ground more and more ; especi-ally as a series of facts have been discovered in support of it.Thus Weidel obtained a pyridine-tricarbonic acid by theoxidation of berberine ;80 Gerichten was able to preparepyridine-dicarbonic acid from narcotine,90 and Ladenburgdibromo-pyridine from atropine ;91 while Hofmann con-verted coniine into propyl-pyridine.92

The results which attended this view of pyridine and ofquinoline, and the recognition which they met with, led tothe introduction of a similar view concerning many othersubstances. In the first place it is necessary to consider theformula which was assigned as early as 1869, by Baeyer andEmmerling,93 to indol,94 the starting-point for most of theindigo derivatives:—

According tcrthis formula indol is represented as a doublenucleus resembling naphthalene and quinoline. This mode ofrepresenting it acquired greater significance when Baeyer andEmmerling,95 somewhat later, regarded pyrrol also as a u ring."The same relation was now assumed between pyrrol and

m Weidel, Annalen. 173, 76 ; Ramsay and Dobbie, Berichte. II, 324.87Ibid» 12, 1506; compare also Ladenburg, ibid. 12, 947. m Studienilber die Alkaloide, Munich 1880. 89 Berichte. 12, 410. m Annalen.2io, 101. 91 Ibid. 217, 148. m Berichte. 17, 825. n Ibid. 2,679. u Baeyer, ibid. 1, 17, m Ibid. 3, 517.

LECT. xxv.] HISTORY OF CHEMISTRY 287

indol as between pyridine and quinoline or between benzeneand naphthalene :

HC

HC

CH

CH

Pyrrol.

The analogues are :-

Benzene.

C5H,N

Pyridine.

Pyrrol.

C10H8

Naphthalene.

C9HrNQuinoline.

CSH7NIndol.

A t the same time, te t ra-phenol% (furfuran), C4H4O, dis-covered by Limpricht, was represented by a formula an-alogous to that for pyrrol, t he N H group of the latter beingregarded as replaced by O. Related to furfuran, there isthiophen, C4H4S,07 (discovered more recently by V. Meyer),which is looked upon as thiofurfuran, and which has alreadybecome of great importance on account of its numerousderivatives. In the case of thiophen the resemblance ex-hibited by it and its derivatives to benzene and the benzenederivatives is particularly noteworthy.

Carbazol,08 discovered by Fritzsche, may also be mentionedhere—a substance which Grabe0 9 regards as fluorene in which

CflHACH 2 is replaced by NH, thus : ] )NH. Further, there is

C6H4 /acridine,100 found in crude anthracene by Grabe and Caro,which is regarded as a derivative of anthracene1 0 1 or of phen-

M Berichte. 3, 90. w Ibid. 16, 1465. 98 J. pr. Chem. 73, 286 ; 101,342. » Annalen. 167, 125, 174, 180. 10° Ibid* 158, 265. 101 RiedelBerichte. 16, 1609 ; Bernthsen and Bender, ibid* 16, 18031

288 HISTORY OF CHEMISTRY [LECT. XIV>

anthrene102 in which an atom of nitrogen has taken theplace of CH.

I have already stated (p. 117) that all these investigationsas to the constitution of organic compounds were occasionedby the numerous cases of isomerism which meet the chemistat almost every step, and the existence of which seems torequire some explanation. It must be acknowledged, thatthe theory of the valency or atomicity of the elements ful-filled this requirement to a large extent, and in this lies thegreat importance. of that theory ; whereas, on the otherhand, it cannot be denied that the principles of the theoryare far from being clearly and precisely worked out—amatter into which I intend to enter more fully in the nextlecture. But attention may here be called to the fact thatnot merely is the possibility of an explanation of these iso-merisms supplied to us by the advancement of our theoreticalknowledge, but this explanation chiefly depends upon themuch more extensive experimental material at our disposal.And this material has^ in great part, been obtained by theapplication of a method which, even although it has beenrecognised for a long time as a possible one, has only attainedto pre-eminent importance within comparatively recenttimes. I refer to the method of synthesis, which is, more-over, in many cases, not merely a means to an end, but isitself the aim of the experiments.

In an earlier lecture (p. 116) the synthesis by Wohler ofan organic compound" (urea) was mentioned, and also theimportance of this synthesis in regard to our whole concep-tion of nature. Similar results were only obtained in thecases of other substances long afterwards, and the value ofthis method was shown in a proper light by Berth elot'scomprehensive work.108 The syntheses of some speciallyimportant substances—marsh gas, ethylene, alcohol, formicacid, benzene, etc.—also originated with Berthelot.

It has been found in many cases that the earlier analytical102 Ladenburg, Berichte. 16, 2063 ; Grabe, ibid. 17, 1370. m Chimie

organique fondle sur Ja Synth&se, Paris i860.


method is not sufficient for establishing the chemical natureof a compound, and that the synthetical method constitutesa necessary complement. The first method usually precedesthe second ; but, in the history of a substance, its synthesis,with rare exceptions, marks a period, and, with it, theinterest which the scientific investigation of the substancepresents is usually at an end.

From this point of view, the syntheses of specially impor-tant substances are worthy of mention here. Thus alaninewas prepared, in 1850, by Strecker, from aldehyde-ammonia,hydrocyanic acid, and hydrochloric acid.104 Five years later,Zinin obtained mustard oil from allyl iodide and potassiumthiocyanate,105 its connection with garlic oil having alreadybeen established much earlier by Wertheim.100 Glycocoll wasprepared synthetically by Perkin and Duppa10T from brom-acetic acid and ammonia, and Hiifner afterwards obtainedleucine108 in an analogous manner. Racemic acid was preparedsynthetically by Perkin and Duppa109 from dibromsuccinicacid, and malic acid by Kekule110 from monobrornsuccinicacid. We are indebted to Kolbe for the synthesis of taurine,111

a substance which he prepared from isethionic acid. Anthra-cene was first prepared artificially by Limpricht, by boilingbenzyl chloride with water;112 and guanidine was prepared byHofmann,lls who obtained it from chlorpicrin, and by Erlen-meyer,114 who obtained it from cyanamide by the action ofammonia. Volhard prepared creatine synthetically fromchloracetic acid,115 by converting the latter into sarcosine bythe action of methylamine and then converting the sarcosineinto creatine by means of cyanamide. Picoline and collidinewere prepared synthetically by Baeyer110 from aldehyde-ammonias ; crotonic acid, by Kekule, from aldehyde ; l i r and

104 Annalen. 75, 29. m Ibid. 95, 128. I06 Ibid. 55, 297.™ Ibid 108, 112. 108 Htifner, J. pr. Chem. [2] I, 6. m Journ.Chem. Soc. 13, 102 ; Annalen. 117, 130. no Ibid. 117, 120. l n Kolbe,ibid. 122, 33. m Ibid. 139, 308. 11S Berichte. I, 145. m Annalen.146, 259. m Zeitschrift fiir Chemie. 12, 3T8. 116 Annalen. 155, 283.i« Ibid. 162, 92.


glycerine by Friedel and Silva, starting from acetone.118

Wurtz110 converted glycol chlorhydrine into choline (neu-rine) by means of trimethylamine, whilst Reimer andTiemannobtained vanilline from guaiacol.120 Grimaux syntheticallyprepared allantoin,121 alloxantine,122 and citric acid ;12S and,more recently, Erlenmeyer prepared tyrosine,124 Ladenburg,piperidine125 and coniine,126 and Horbaczewsky, uric acid.127

The synthesis of indigo blue by Baeyer128 also deserves to bementioned, since not only did it familiarise us with the pre-paration and furnish us with an explanation of the constitutionof an important colouring matter, but it was accomplished,besides, by means of new and peculiar reactions.

Special attention is due to the general methods whichpermit the synthesis of whole groups of substances. Themost important of these methods will be specified here.

Frankland was the first who succeeded in building uphydrocarbons.129 He obtained dimethyl (ethane) from zincand methyl iodide, and diethyl (butane) from zinc and ethyliodide. This reaction was extended by Wurtz, who treatedmixtures of alkyl iodides with sodium130—a method whichFittig and Tollens turned to account in the synthesis of aro-rriatic hydrocarbons.131 It had already been found possibleto obtain hydrocarbons, according to a reaction discoveredby Berthelot, by the distillation of benzoates with salts ofthe fatty acids.13*3 A synthetical method was elaborated byZincke, which permits of the preparation of hydrocarbonswith two phenyl groups, and depends upon the action ofbenzyl chloride upon aromatic hydrocarbons in presence ofzinc dust.183 These compounds can also be obtained, accord-ing to Baeyer, from aldehydes and aromatic hydrocarbons, by

118 Bull. Soc. Chim. [2] 20, 98. 1W Annalen. Supplementband 6, 116.W0 Berichte. 9, 424. 121 Ann. Cbim. [5] 11, 389. m Jahresbericht1878, 361. 123 Comptes Rendus. 90, 1252. 124 Annalen. 219, 161.125 Berichte. 18, 2956 and 3100. m Ibid. 19, 439 and 2578. 127 Monats-hefte. 3, 796; 6, 356- m Berichte. 13, 2254. ' m Annalen. 71,171; 74, 41; 77* 22i- 130 Ibid- 96, 364. m ibid. 131, 303,132 Ann. Chim. [4] 12, 81, m Annalen. 155, 59, etc.


the aid of substances which remove the elements of water.134

A method of very general applicability is that discovered byFriedel and Crafts,135 which renders it possible, by the helpof aluminium chloride, to introduce groups of very differentkinds into an aromatic substance, with simultaneous separa-tion of hydrochloric acid or of water, and thus permits thesynthesis of hydrocarbons, ketones, acids, etc.

The possibility of ascending in the series of the primaryalcohols, from one term to the next higher term, was shownby the investigations of Pelouze,180 Kolbe and Frankland,137

Piria,138 and Wurtz.189 The desired end is attained by theconversion of the alcohol into cyanide, acid, aldehyde, andalcohol, in accordance with the following equations :—

CNK + C2H5SO4K - C2Hr>CN + SO4K9

CoHBCN + KOH + H2O = COH6CO2K + NH8

C2H5CO2K + CHO2K - C*HSCOH + CO3K,C2H5COH + H3 = CaH6CH2OH.

Lieben and Rossi ascertained the general applicability ofthe methods.140 There is also a second mode for obtaining,from one alcohol, the next term in the homologous series, viz.,by converting the cyanide (nitrile) into an amine by means ofnascent hydrogen (Mendius141), and then decomposing thisby means of nitrous acid (Hunt l42). A statement has alreadybeen made about the synthesis of secondary and tertiaryalcohols (pp. 262 and 264). The preparation of the phenolsfrom the hydrocarbons is accomplished by a process whichDusart, Kekule, and Wurtz148 announced simultaneously.

Aceto-acetic ether has become of great importance in thesynthesis of acids, as already stated (p. 264). Malonicether144 and benzoyl-acetic ether145 have also been made use

184 Berichte. 5, 1094. rM Comptes Rendus. 84, 1392, 1450; 85, 74,etc. 138 Annalen. 10, 249. 18T Ibid. 65, 288; see also Fehling, ibid.49> 95- 188 Ibid. 100, 104 ; compare also Linipricht, ibid. 97, 368.m Ibid. 123,, 140. 140 Ibid. 165, 109. 141 Mendius, ibid. 121,129. m Liebigr and Kopp's Jahresbericbt 1849, 391. m ComptesRendus. 64. IU Conrad and Bischoff, Annalen. 204, 121, 14fi Baeyer,Berichte. 15, 2705 ; Baeyer and Perkin, ibid, 16, 2128,


of in a similar manner ; whilst it has been possible, on theother hand, to obtain synthetically some interesting nitrogencompounds by the aid of aceto-acetic and malonic ethers.146

Perkin's reaction,147 which is connected with observationsmade by Bertagnini,148 and depends upon the action ofaldehydes on the salts of organic acids in presence of agentswhich remove the elements of water, has led to the preparationof a large number of acids. It was first employed in a some-what more complicated form, however, in the synthesis ofcumarine.149 The conversion of nitriles into acids, alreadyreferred to above, has also been employed in the preparationof polybasic acids ; and, for this purpose, it is possible to start,as Simpson did,150 from the cyanogen compounds of polyatomicradicals, or, as was shown by Kolbe151 and by H. Miiller,152

from the cyanogen derivatives of acids. The first conversionof a nitrile into an acid was carried out, however, by Pelouze,153

who, in 1831, converted hydrocyanic acid into formic acid,and who reconverted the ammonium salt of the latter intohydrocyanic acid by the action of heat. Winkler,154afewyearsafterwards, converted oil of bitter almonds containing hydro-cyanic acid, into mandelic acid—a reaction which was correctlyinterpreted by Liebig.155 Polybasic acids can also be obtainedby a process published by Wislicenus,160 whilst Kolbe's re-action, which consists in treating phenates with carbonicanhydride, is of great importance in the synthesis of phenolacids.157 Related to this, there is Reimer's synthesis ofphenol aldehydes from phenates and chloroform.158

Finally Hofmann's method for the formation of alkyl

146 Compare especially Hantzsch, Annalen. 215, 1; Knorr, Berichte.17, Referate. 148, 540, 1635 etc,; Riigheimer, ibid. 17, 736. U7 Ibid.8, 1599; compare also Fittig, Annalen. 216, 115 ; 227, 48. 148 Ibid.IOO, 126. 149 Ibid. 147, 229. m Ibid. 118, 373 ; 121, 153. Ul Ibid.131, 348. m Ibid. 131, 350. 18S Ann. Chim. [2] 48, 395- m Annalen.18, 310* 155 Ibid. 18, 319. lm Ibid. 149, 215. w Kolbe andLautemann, ibid. 115, 201 ; Kolbe, J. pr. Chem. [2] 10, 93. m Ber-ichte. 9. 42 3.

LECT. xiv.] HiSTO&Y OF CHEMISTRY 293

bases159 may here be referred to. This method he afterwardsaltered and considerably improved.100

In many of these investigations an idea was turned toaccount which has already borne much fruit, and which will,no doubt, also be of great service in future. I refer to the so-called condensation processes ; that is, to the very frequentlyoccurring formations, both in nature and in artificial reactions,of complex substances from simple ones, where several iden-tical or similar molecules unite to form one molecule, usuallywith the simultaneous elimination of hydrogen, water, am-monia, etc. Gerhardt drew attention to reactions of this kindwhen he formulated his theory of residues (p. 180), but it isonly in comparatively recent times—within the last thirtyyears or thereabouts—that the necessary attention has beenbestowed upon these processes. Berthelot was probably the firstwho closely studied such reactions, and he obtained valuableresults by doing so. Amongst these results are the syntheses,discovered by him, of benzene C6H0from acetylene, of diphenylfrom benzene, of anthracene from toluene,161 etc. In theseexperiments he established, amongst other things, the fact,which has since been frequently confirmed, that at a hightemperature several molecules of a hydrocarbon may unite toform a new molecule with the elimination of hydrogen.

Some time afterwards, Baeyer began to work at this subject.He regards the difference between condensation and poly-merisation as consisting in the fact that in the former themolecules combine by virtue of union with carbon atoms, andin the latter of union with oxygen or with nitrogen atoms.162 It isalready clear to him that for purposes of synthesis, condensationis alone of importance. He draws attention, besides, to veryimportant syntheses which have already been carried out, suchas the formation of mesitylene from acetone by Kane,163 andChiozza's synthesis of cinnamic aldehyde from bitter almond

ir>9 Annalen. 66, 129 ; 67, 61 and 129 ; 70, 129 ; 73, 180 ; 74, 1, 33, 117 ;75> 3$6; 78, 253; 79, 11. im Berichte. 14, 2725 ; 15, 407, 752, 762.161 Bull. Soc. Chim. [2] 6, 268. lffii Annalen. Supplement band 5, 79.m Ibid. 22, 278.

594 HISTORY OF CHEMISTRY [LECT. XIV>

oil and aldehyde by the action of hydrochloric acid.104 Hethen turns the theoretical views advanced at that time toimmediate account in the synthesis of picoline and collidine,which are obtained by the condensation of acrolein-ammoniaand of aldehyde-ammonia :— m

2C3H4ONH8 = C6H7N + 2H2O + NHB

4C2H4ONH3 = C8HnN 4- 4H2O + 3NH3.Kekule, a few years later, condensed two molecules of

aldehyde so as to form crotonic aldehyde,100 thereby throwinglight upon the chemical nature of the so-called acrylic alde-hyde already examined by Lieben.107 This reaction was after-wards studied by Wurtz,168 who showed that the two aldehydemolecules unite in the first place, without the elimination ofwater, to form aldol, the aldehyde of yff-hydroxybutyric acid;and that crotonic aldehyde is then formed from the latter bythe loss of water. The general character of this interestingreaction was established, subsequently, by various investiga-tions, and especially by the researches of Claisen.169

The idea of condensation has been greatly extended inrecent times, every reaction in which union of carbon to carbonoccurs amongst the molecules that act upon one another beingdesignated a condensation. The word thus became synony-mous with synthesis, and lost all independent meaning and allmeaning corresponding to its etymology. It is due to thisthat Baeyer's reaction for the formation of hydrocarbons fromaldehydes and benzene anditsderivatives,and likewise Perkin'smethod of forming unsaturated acids from aldehydes and thesalts of fatty acids, came to be designated as condensations.

The idea of condensation has also undergone change inanother direction inasmuch as internal condensations havebeen contrasted with the processes just mentioned, whichhave in turn been called external condensations. By internalcondensations we now understand reactions in which a single

164 AnnaleD. 97, 350. m Ibid. 155, 283 and 297. 1<5<5 Ibid. 162, 77.167 Ibid. 106, 336 ; Supplementband. I, 114. 16S Jahresbericht 1872, 449 ;1873, 474; 1876, 483; 1878,612. m Annalen. 180, I; ibid. 218, 121.

rr. xiv.] HISTORY OF CHEMISTRY 295

lecule of a substance becomes converted into a new mole-^ by parting with some of its atoms, which unite to form;£} a molecule as H2, HC1, H20, NH3, etc. ; i,e., reactions£ch occur within a molecule. -As applied to such reactions,

word condensation so far retains its meaning, that theare related more intimately (that is, by a greater

of valencies) to one another. Amongst these re-Lons there are many processes which have long been>wn, such as the formation of ethylene from alcohol, of~Z\ from C2C1O, of aldehydes or of ketones from alcohols,ethylene oxide from glycol, of anhydrides from polybasici s , etc. But the formation of the anhydrides of mono-ic acids, of the lactones and of the lactone acids, whichre been minutely studied by Fittig, comparativelyently, must also be regarded as internal condensations.

the same class of reactions belong, further, the formationcumarine, and that of the oxycumarmes (urmbelliferone,>linetine, etc.), of isatine, indol, rosaniline, ofrosolic acid,t h e phthaleines, of the aldehydines, of quinoline, naph-.lene, anthracene, etc. Consequently these processes haveyed an important part in more recent investigations, andry will engage our attention here a little longer.The formation of ethenyl-xylene-diamineand of ethenyl-

L^ylene-diarnine by the reduction of nitro-acet-ixylid and ofro-acet-toluid, observed by Hobrecker,170 first attracted."foner's attention to this matter. The latter chemist pre-•ed a large number of analogous compounds, and was ableshow that this abnormal course of the reduction only

:urred with the ortho-benzene derivatives, and not withi meta- or para-derivatives.171 This was entirely con-oed by Ladenburg's investigations.172 The latter chemistcovered quite a number of reactions which proceed in an3gether different manner in the ortho-series from that inIch they proceed in the other isomeric series. In the

170 Berichte. 5, 920. m Ibid. 8, 471; Annalen. 208, 278 ; 209, 339 ;t 328. " Benchtc. 8, 677 ; 9, 219 and i $24 ; IO, 1123, 1260, etc,

296 HISTORY OF CHEMISTRY [LECT. A-IV.

case of the diamines, he showed, by means of reactions ofthis very kind, how the ortho-compounds may be distin-guished from their isomers ; and he was the first to pointout that the formation of the above-mentioned substancesdepends upon " ortho-condensation.'r Baeyer then turnedhis attention to this subject, and the syntheses of quinoline(already referred to) and of oxindol constitute the valuablefruits of his studies.

Closely related to this internal condensation, is internaloxidation. Reactions involving the latter change are thosein which oxygen atoms already present in the molecule, andgenerally belonging to NO2 groups, oxidise, by the dissolu-tion of existing unions, other groups belonging to the samemolecule. The first reaction of this kind was observed byWachendorff,173 but GreifT174 was the first to explain it.The matter involved was the action of bromine on ortho-nitrotoluene, which the latter of the above-named chemistsrepresented in the following manner :—

C H ! *«*CH! ^ 6 2 2 ccffa

This reaction furnishes the explanation of the importantmethod, discovered by Baeyer,175 for preparing isatine fromortho-nitro-phenylpropiolic acid, by boiling this acid withalkalies:—

6H4N"O2 +2K0H

/CO—CO= C6H4 / / +CO8K2 + H2O.

The formation of indigo from ortho-nitro-phenylpro-piolic acid, depends upon similar rearrangements.

"* Berichte. 9, 1345. m Ibid. 13, 288. m Ibid. 13, 2259.

L E C T U R E XV.

THE FUNDAMENTAL CONCEPTIONS OF CHEMISTRY—PHENOMENA OFDISSOCIATION—ABNORMAL VAPOUR DENSITIES — CONSTANT ORVARIABLE VALENCY—THE DOCTRINE OF VALENCY IN ORGANICCHEMISTRY-—THE PERIODIC LAW—LATER DEVELOPMENT OF THEDOCTRINE OF AFFINITY — SPECTRUM ANALYSIS — SYNTHESIS OFMINERALS—CONTINUITY OF MATTER IN THE LIQUID AND GASEOUSSTATES—LIQUEFACTION OF THE SO-CALLED PERMANENT GASES—THERMO-CHEMISTRY—ELECTRO-CHEMISTRY — PHOTO-CHEMISTRY—MOLECULAR PHYSICS—MORPHOTROPY.

HAVING- now followed ojganic chemistry in some of its morerecent discoveries, and having obtained a knowledge of itsremarkable progress under the influence of the theory ofvalency, it is appropriate to suggest and to discuss thequestion whether this theory is capable of serving as afundamental principle in mineral chemistry; and also torecount some of the most important results of investigationsin general chemistry.

Before passing, on, however, to this part of our task, thetheories themselves must be subjected to a more minute con-sideration and scrutiny. In describing how they have comeinto existence we have not always been able to enter into theexact significance of their fundamental conceptions. Weshall now turn our attention to this matter, although,naturally, it is only possible to bring forward the most im-portant points. For the remainder, the reader is referred tothe standard text-books of theoretical and general chemistry.

Our views rest essentially on the precise formulation anddistinction of the conceptions of atom, molecule, andequivalent.

An atom is defined as the smallest indivisible quantity ofan element which exists under any circumstances ; and mostgenerally it only exists in combination with other atoms.

207

298 HISTORY OF CHEMISTHY |I.KCT. XV.

A molecule is defined as the smallest quant it y of achemical substance that occurs in the free Mule, whether thesubstance be elementary or compound. The determinationof the molecular weight, depends essentially upnn our com-bining the conceptions of the physical and of the chemicalmolecule ; that is to say, we apply the word molecule to thesmallest quantity of a substance which occurs free in thegaseous state, as well as to the smallest quantity that entersinto a reaction.

With respect to determinat it »nsctf atomicwcightsT it is to beremarked here that the numbers proposed by Gerhardt ! weresubjected to an important alteration if* sof.tr that the atomicweights of all the metals were doubled, except those of themonatomic ones {/>., the alkali metals and silver). As early as1S40, when the atomic weights of Ber/.elius were still in use,Regnault had proposed to halve the atomic weight of silver,and, in accordance with ihh proposal, to assume two atoms ofmetal in silver oxide for one atom of oxygen.- He afterwardsmade a similar proposal with respect to the atomic weights ofpotassium, sodium, and lithium/1 The reason was, that hisclassical experiments on specific heat had shown him thatDulong and Petit *s law only applied to theae metals when thisassumption wan made. Had this proposal of Keguault's beenadopted at that time* our present atomic weights would (withfew exceptions) have been obtained. But since, following Gcr-hardt's lead, the atomic weights of all the metal* were halved,it was afterwards necessary (when the desirability of Rcgnault'sproposal had been shown upon new grounds, especially by H.Rose4 and by Cannizzaro*) to double them again, with theexception of those of the metals mentioned above, Cammzaroin particular, showed, in his pamphlet referred to below, thatthe law of Dulong and Petit was a guide in the determination

1 Compun, for vx»impli% G*?tlintclf, lriirtKluctioin it !*I%III*?C 4c l;i Chitnie*184$, 29. *4 Ami, Chim. \z | 73, 5; Artnaten. j6» no. -1 Ann. CAtiw.[3j 26, 2&u 4 Togg. Ann. xoo, 70, * Mtiovo Cimenni, 7, 31 f ; dmRupert tie Chtmie pure* X, 201 ; compare Stinto dl 1111 Ceno ill Fitot»(t»Chll 1858, 3$.

LECT. xv.] HISTORY OF CHEMISTRY 299

of the atomic weights, just as the hypothesis of Avogadro wasin that of the molecular weights. Even at this time the carryingout of these principles was still confronted by great difficulties.It is true that Deville and TroostG showed, at this date, thatthe vapour density of sulphur, at about iooo°, was only one-third of the number previously found by Dumas and Mitscher-lich (compare p. 105) at lower temperatures ; so that it waspossible to adopt the molecular formula S2 for sulphur. Theanomalies previously observed in the cases of mercury, phos-phorus, and arsenic, remained, however ; but these were notan obstacle to Cannizzaro. The chemical relations had tostand aside in order to procure acceptance of the principle.He assumed that only one-fourth of a molecule of phosphorusand of arsenic, respectively, was contained in two volumes ofphosphuretted and of arseniuretted hydrogen ; whilst half amolecule of nitrogen is present in two volumes of ammonia,and a molecule of mercury in two volumes of mercuricchloride. Consequently the divisibility of the molecule isdifferent, according to Cannizzaro, even in chemically ana-logous substances. Even although this appeared to be a boldview, still no decisive reasons could be established against it.

The assumption of differences of constitution amongst theelementary molecules, although striking at first, seemed aftera time to be fully justified. Why should not a state ofmatters be met with in the case of the elements similar tothat observed amongst compound substances, the moleculesof which are known to present the greatest variety withrespect to the numbers of their atoms ? Cannizzaro veryaptly compares the elements with the hydrocarbons—-themolecules of hydrogen, oxygen, etc., with the so-called alcoholradicals, methyl, ethyl, etc., and the molecules of mercury,zinc, and cadmium, with the olefines, a view which may also beextended to the derivatives of both classes of substances ;—7

0 Comptes Rendus. 49, 239 ; Annalen. 113, 42. 7 Wurtz, Lemons dephilosophic chimique, 172.

300 HlSTOttY Of CHftMISTkY [user, XV.

H21 O2, Ns

Hg, ZnK2O, H2OCaO, ZnO

Bi2O3, Sb2O3

SnO2, SiO0KOH

Ca(OH)2

Bi(OH)3

Sn(OH)4

corresponding

3?1}}>>)1)7J)>?}

to (CH3)2, (C2H5)2

(CH3)2O, (C2H5)2OC2H4O, CSH6O

(C3H6)2O3

CO,,CH3OH, C,H5OH

C,H4(OH)2, C3H6(OH)2

C3H6(OH)3

C4H0(OH)4

Of decisive importance, however, as regards the nature ofthe molecule of mercury, are the experiments of Kundt andWarburg,8 which may be adopted as a direct proof of Canniz-zaro's view. These physicists, by observing the velocity ofsound in mercury vapour, determined the ratio of the specificheats at constant pressure and at constant volume to be 1.67—a number furnished by the mechanical theory of heat, on theassumption that the total energy of the gas consists of thetranslatory motion of the molecules. The demonstration,furnished by Victor Meyer, of the variable vapour density ofiodine,9 which, as Crafts in particular has shown,10 eventuallysinks to one-half of the original density and then remainsconstant, can only point to the fact that the molecule ofiodine, at high temperatures, consists of a single atom.

A great deal more trouble was experienced in fixing themolecular weights of compounds, in those cases where thenumbers calculated from the vapour densities did not agreewith those deduced from the chemical relationships of thesubstances. In his determinations of the relative densities ofvapours, Bineau obtained such remarkable numbers that heconsidered decomposition to be the cause of the peculiarvolume relations.11 Thus he found the density of ammoniumcarbamate (anhydrous carbonate of ammonia, as he calls it) tocorrespond to six volumes, whence he assumes a decomposition

8 Berichte. 8, 94$. Pogg. Ann. 157, 353. fl Berichte. 13, 394-Comptes Rendus. 92, 39. n Ann. Chim. [2] 68, 434 ; 70, 272


into four volumes of ammonia and two of carbonic anhydride.Mitscherlich made a similar assumption in the case ofantimony pentachloride,12 and so did Gladstone13 in that ofphosphorus pentabromide. In both of these cases it wasassumed that, besides the halogens, the trichloride and thetribrornide of the respective elements had been produced.Cahours14 also expressed the same view in 1847, in explana-tion of the low vapour density of phosphorus pentachloride.

In thesameyear, Grove15 made the remarkable observationthat water is decomposed into its elements by contact withbrightly glowing platinum,—a circumstance which he soughtto explain as a result of the high temperature. This viewmet, however, with little acceptance, the fact that platinumcan be melted by means of the-oxy-hydrogen flame beinglooked upon as opposed to it. . Consequently, Grove's experi-ment was regarded as a result of the action of affinity ; and itwas explained as exactly similar to the decomposition of water(observed by Regnault) by means of melting silver, wheresilver oxide and hydrogen were supposed to be formed.16

Grove's way of regarding the matter was first definitelyproved by Henry St. Claire Deville as the result of a verydetailed investigation which constitutes the basis of thetheory of dissociation.

Before passing on to describe more minutely these pheno-mena, which are highly important for chemistry, I must herepoint out that the views with respect to them have beenaffected by the advances which have meanwhile been madein our knowledge of heat. These advances have beencalled forth by the law of the conservation of energy, which,as is well known, was first clearly formulated by J. R.Mayer ;17 and they find their expression especially in themechanical theory of heat and in the kinetic theory of gases,developed chiefly by Clausius, Joule, Rankine, Thomson,Helmholtz, Maxwell, and others.

12 P°£g- Ann- 29, 227. 1S Phil. Mag. [3] 35, 345- 14 Ann. Chim.[3] 20, 369. 15 Phil. Trans. 1847, I ; Annalen. 63, 1. 16 Ann. Chim.[2] 62, 367. 17 Annalen. 42, 233,

;]Oii IPSTOKY <>I; CHF.MISTHY imT. xv.

After drawing attention to the fact that the affinity uf silverfor oxygen could not come into play in Rt'giiatilt1^ experiment(since silver oxide breaks up into its constituents at muchlower temperatures, and thesune thinj* imiM. certainly lakeplace in presence of hydrogen), Deville shows that the tiecoin-position of water by means of strongly healed loci oxide (at1200" to 1300'") is also observed. He succeeds in effectingthe same decomposition by means ot ingeniously contrivedapparatus, without the action of a foreign Huhhtaucc : and inthis way his opinion that the decomposition is a result ofthe high temperature is confirmed in an elegant manner.**

The difficulty in these investigations arises from the factthat the constituents separated during the decomposition,combine again at lower temperatures, so ili.il the decom-position winch has occur red is not recognisable under ordinarycircumstances. The proof that decomposition has occurredmay be furnished, as Dcvillc shows, (1) by diluting theproducts of the decomposition by means of a rapid currentof an indifferent gas, so thai complete recombination is pre-vented ; (2) by diffusion, whereby the composition erf thegaseous mixture is altered ; or (i») by means of the so-calledtube chand etjhiki; *>., by audden cooling of the productsof decomposition.

In the forms of apparatus constructed to carry out thesemethods, Deville succeeded in proving not only the decom-position of water into hydrogen and oxygen, bin aluo thatof carbonic anhydride into carbonic oxide *md oxygen, "ofcarbonic oxide into carbon and cutbotiic auhvrfridi*, of hydro-chloric acid into chlorine and hydrogen, of nulphtirouft an-hydride into sulphuric anh)dritfe and sulphur, etc*.

Supported by these experiments*, Devillt! compares theformation of compounds with the t*'»n<it*!i»;ttion nf vapour*.According to him, both change* begin at definite trfitprrat urenand both proceed gradually. Certain qtmttttties ut he*tt aregiven out during the condensation of v ipmr^ and f fit*

m Comptet Rencliw* 56. 1 5. |ti» 739;sur la. dissociation, 1864,


thing takes place (frequently to a much greater extent) in thecombination of two substances. But further, in exactly thesame way that evaporation begins below the condensing point,the decomposition of substances can be observed below thetrue combining temperature. As every degree of the thermo-metric scale corresponds to a definite vapour pressure, so, incertain cases at least, the pressures of the products of decom-position can be stated. Deville distinguishes between decom-position by the action of heat and decomposition by chemicalmeans. He applies the name dissociation to the formeronly.19 It is characterised by the facts that its different phasescan be observed ; that it begins at one definite temperatureand is completed at another ; and that between these limitsthe pressures increase from o to 760 mm. and more of mer-cury, so that a definite pressure, due to the gaseous productsof the decomposition, corresponds to every temperature.

Subsequent experiments on this subject (of a very detailedcharacter) have confirmed Deville's views, in general at least.Only those decompositions are now regarded as examples ofdissociation which take place in opposition to the chemicalforces and are accompanied by the absorption of heat.20 Thecomparison of these phenomena with evaporation, even if itis not quite generally applicable, still holds in the decom-position of solid substances with the formation of gaseousconstituents, as was shown by Debray in the case of calciumcarbonate,21 by Naumann in that of ammonium carbonate,22

by Isambert in that of ammonium hydrosulphide,23 and byothers. Investigations of the compounds of silver chloridewith ammonia24 and of compounds containing water ofcrystallisation, came to be of special importance, because inthe cases of these substances the different compounds withammonia and the different stages of hydration of the salts,

19 Annalen. 105, 383. ° Compare Horstmann, Theoretische Chemie.666. ai Comptes Rendus. 64, 603 ; Bull. Soc. Chim. 7, 194. t22 Beirichte.4, 779. & Comptes Rendus. 92, 919 ; 93, 73T. ^ Isambert, Ladenburg'sHandw5rterbuch der Chemie. 3, 400 j Horstmann, Berichte. 9, 749.

304 HISTORY OF CHEMISTRY [I.F.CT. XV.

respectively, were indicated by the abrupt variations of thepressure.25

Pfaundler26 endeavoured to explain the, at first, surpris-ing fact of a partial decomposition which gradually increasedwith rise of temperature, involving, as it did, the differentbehaviour of similar molecules under the same conditions.Naumann27 further developed these views, and they weremore definitely formulated by Horstmann,28 who made useof Maxwell's probability theory29 of the distribution of thevelocities.30 A close agreement between this theory andthe observations was noted in various cases.

Horstmann was the first who tried to establish a generaltheory of dissociation,31 starting from the principles of themechanical theory of heat—especially from the so-calledsecond law. This was found to be in complete accord withthe results of experiment in one case.82 The researches ofGibbs33 and of Helmholtz,34 which were based upon similarprinciples, were more comprehensive and were highly pro-ductive.

These investigations, which belong, in part, to the domainof physics, have become of great importance with respect tothe question we have here to consider. Shortly after the firstresearches of Deville, the opinion was stated by three differentchemists—Cannizzaro,35 Kopp,36 and KekuleS7—that the so-called abnormal vapour densities were to be explained as clueto the substances concerned breaking up into two or more con-stituents. The latter were supposed to re-combine on cooling,so that no decomposition was perceptible upon distillation.

The difficulties which thus stood in the way of a direct

25 Debray, Comptes Rendus. 66, 194; G. Wiedemann, Pogg. Ann.Jubelband 1874, 474. 26 pogg# AniL I3 I ? 6 a 27 Annalen. Supplement"band5, 341- * Berichte. I, 210. 29 phiL Mag> r -j i9, 22 . 3^ xg5,30 Compare also Boltzmann, Wiedem. Ann. 22, 31. « Annalen. SuppJe-mentband 8, 112 ; 170, 192. *2 Ibid. 187, 48. * Silliman's Journal l6,441 ; 18, 277.. ^ Berlin. Akad. Ber. 1882, 22, 825 ; 1883, 647, m NuovoCimento. 6, 428 ; 7, 375 ; 8, 71. Compare also Rupert, de Chimk pure.I, 2or. 36 Anqalen. 105, 390. ** Ibid. 106, 142.


proof of decomposition, were only overcome some yearsafterwards by Pebal,38 who based his experiments on thestatement, first made by Bunsen,39 that it was only possibleto distinguish mixtures of gases from homogeneous gases byphysical methods (diffusion or absorption). On causing themixture of gases obtained by heating ammonium chloride todiffuse through an asbestos plug, Pebal was able to show, bythe colours imparted to litmus, that the gas in one part ofthe apparatus possesses an alkaline and in another part anacid reaction.

In a similar manner, by means of diffusion, Wanklyn andRobinson40 endeavoured to show the breaking up of sulphuricacid into sulphuric anhydride and water, and of phosphoruspentachloride into phosphorus trichloride and chlorine.

Deville attacked the conclusions which these chemistsdrew from their experiments.41 According to him, completedecomposition was not necessary in order to accomplish aseparation of the constituents by means of diffusion, a dis-sociation involving a slight increase of pressure being quitesufficient. As the products of decomposition are carriedforward, further quantities are formed, so that, given asufficiently long duration of the experiment, a completeseparation of the constituents is attained at a temperaturewhich only corresponds to a very slight decomposition.Deville points out that the vapour density of water is stillnormal at iooo°, while at this temperature it can be shown bydiffusion that dissociation has already taken place ;42 andhence he considers that the abnormal vapour density mustbe ascribed to the undecomposed vapour of ammoniumchloride. He finds what he regards as a positive proof ofthis, in the considerable rise of temperature which he believeshe can recognise upon the intermixture of ammonia and hy-drochloric acid gases in a vessel previously heated to 3500.43

Wanklyn and Robinson having raised the objection that the38 Annalen. 123, 199. * Bunsen, Gasometrische Methoden, 1857, 242.

40 Comptes Rendus. 56, 547. ^ Ibid. 56, 729. 42 Deville, Lemons sur ladissociation, 365. m Comptes Rendus. 56, 729.

U

306 HISTORY OF CHEMISTRY [LECT. XV.

gases had not been sufficiently heated prior to their inter-mixture,44 Deville afterwards repeats the experiment in amanner which no longer permits of this objection being raised,and again observes a rise of temperature, the amount of which,however, he does not state.45 He finds a further argumentin his favour in the fact that ammonia, when heated toIIOO°, breaks up into nitrogen and hydrogen. In his opinionammonium chloride, after having been heated to this tem-perature, ought to yield these two gases on cooling as evi-dence of the formation of ammonia ; but this is not the case.

In opposition to this argument, Than adduces the fact thata gaseous mixture is much more difficult to decompose thana pure gas,46 and this is in complete agreement with Deville'sviews regarding dissociation.47 By diminishing the partialpressure, the temperature at which dissociation begins israised ;4S or, the temperature remaining the same, the pres-sure due to decomposition is diminished. Further, Thanobserved no rise in temperature on mixing hydrochloricacid and ammonia at 3600. Even if the errors were greaterin his arrangement of the experiment, and assuming that hewas unable to measure very small differences of tempera tureT

still it is placed beyond doubt by his statements that onlyinconsiderable quantities of heat are liberated by the inter-mixture of ammonia and hydrochloric acid at 3600. This isconfirmed by an experiment by Marignac,49 who was able toshow that just as much heat is evolved in the formation ofammonium chloride from ammonia and hydrochloric acid asis required for its volatilisation. Hence it may be lookedupon as fully proved that ammonium chloride does notexist in the gaseous state, but that it breaks up, on volatili-sation, into its components.

Similar facts, even if not always so convincing, have alsobeen observed in the cases of many other compounds whosemolecules in the gaseous state correspond to four volumes ; as,

44 Comptes Rendus. 56, 1237. ™ Ibid. 59, 1057. m ^a^Tx^129. 4? Deville, Lemons. 364. <* Compare Naumann, Annalen. Stipple-mentband 5, 341. « Comptes Rend as. 67, 877.

L£CT. xv.] HISTOEY OF CHEMISTRY 307

for example, phosphorus pentachloride,50 ammonium sul-phide,61 ammonium carbamate,52 etc. A lengthy discussiontook place between Wurtz on the one hand,53 and Troost,54

Deville,55 and Berthelot56 on the other, regarding the natureof the vapour obtain ed from chloral hydrate. Thi s discussionended in favour of the former, and led to the proof of thedecomposition of chloral hydrate upon vaporisation.

Returning now to the definitions of our fundamental con-ceptions (compare p. 297), we designate as the equivalent, orbetter, the equivalent weight, that quantity of an element orof a radical which can replace or combine with one atom ofhydrogen. This conception, however, no longer plays anyessential part ; another one, which stands in close relation-ship to it, having been introduced instead of it,—that, namely,of valency or atomicity. By this term we understand thequotient obtained by dividing the atomic weight by theequivalent, and it was discussed at length in the precedinglecture. The question as to whether the valency of anygiven element is constant or variable is one of particularimportance. So long as we are satisfied to formulate and tomake use of our conception of valency in harmony with theabove definition, constant valency may, of course, be assumed.As soon, however, as we compare (as it is necessary that weshould do) the valencies of the multivalent elements with oneanother, we can no longer assert the absolute constancy of thevalency of any element. Even in the case of carbon, where theassumption of a uniform quadrivalence encounters relativelyfew exceptions, the existence of carbonic oxide is at variancewith its universal accuracy. We find a similar thing, only to agreater extent, in the cases of the other elements, and are there-fore obliged to admit the possibility of exceptions in every case.

00 Cahours, Ann. Chim. [3] 20, 369 ; Deville, Comptes Rendus. 62,1157. 61 Horstmann, Annalen. Supplementband 6, 74. 52 Naumann, ibid,160,1. 53 Comptes Rendus. 84, 977, 1183, 1262, 1347 ; 85, 49 ; 86, 1170 ;89, 19?, 337, 429, 1062 ; 90, 24, 118, 337, $72. u Ibid. 84, 708 ; 8g,32, 144, 400 ; 86, 331, 1394. 5S Ibid. 84, 711, 1108, 1256. 66 Ibid*84 1189, 1269 ; 85, 8 ; 90, 112, 49T.


Two different methods have been proposed in order to bringthese exceptions as far as possible into harmony with thesystem, but neither of them wholly gets rid of the difficulty.

One party, under the leadership of Kekule,57 adheres to thedefinition of valency given above, but admits that there is alarge class of substances to which it is not applicable. Thisclass is composed of the molecular compounds, the smallestparticles of which consist of aggregates of molecules held to-gether by means of molecular forces. Examples of the classare the compounds containing water of crystallisation (alsoalcohol, benzene, etc., of crystallisation), the majority ofdouble salts, the ammonium salts, phosphorus pentachloride,iodine trichloride, etc. No precise definition of them canbe given, but they are characterised generally by the facts thatthey cannot pass, undecomposed, into the state of vapour(although Thorpe found phosphorus pentafluoride to be anexception to this rule58), and that they are easily formedfrom and decomposed into their molecular constituents.

The adherents of constant valency are further obliged torecognise the unsaturated compounds as exceptions. Evenalthough there are not a very great many of these com-pounds, still their existence constitutes a serious objectionto the doctrine; and fruitless endeavours have been madeto weaken this objection on the ground of the tendencyexhibited by such substances to become saturated.60

The opponents of these views, whose first representativesare Frankland and Couper,60 define the valency of an elementas its maximum saturating capacity, and under this definitionthe unsaturated compounds cease to occupy an exceptionalposition. In view of the fact that they further assume thevalency in the case of many elements to be considerably higherthan had previously been assumed—nitrogen and phosphorus,for example, as quinquivalent, sulphur as sexivalent, iodine as

57 Lehrbuch der Chemie. I, 142, 443 ; Comptes Rendus. 58, 510.68 Annalen. 182, 204* 89 Compare p. 269 and Horstmarm, TheoretischeChemie. 295* 60 Compare pp. 231 and 254.


quinquivalent or septivalent—it is possible for them to includein the system a large number of molecular compounds. But italso becomes necessary for the adherents of this view to explainthe change in the saturating capacity, or at least to establishthe conditions which bringabout this change in the propertiesof the elements, if their hypotheses are to deserve the name ofa theory. Very little has yet been done in this direction, how-ever, and the little that has been done is scarcely capable ofany general formulation.61 On the other hand, a number offacts have become known which can only be explained withdifficulty on the assumption of constant valency ; such, forexample, as the identity of the naphthyl-phenyl sulphones andof the tolyl-phenyl sulphones which can be prepared indifferent ways ;62 or such as the isomerism of the two tri-phenyl-phosphine oxides, one of which, P(C6H5)3O, is sup-posed to correspond to phosphorus pentachloride; and theother, P(CCH5)2OCGH6, to phosphorus oxychloride.63

It is clear from these few observations that the subject ofvalency, quite apart from any mathematical basis (whch isat present altogether wanting64), must still be called a veryanomalous and uncertain one, and that there is no existingconception of it which is capable of dealing in a logicalmanner with the whole domain of chemistry.

That the idea is still retained, in spite of this, and thatit is even yet regarded as one of the most important prin-ciples, is explicable, in organic chemistry at least, onaccount of the almost marvellous consequences which thelatter branch of the subject is able to show as the result ofits assistance during the last fifty years. In inorganicchemistry, however, the state of matters is very different.

No doubt a favourable and helpful influence may be ob-served in inorganic chemistry also ; and, in particular, classifi-

61 Compare, however, Horstmann, Theoretische Chemie. 327 et seq.;Van 't Hoff, Ansichten iiber die Organische Chemie. I, 3. &2 Michaeland Adair, Berichte. 10, 583 ; 11, 116. ** Michaelis and Lacoste, ibid.18, 2118. u Compare, however, Kekule Annajen, i<&, 86 ; Baeyer,Berichte. 1$, 2277,


cation has become essentially clearer, as I wish to illustrate inindividual cases. The possibility of classifying the elementsthemselves according to their valencies indicates a step inadvance, since analogies were thereby brought out which hadonly been partially recognised previously. The analogy ofcarbon with silicon had already been pointed out, but boron hadalso been placed along with these two. The analogy of the twoformer was now established much more clearly, whilst boronwas recognised as belonging to another series altogether. Onthe other hand, titanium, zirconium, and tin were classed alon gwith carbon and silicon. Similarly arsenic, antimony, and bis-muth took their places beside nitrogen and phosphorus, thenvanadium also, as a consequence of Roscoe's careful investiga-tion,65 and, finally, niobium and tantalum when the results ofMarignac's researches were published.00 A similar thing tookplace with the metals, which had hitherto been arranged eitheraccording to their relative densities or to their analytical be-haviour. The theory of valency exercised, further, a decided in-fluence upon the views respecting many classes of compounds.Thiswasthecasewiththesilicatesinparticular. Wurtz showedhow the facts ascertained by him with respect to the condensa-tions of glycol might be extended to the derivatives of silicicacid,67 and, by so doing, he brought sudden light into a hithertoobscure region. Soon afterwards this region was further illumi-nated by Tschermak's68 important research on the felspars, inaccordance with which these substances must be regarded asisomorphous mixtures of orthoclase, albite, and anorthite. Thenumerous metal-ammonia and metal-ammonium compoundsnow found a place in the system also, being looked upon asammonia or as ammonium chloride with hydrogen atoms re-placed by metal or metallic oxide. Hofmann was the first toattempt this classification,60 and in doing so, he turned toaccount the results of his researches on organic bases. The

65 Annalen. Suppiementband 6, 77. 66 Ann. Chim. [4] 8, 5 and 49(abstract) ; Annalen. 135, 49 ; Ann. China. [4] 9, 249 ; Annalen. Suppie-mentband 4, 350. G7 Ripen, de Chimie pure. 2,449 ; Legons de philosophic<?h,imkjue, 181. ^ Pogg. Ann. 125, 139. 69 Annalen. 78, 253; 79, u,_


same idea was more fully carried out by Weltzien,70

H. SchifF,71 Cleve,72 and many others.Despite all this, it cannot be said that the theory of valency

has proved very productive in inorganic chemistry. In thefirst place, the number of researches which it has occasionedis by no means very considerable ; and, further, a systematictreatment of the subject, based upon valency, is not capableof being uniformly and logically carried out. Another hypo-thesis has had a far more important and lasting effect, and isnow able to show the most brilliant and undreamt-of results.I must here discuss the relations which have been foundto subsist between the atomic weights and the propertiesof the elements.

The more recent investigations on this subject are relatedto Prout's hypothesis, which has already been considered.73 Itis true that this hypothesis never became generally accepted,but nevertheless it occasioned speculations from time to timein the same direction. I only mention here Dobereiner, who,in 1829, first drew attention to what he called the triads,74(thatis, to groups of three analogous elements possessing atomicweights such that one of them might be regarded as the arith-metical mean of the other two). Gmelin,75 Pettenkofer,76

Dumas,77 and Lenssen 78 further elaborated these ideas, with-out arriving at any results specially worth mentioning.

A valuable result was, however, attained by the proof thatthe properties of the elements are periodic functions of theiratomic weights. For this, we are indebted to the investigationsof Newlands,79 Lothar Meyer,80 and Mendelejeff.81 The chiefmerit unquestionably belongs to the latter, who first gave pro-minence to the existing relations in a quite general form, and(what must be regarded as specially important) pointed out

70 Annalen. 97, 19. 71 Ibid. 123, I. 72 Bull. Soc. Chim. [2] 7, 12 ; 15,161 ; 16, 203 ; 17, 100, 294. 73 Compare p. 102. 74 Pogg. Ann. 15,301. 75 Handbuch. Third Edition, 1, 35. 70 Annalen. 105, 187. 77 Seep. 102. 78 Annalen. 103, 121 ; 104, 177. 79 Chem. News. 10, 59, 94 ;13, 113. 80 Moderne Theorien. First Edition, 136; Annalen. Supple-mentband 7, 354. 81 Zeitschrift fur Chemie. 12, 405 j Annalen,Supplementband 8, 133*

;U2 HISTORY OF CHEMISTRY [LECT. XV.

clearly the advantages of considerations of the kind. It is forthis reason that his paper at once made a great sensation,whereas that of Newlands remained quite unnoticed.82

In the tabulation adopted by Mendelejeff, the elementsare arranged according to their atomic weights ; but they arefurther arranged into divisions in such a way that the elementswhich are analogous to one another fall into vertical columnsand form groups, whilst each set of from seven to ten elementssucceeding one another in a horizontal line in the order oftheir atomic weights, constitutes a short period within whichthe properties (physical as well as chemical) progressivelyvary. Two successive horizontal series form a long periodin connection with which it is to be noted that, in the groups,the analogies between elements of the even, and also of theuneven, horizontal series, are greater amongst themselvesthan those between elements which belong partly to the evenand partly to the uneven series.

Of the applications of the " periodic law/' the two followinghave attained special importance :—(i) The determination orcorrection of the atomic weights of insufficiently investigatedelements, and (2) the prediction of the properties of unknownelements.

With respect to the first of these applications the followingfacts must be mentioned here. In conformity with the pro-posal of Awdejeff,83 Mendelejeff assumed the atomic weight ofberyllium to be 9, and placed this element in a group alongwith magnesium ; whereas it had hitherto been regarded bymany chemists as a metal akin to aluminium, and of atomicweight 13.5. This new view called forth a prolonged dis-cussion, which terminated, however, with the completetriumph of MendelejefFs opinion.84

The atomic weight of indium was assumed to be 113, or82 With respect to the question of priority, compare Newlands, Chem.

News. 32, 21, 192; L. Meyer, Berichte. 13, 259; Mendelejeff, ibid. 13,1796. &* Pogg. Ann. 56, 101 ; compare also Klatzo, J. pr. Chem. 106,227. & Nilson and Pettersson, Berichte. 11, 381 ; 13, 14 51 ; 17, 987 ;L, Meyer, ibid. 13, 1780 ; Reynolds, ibid. 13, 2412 ; Nilson, ibiJ. 13, 2035,


one and a half times as great as previously, and this numberwas very soon confirmed by the determination of the specificheat of the metal by Bunsens5 and by Mendelejeff.86 Theatomic weight of uranium was doubled—a proceeding whichwas found by the excellent and detailed investigations ofZimmermann 87 to be in complete agreement with the facts.Finally, it may be pointed out that Mendelejeff adopted 125as the atomic weight of tellurium, in opposition to the previousdeterminations which had furnished the number 128. Thiswas also apparently confirmed by the redetermination of theatomic weight in a specially purified sample,88 but Brauner'smost recent experiments gave essentially different results.

ButMendelejeff'spredictionsrespectingnewelementshavebeen followed by results which are simply marvellous. Inorder to render p ossible the arrangement into groups and series,and to attain approximately equal differences in successivemembers, blanks had to be left, which, according to Mendelejeff,would be filled up by existing, but at that time unknown,elements. He was able to foretell the atomic weights andother properties of these elements from their position in thesystem, with the aid of the properties observed in the groupsand series, which, like a system of co-ordinates, could be calledin to assist. Three such blanks occurred in the first five series,and these he indicated as representing the positions of eka-boron (at. wt.44),eka-aluminium (at. wt. 68), and eka-silicon (at.wt. 72). Since that time, these three elements have been dis-covered, and they have been found to possess, approximately,the properties predicted by Mendelejeff. They are: scandium,discovered by Nilson,89 with atomic weight 44.1; gallium, dis-covered by LecoqdeBoisbaudran,90 with atomic weight 70; andgermanium, discovered by Winkler,91 with atomic weight 72.

But these are not the only results which render this theorymost valuable. The theory has so thoroughly permeated the

89 Pogg* Ann. 141, 1. m Bull, de l'Acad. Imp. de St. Petersbourg.16, 45. 87 Annalen. 213, 285 ; 216, I. m Brauner, Berichte. 16,3055; compare p. 349. 89 Berichte. 12, 550, 554 ;.13, 1439- 90 ComptesRend us. $1, 193, 1100 ; $6, 475, etc, w J. pr. Chem. [2] 34, 177,

314 HISTORY OF CHEiMISTRY [LECT. XV.

whole of chemistry, that investigations of the elements andof their compounds have gained a new significance. As aresult of the bond which it establishes between the individualelements, it lends to every such special investigation the charmof a research of universal interest.

Investigations which deal with the subject of affinity, pre-senting this subject in a mathematical form and combining itwith Berthollet's doctrine of affinity, are of equal importance.

We possess investigations of this kind, in a finished state,which have been tested in the most different directions andhave been found accurate ; so that in this respect also greatresults have been attained. An investigation by Guldbergand Waage 92 constitutes a new departure foF advancement inthis direction. In place of Berthollet's notion of chemicalmass, these investigators introduced that of active mass, bywhich they understand the quantity of a substance containedin a unit of space. The chemical energy with which twosubstances act upon one another is then equal to the productof their active masses multiplied by the affinity coefficients ;and by the latter Guldberg and Waage understand valueswhich are dependent upon the chemical nature of the sub-stances and upon the temperature.93

When the substances A and B are transformed, in achemical operation, into A' and B', and where, conversely, A!and B' can be transformed into A and B) equilibrium is estab-lished when the forces acting between A and B are equal tothose acting between A' and B'. If the active masses A and Bare represented b y / and q, and those of A and B b y / and q\and further, if the affinity coefficients are k and i', then inorder that equilibrium may be established we must have—-

kp q=k'p' q'

In applying this equation it is advisable to introduce,instead of the quantities/, q,p', and q, the relative numbersof molecules ; that is, the quotients obtained by dividing thequantities present by the molecular weights.

92 Etudes sur les affinity chimiques. Christiania I^^J7^19, 69, *>3 Compare also VWt Hoff, Perichte, JQ, 669.


This "law of chemical mass action" has been tested invarious ways, and the observed facts have repeatedly beenfound in agreement with it. In this connection, mentionmust first be made here of investigations which were carriedout by Wilhelmy, by Berthelot and Pean de Saint Gilles,and by Vernon Harcourt and Esson, all of whom must beregarded as precursors of Guldberg and Waage.94 Wilhelmyintroduced the idea of velocity of reaction as early as 1850,and thus more than fifteen years prior to the investigationsof Guldberg and Waage. In a highly interesting researchon the inversion of cane sugar,95 he showed that the quantityof sugar inverted in unit time is proportional to the totalquantity of sugar present. Berthelot and Pean de SaintGilles 96 studied the limit of ester formation and the velocityof the reaction, and the numerical values found are sufficientlyclose to those calculated from theory. These experimentswere continued and extended by Menschutkin,97 who ex-amined the ester formation in the direction referred to, in thecases of the most different alcohols and acids, and thus fur-nished an important contribution to the varying behaviourof substances of different structure. Vernon Harcourt andEsson arrived at results which are in conformity with thegeneral law, in their investigation of the reduction ofpotassium permanganate by oxalic acid added in great excess,98

and in their later work on the action of hydrogen peroxideon hydriodic acid.99 Bearing also upon this subject are thedetailed thermo-chemical researches of Thomsen,100 and thestudies of Ostwald101 on chemical volume, which not merely

94 According to Ostwald (Allgemeine Chemie, 2nd Ed. II. part 2, 40),Wenzel had already stated the most important proposition of the doctrineof affinity, that the intensity of the chemical action is proportional to theconcentration of the acting substance. 95 Pogg. Ann. 81, 413 and 499.96 Ann. Chim. [3] 65, 385 ; 66, 5 ; 68, 225. 97 Berichte. 10, 1728, 1898 ;II, 1507, 2117, 2148; 13, 162, 1812 ; 14, 2630. Annalen. 195, 334', *97>193. Ann. Chim. [5] 20, 289; 23, 14 ; 30, 81. 98 Phil. Trans. 1866,193.98 Ibid, 1867, 117. 10° Pogg. Ann. 138, 65 ; and Thermochemische Unter-auchungen. I. m J. pr. Chem. [2] 16, 38$ ; 18, 328 ; 19, 468 5 2S» I IPogg. Ann. Erg&nsnungsband §, 154 ; Wiedem. Ann, 2, 429» 671,


confirmed the theory but also extended it. These investiga-tions are chiefly connected with the affinity relations betweenacids and bases. The conception of avidity is here introduced—an idea approximately corresponding to what used to besomewhat less precisely designated as the strength of acids orof bases. What is understood by this term is the proportionin which two substances are shared by a third, the quantityof which is insufficient for their complete saturation ; and itappears that the avidity is proportional to the square root ofthe affinity coefficient.

Other important investigations are those of Horstmann102

on the incomplete combustion of carbonic oxide and ofhydrogen : that on the partial decomposition of ferrous saltsby water, which G. Wiedemann 1()3 carried out by the aid ofa magnetic method ; and that on the ratio of the distribu-tion of acids between two alkaloids, which Jelett104 ascer-tained by determining the optical rotatory power. But it isnot possible to enter more particularly here into these andmany similar researches.105

To a certain extent in contrast with these investigations,which are chiefly theoretical, there is the discovery of amethod of investigation which may certainly be regarded asone of the most brilliant that has been brought forward inrecent times as the result of experimental research. I refer tothe method of spectrum analysis, which has enabled us to drawconclusions- regarding the chemical composition of distantheavenly bodies whose material constitution was previouslyaltogether unknown, and by the aid of which the number ofthe known elements has been very considerably increased.

It would take too much space to deal with the early re-searches prior to the classical investigations of Kirchhoff andBunsen ;106 and therefore I refer, with respect to these, to the

102 Annalen. 190, 228. «» Wiedem. Ann. 5, 45. i« Trans# Roy> W g hAcad. 25, 371- 105 Compare the article " Affinitat" by E. Wiedemann, inLadenburg's Handworterbuch der Chemie, 1, 114. m Pogg. Ann. no, 161 ;H3) 337 J compare also Kirchhoff, Berlin. Akad. Abhandl. 1861,63 ; Untersu-chungen uberdas Sonnen spectrum und die Spectren derchemischen Elements


historical treatment of the subject by Mousson,107 to somenotices by Tyndall,108and especially to a paper by Kirchhoff,109

which deals with them. I must content myself by simplymaking a few remarks on the matter here.

Wollaston, in 1802, first observed the dark lines in thespectrum of the sun.110 These were more fully examined anddetermined in 1814 by Fraunhofer,111 to whom Wollaston'sobservations were unknown. In 1822, Sir J. Herschel ob-served the bright bands112 which the light of a flame colouredby metallic salts exhibits when decomposed. This pheno-menon was folio wed up further by Talbot113 and by Brewster.114

Swan pointed out the great delicacy of this reaction, especiallyin the case of common salt.115

Fraunhofer drew attention to the coincidence of the D linewith the yellow sodium line. Brewster found the potassiumlines to correspond to others of the Fraunhofer lines, andFoucault116 also made similar observations.

There are two points in particular which are of fundamentalimportance with respect to spectrum analysis, and these weowe to the researches of Kirchhoff and Bunsen. The first ofthem is the fact that every element, in the state of incandescentvapour, is characterised by giving a definite discontinuousspectrum, afact which even Swan did not venture to state withcertainty; and the second is the law of selective absorption,which Angstrom117 and Balfour Stewart approached veryclosely, without actually grasping it fully and clearly.118

This law is now known as KirchhofFs law, as it was provedmathematically by Kirchhoff,119 and experimentally by Kirch-hoff and Bunsen by means of the celebrated experiment of

107 Connaissances sur le spectre ; Bibl. Univers. de Geneve [2] 10, 221.108 Phil. Mag. [4] 22, r5$. 109 Fogg. Ann. n8r 94. 110 Phil. Trans.1802, 378. m Gilb. Ann. 56, 278. 112 Treatise on Light. U3 Brewster,Edinburgh Journal of Science. 5, 77 ; Phil Mag. [3] 3, 35 ; 4, 114 ; 9, 3.114 Phil. Mag. [3] 8, 384; Pogg. Ann. 38, 61 ; compare Comptes Rendus.62, 17. 115 Trans. Roy. Soc. Edin. 21, 411. 1]6 Ann. Chim. [3] 58,476. 117 Pogg. Ann. 94, 141. 118 Trans. Roy. Soc. Editu 22, I, 59.119 Pogg. Ann. 109, 275.

318 HISTORY OF CHEMISTRY

the reversal of the lines. The law states that the rat io betweenthe emissive power and the absorptive power is the same forall substances at the same temperature, for rays of the *amewave-length. From this it follows that nil opaque mint ancesbegin to glow at the same temperature—that is, thai I Jivy giveout light of the same wave-length—and that iiic;imk**ccntsubstances only absorb such rays as they themselve* emit.Since, however,incandescentgases possess rnaxiinaaniiniitiimaof light intensity, while solid and liquid sub<ttamc<>uml litfhtof every kind when sufficiently heated, the former i«ni4 al*npossess a selective absorptive power, and \hh i* not I lit* t *T«»ein general with the latter.1*' The Kraun holer lines an: thu*explained as consequent upon absorptions hy mean*, at in-candescent vapours. Their existence led to the Huutlafiunof the physical nature of the sun, while the det< rmtuatit*u t#ftheir positions (wave-lengths) and the compaiis<i« *#f tin mwith the emission spectra of the elements in tin: guM'otihstate, led to the fixing of its chemical composition. Kirch*hofFthus became the founder of a new branch of chtmiualscience—that of stellar chemistry. Although this branch hstill comparatively recent, it is already in a poshii>n In ^Utwgreat results. By means of it astronomy has rsnt with newproblems, and has been furnished with new method*, whichhave immensely widened its sphere of activity ; but it m tintpossible to enter into this subject more fully here*

The discoverers of the spectroscopic mcthml til a«;ily*4*fwere themselves able to establish its importance in c hernia rynot only by showing its application to analytical chemistry,but also by the discovery of two new elements ~-c;u'<iitint rfiitlrubidium. We are indebted to the mnm method lut ttitidiscovery of thallium by Crookes,131 and of incliuttt hy Kcichand Richter ;122 as well as for that of gallium aof which mention has already been

120 An exception is furnished by the salts of dldyitifuiii, whit It f «*e§a selective absorptive power, *« Phil. Miig. [4] ai. )OI j Anft.i!«n124, 203. m J. pr. Chera. 89, 441. » Coropwe p. $15


Amongst the more recent investigations in this depart-ment some which deserve to be specially mentioned are thoseof A. Mitscherlich, who was able to show that not merelyevery element, but every compound possesses, in thegaseous state, a spectrum peculiar to itself,124 and those ofPliicker and Hittorf, who showed that there are two spectracorresponding to every element; i.e., that besides the line-spectrum there is also the band-spectrum.125 Further, thereare the investigations on quantitative spectrum analysis,especially those of Vierordt126 and of Glan.127 Finally, thereare the numerous researches which are directed towardsestablishing relations between the emission spectra of theelements, such as those of Lecoq de Boisbaudran128 and ofCiamician ;129 or between the absorption spectra of compounds,such as those of Abney and Festing,180 of Krtiss,lsl and others.

Forming a counterpart to this analytical method there isalso a synthetical one, to which, however, the same generalapplicability that the former possesses cannot be attributed.I refer to the synthesis of minerals. A stimulus to makingexperiments of this kind was supplied by observations madeby Koch (1809),182 and especially by Hausmann and Mitscher-lich, who found, amongst the slags obtained in metallurgicalprocesses, products which proved to be identical with knownminerals. The first successful experiment of this kindoriginated with Sir James Hall, who prepared crystallisedcarbonate of lime (marble) by heating the carbonate underpressure.ias Berthier and Mitscherlich obtained artificalmica,"'pyroxene, and similar minerals by fusing silica withlime, magnesia, and ferric oxide.184 Gaudin prepared small

124 pOgg> Ann. 116, 499 ; 121, 459. im Phil. Trans. 1865, 1. m An-wendung des Spectralapparates zur Photometrie der Absorptions-spectren.Tubingen 1873. 127 Wiedem. Ann. 1,351; compare also Hiifner, J» pr.Chem. [2] 16, 290. 128Comptes Rendus. 69, 445, 606, 657, etc. 129 Ber.Wien. Akad. 76 (2), 499 ; 79 (2), 8 ; 82 (2), 425. m Journ. Chem. Soc42, 130-131. 131 Berichte. 16, 2051 ; 18, 1426. 132 Ueber krystall.Hiittenproducte. 183 Trans. Rcy. Soc. Edin. 6, 71 ; compare Gehlen'sJournal fur die Chemie, etc. 1, 271. 1S4 Ann. Chim. [2] 24, 35$.


rubies by fusing alumina (obtained by heating ammoniaalum) in the oxyhydrogen blow-pipe, after the addition ofsome chromic oxide.135 Gay-Lussac obtained crystallisedhematite by the action of water vapour on ferric chloride.130

Ebelmen succeeded in preparing an extensive series of diffi-cultly fusible or infusible crystalline minerals by employingborax or boracic acid as a material from which to crystallisethem.137 Becquerel obtained, in the crystalline state, sub-stances insoluble in water, such as silver chloride, silversulphide, cuprous oxide, basic cupric carbonate, etc., by-making use of slowly progressing chemical reactions.138

Although I cannot enter into further details here, I mayadd that Senarmont attacked and, partially at least, solvedthe problem of determining, and of realising, the conditionsunder which those naturally occurring minerals are formedwhich are met with crystallised in veins. In particular, heemployed water for this purpose, and he caused it to actunder pressure at about 3500.139 It may further be statedthat Sainte Claire Deville140 and his pupils discovered andmade use of the favourable effect of hydrofluoric acid and ofother fluorine compounds in promoting crystallisation ; andthat Hautefeuille141 was the first who artificially preparedpotash and soda felspars. Mention must also be made hereof the numerous syntheses of minerals that were carried outby Friedel and his pupils.

The introduction of the idea of kthe critical temperatureor of the absolute boiling-point signalised a great advancein our knowledge of the connection between the differentstates of aggregation of substances.

Cagniard de la Tour U2 observed, as long ago as 1822, thaton heating liquids in sealed tubes which they almost fill, atemperature can be attained at which the meniscus dis-

138 Annalen. 23, 234. m Ann. Chim. 80, 163 ; [2] 1, 33. w ComptesRendus. 25, 661; Ann. Chim. [3] 22, 211. ws Ann. Chim. [2] 51, xox.^Annalen. 80, 212. 140Caron and Deville, ibid. 108, 55; xog9242, etc. MI Comptes Rendus. 90, 830. 142 Ann. China. [2] 2X, 127"178; 22, 410. L J , /»


appears, and the whole presents a perfectly homogeneousappearance. From this he concluded that at this tempera-ture the liquid is converted into gas notwithstanding thepressure. Although these experiments were highly note-worthy, still they did not attract any considerable attention ;and it was only thirty-three years later that Wolf143 andDrion144 tried to determine, in the cases of a few liquids,the temperatures at which they pass into the state observedby Cagniard de la Tour. Mendelejeff, in 1861, introducedthe very appropriate name u absolute boiling-point" todesignate this temperature; and he defined it as thetemperature at which both the cohesion of the liquid andits heat of evaporation vanish, and the liquid itself is con-verted into vapour irrespective of pressure and of volume.145

Eight years later, the celebrated paper of Andrews140 ap-peared, in which the connection between pressure, volume,and temperature was accurately examined in the case ofcarbonic anhydride, which, it was shown, could not beliquefied at temperatures above 30.920 C. Andrews calledthis the critical temperature ; and, further, he designated ascritical pressure the pressure which is just sufficient to bringabout liquefaction at a temperature infinitesimally below thecritical temperature. Andrew's observations enabled him todraw isotherms for carbonic anhydride at different tempera-tures, which exhibited the relations between pressure andvolume. When this had been done, it appeared that thecurves were discontinuous below 30.920, and consisted ofdifferent parts. Although slight changes of curvature areobserved in the isotherms for temperatures just immediatelyabove 30.920, these are no longer observed at 480, the curveat that temperature approximately corresponding, through-out its entire length, to the equation which holds for

S a s e s - pv-C,

that is, it approximates to a rectangular hyperbola.143 Ann. Chim. [3] 49, 265. 144 Ibid. [3] 56, 33- 146 Annalen. 119, 1.

148 Phil..Trans. 1869, 57$.X

322 HISTORY OF CHEMISTRY [LKCT.. XV.

As a result of this investigation the definitions ofvapourand of permanent gas which had previously been adoptedwere abandoned, and the word gas is now applied to. everysubstance in the gaseous state when heated above it? criticaltemperature.147 The continuity of the liquid and gaseousstates is observable when a liquid is heated under a pressuregreater than the critical pressure. In such a case a separa-tion into liquid and gas never takes place, but the liquid istransformed into gas without the change giving rise to anynoticeable heterogeneity.148

These investigations exercised a decided influence uponthe experiments on the condensation of gases. Faraday, asis well known, was the first to turn his attention to thismatter with any considerable result; and quite a number ofgases were liquefied by him in an extremely simple and in-genious manner.149 He operated upon the small scale only,whilst carbonic anhydride was first liquefied in considerablequantities by Thilorier.150 Faraday then continued his in-vestigations,161 making use of the knowledge already gainedby Thilorier, but without obtaining any result in the casesof hydrogen, oxygen, nitrogen, carbonic oxide, nitric oxide,etc. Natterer was likewise unable to liquefy hydrogenalthough he exposed it to a pressure of 2790 atmospheres.152

It was only in 1877 that Pictet153 and Cailletet154 suc-ceeded, nearly simultaneously, in liquefying the majority ofthe so-called permanent gases ; but it was not possibleat that time, by the aid of the methods and appliancesemployed by these investigators, to obtain the liquids inquantity and to determine their physical constants (boiling-point, critical temperature, density, etc.). This was first

147 Wroblewsky (Monatshefte. 7, 383) afterwards attacked the concep-tion of critical temperature; but the matter cannot be further discussedhere. 148 Compare Ostwald, Allgemeine Chemie 1, 267. 149 Phil. Trans.1823, 160, 189; Alembic Club Reprints, 12, 5, 10. 150 Ann. Chim. [2]60,427. ]51 Phil. Trans. 184$, 155 ; A.C.R. 12, 33. *« Pogg. Ann.94, 436. m Comptes Rendus. 85, 1214, 1220 ; Ann. Chim. [5] 13, 145.15i Comptes Rendus. 85, 851, 1016, 1213 j Ann. Chim. [5] 15, 132.


accomplished by Wroblewsky,155 whose research may beregarded as an example of finished skill.

The intimate connection between physics and chemistry,which is apparent in the matter just dealt with, makes itselfstill more clearly manifest when we pass on to thermo-chemistry. This is a subject which is of equal importanceto both branches, and, moreover, it has been almost entirelyelaborated by men belonging to the two sciences. Lavoisierand Laplace may be looked upon as the founders of thermo-chemistry, not only on account of their experimental re-searches on specific and latent heats and on heats of com-bustion, but also on account of their masterly definitions,150

and, especially, of the fundamental (if not quite pre-cisely formulated) principle which they deduce from themechanical law of the conversation of energy :—

The heat liberated during combination or change of stateis consumed again during decomposition or return to theoriginal state, and vice versa,157

This principle was enunciated by Hess, in 1840, inanother form which is very important and strictly accuratefor practical thermo-chemistry ;—-The evolution of heatcorresponding to any chemical process is the same whetherthe process is accomplished in different stages or all atonce.158

Hess established this principle empirically, and heemployed it extensively in order to determine quantitiesof heat which were incapable of direct measurement—pro-ceeding, therefore, in exactly the same manner as is done atthe present day.

The comprehensive researches of Favre and Silber-mann159 are of great value. These consist, in part, of veryexact thermal determinations, especially of heats of combus-tion, and, up till about forty years ago, they constituted the

155 Monatshefte. 6, 204. 156 Compare p. 26. 157 Lavoisier, Oeuvres.2, 287. 158 Pogg. Ann. 50, 385 ; 52, 97- 159 Ann. Chim. [3] 34, 357 ;36, I ; 37, 406.


empirical basis of thermo-chemistry. They have now beensuperseded, however, by the excellent experiments of J.Thomsen and of Berthelot, which embrace almost thewhole domain of chemistry.

Thomsen160 recognises that the principle of Hess is adeduction from the first law of the dynamical theory ofheat, which he adopts as the basis of his theoretical con-siderations. He then advances a second principle, accord-ing to which every simple or complex action, of a purelychemical nature, is accompanied by the evolution of heat;and this he endeavours to establish both theoretically andempirically. The principle in many cases agrees with theresults of experiment, but still exceptions are known. These,however, can perhaps be otherwise explained. It may there-fore be asserted that chemical forces, when acting indepen-dently, always tend to bring about exothermic reactions ;whilst endothermic reactions are regarded as consequentupon the action of heat.161

Thomsen has endeavoured, more recently, to determinefrom the numbers found empirically the values of theaffinities of carbon, expressed in calories ; and from thesevalues, to further deduce the heats of formation of manyorganic compounds. Here likewise a remarkable agreementhas been observed, but exceptions have to be noted in re-spect to this matter also. It has already been pointed outat the proper place162 that considerations of this kind can beemployed in confirming the structure of organic compounds.

Berthelot163 advanced three principles, the first of whichstates that the evolution of heat in chemical processes is a

160Pogg. Ann. 88, 349 ; 9<>> 2fil J 9h «3 ,* 92, 34 ; 138, 65. See alsoBerichte. 2, 482, 701 ; 3, *87, 496, 7*6, 927 ; 4, 308, 586, 591, 597, 941,;5, 170, 181, 508, 614, 1014 ; 6, 233, 423, 697, 710, 1330, H34 *, 7> 3i» 379452, 996, 1002; 9, 162, 268, 307; 10, 1017, etc.; ThermochemischeUntersuchungen, Four Vols. Leipzig, 1882-86. 181 Compare Hortsmann,Theoretische Chemie. 612 et seq. 16<i Compare p. 278, 168 Ann. Chim.[4] 6, 290 ; 18, 103 ; 29, 94. ; [5] 4, 5, etc.; Mecanique Chimique fondlesui la Thermochimie, Two Vols. Paris, 1879.

Lief, xv.] HISTORY OF CHEMISTRY 325

measure of the chemical and physical work done during theactions. The principle is thus an application of the firstlaw of the dynamical theory of heat. According to thesecond principle, the evolution of heat in a chemical pro-cess in which no external work is done, depends only uponthe initial and the final states of the system. This is a moreprecise statement of the principle of Lavoisier and Laplace(compare p. 323). The thirfl principle states that everychemical transformation which is completed without the aidof any external energy, tends to produce that substance orsystem of substances in whose formation the maximumevolution of heat takes place.

This "principle of maxim urn work'7 raised a great dealof commotion. Not only was its originality contested, sinceit was looked upon as a repetition of Thorn sen's principle164

(compare p. 324), but its accuracy was also attacked.Berthelot defended it on both grounds, but still he was notable to prove the general accuracy of the principle.105

It may be further remarked here, in passing, that theterm thermal effect (Warmetonung), and the words exo-thermic and endothermic as applied to reactions were intro-duced by Thomsen166 and by Berthelot167 respectively.

Endeavours to discover the relations between electricaland chemical forces have occupied the attention of the mosttalented investigators since Davy and Berzelius, without, asyet, throwing full light upon this important department.Faraday's electrolytic law108 is an empirical one. It may to-day be looked upon as one of the most powerful supports ofthe theory of valency,109 and still a clear theoretical appre-hension of it has not yet been obtained. Even the pheno-menon of electrolysis itself remains a riddle still unsolved,

164 Thornsen, Berichte. 6,423 ; Berthelot, Bull.Soc. Chira. [2] 19, 485 ;Ostwald,' Allgemeine Chemie. 2r 20. 165 Compare Rathke, Ueber diePrincipien der Thermochemie, Halle 1881 ; and especially Helmholtz, ZurTherrnodynamik chemischer Vorgange, Berlin. Akad. Ber. 1882, 22, 82$.166 Pogg. Ann. 88, 351. 3<i7 Me"can. Chim. 2, 18. m Phil. Trans.1834, 77; Pogg. Ann. 33, 301.' m Ladeuburg, Berichte. 5, 753.

MS HISTORY OF CHEMISTRY [LECTV XV.

although various explanations have been furnished by thefine investigations of Daniell and Miller,170 of Hittorf,171 andof Kohlrausch.172 One fundamental point, nevertheless,appears to have been settled. I refer to the application ofthe law of the conversation of energy to electrolytic pro-cesses ; and to the connection between chemical energy andelectro-motive force. The elucidation of these relations isdue to the investigations of Braun173 and of Helmholtz.174

The former showed that there are galvanic elements whoseelectro-motive force is less, and also those whose electro-motive force is greater, than that corresponding to thechemical transformation of energy. Helmholtz establishedthe principle that the energy of the current is equal to thechemical energy only when the electro-motive force of thebattery is independent of the temperature. When theelectro-motive force increases with rise of temperature, heatas well as chemical energy is used up in the production ofthe current; while in the converse case a part of thechemical energy is liberated in the form of heat.

The accuracy of this principle, besides having been provedexperimentally by Helmholtz himself, has also been provedexperimentally by several other investigators,175 especiallyby Jahn.176

The relations between optical and chemical properties,which are equally important in theory and in practice, canonly be touched upon here, as there are but few results ofgeneral importance to be brought forward. The chemicaleffects of light have been closely studied, especially in threecases:—i, That of the silver salts ;177 2, That of the process

170 Fogg. Ann. Erganzungsband I, 565 ; 64, 18. m Ibid. 89,176 ; 98,1 ;103, I; 106, 337, 513. 172 Wiedem. Ann. 6, I, 145. 17!* Ibid. 5, 182 ;16, 561 ; 17, 593. W4 Berlin. Akad. Ber. 1882, 22, 825. GesammelteAbhandlungen. 2, 985. 17S Czapski, Wiedem. Ann. 21, 209; Gockel,ibid. 24, 618. m Jahn, ibid. 28, 21. 177 Scheele, Chemische Abhand-lung von der Luft und dem Feucr, Ostwald's Klassiker, 58, 48 et seq.;Senebier, M&noires phys. cirim. ; Draper, Phil. Mag. [3] 19, 195 ; etc.

. xv.] HISTORY OF CHEMISTRY 327

similation by the green parts of plants ;178 and 3, Thati e mixture of chlorine and hydrogen.179

draper endeavoured to prove that the chemical effectt-uced is proportional to the intensity of the light ; andproof was completed by Bunsen and Roscoe. It had

ady been recognised by Scheele that all the rays do noticipate alike in the action of the light. This was con-ed by the various subsequent investigators, and it wasmore definitely settled by Bunsen and Roscoe. As the

cipal action was repeatedly found to take place in thee t rays, the idea of specific chemical rays arose ; but thislow entirely dropped. It may be looked upon asWished that rays of every wave-length can bring aboutnciical effects, although not with the same intensity ;

the effects vary according to the chemical nature of thesitiye substance concerned. It is further of importancet the maximum effect in different chemical processes

been found at different parts of the spectrum. It islarkable that the numerous experiments designed toirtain the maximum effect of the different parts of the2trum in the process of assimilation in plants have not

to uniform results. Some find this maximum in theow, and others in the red. The question is one ofsiderable importance.T h e fact recognised by Ingenhousz that the decomposi-1 of carbonic anhydride takes place in the green parts ofats , soon led to the supposition that a connection existedween the chlorophyll colouring matter and the chemicalcess of assimilation ; and Dumas, as early as 1844, stated•• view that the violet rays, which are the principal ones

178 Senebier, loc. cit,; Ingenhousz, Versuche mit Pfianzen, LeipzigD ; Daubeny, Phil. Trans. 1836, 149 ; Draper Phil. Mag. [3] 23, 161 ;fcis, Bot. Zeitung 1864, 59 ; Miiller, Bot. Untersuchungen 1872 ;Efer, Pogg. Ann. 148, 86; Pringsheim, Berlin. Akad. Ber. 1881, 504;jelmann, Bot. Zeitung 1882, 663 ; 1883, I, 17 ; 1884, 81, 97 ;like, Bot. Zeitschr. 1884, 1 ; etc. m Bunsen and Roscoe, Pogg.1. 100, 43 ; 101, 254 ; 117, 529; Erganzungsband 5, 177-

3'28 HISTORY OF CHEMISTRY [LECT. XV.

absorbed by that colouring matter, must be the most activein effecting assimilation.180 Lomrnel, on the other hand,advocated the opinion,181 that the rays lying between thelines B and C, of Fraunhofer, might play the chief part inassimilation, because they possess the greatest intensity, andalso because they correspond to a maximum of absorptionby chlorophyll.

Since the facts actually observed were not favourable toeither of these views, Pringsheim, in connection with hisinvestigations into the effect which light exercises upon theprocesses of oxidation within the plant organism, advancedthe hypothesis and endeavoured to establish it, that thechlorophyll colouring matter is not the chemically activesubstance, but that it merely serves as a screen in moder-ating the breathing in the plant which would otherwisebecome excessive.182

Draper endeavoured to prove that, during its action, thelight must be absorbed (Joe. cit, Note 173). Bunsen andRoscoe instituted quantitative experiments on this point,from which it appears that, in the case of chlorine andhydrogen, about one-third of the rays absorbed are used upin effecting chemical work. But there are two kinds ofcases which must be distinguished : namely, those in whichthe light must supply the energy necessary for the chemicalprocess (which proceeds with absorption of heat), as in thecase of the assimilation by the green parts of plants, andthose in which the chemical process takes place with theevolution of heat, as in the case of chlorine and hydrogen.The light appears, however, to do work in both cases,although, in the latter case, it is merely preparatory work,by which the obstacles to combination are overcome. Forthis effect a certain time is required, and Bunsen andRoscoe proposed to indicate this by the term photo-chemicalinduction.

180Essai de statique clumique des Sires organises, 24. lrtl Pogg.Ann. 143, 581. *& Lichtwirkung und Chlorophyllfunction in der Pflanze!1879.


Finally, there still remains a large department to be dealtwith, namely, that of molecular physics. This department hasto do with the determination of the physical constants ofchemical substances, and with seeking the relations betweenthese and the chemical composition and constitution. Her-mann Kopp may be looked upon as the founder of thisbranch of science. From the year 1842 onwards, he oc-cupied himself with the determination of the boiling pointsand of the specific or molecular volumes of liquids.183 Inorder that the numbers obtained might be compared withone another, they had to be determined under comparableconditions—the boiling-points under the same pressure, andall the specific volumes at the boiling-points, so that thecorresponding vapours should be under the same pressure.The guiding idea in the comparison was that the samedifference in composition corresponds to the same variationin the property under investigation, or that the particularproperty of a compound is the sum of the properties of itselementary constituents. The values pertaining to theatoms of the elements, with respect to this property, werecalculated empirically, and, by means of these numbers andof the composition, the theoretical value of the property wasdetermined for the compound. This value was then com-pared with that obtained by observation. Investigations ofthis kind were carried out in the case of molecular volumes, inparticular, and harmonious results were frequently obtained.Deviations were afterwards observed, however, and it provednecessary to take the constitutions of the compounds intoconsideration also ; so that the value for the atomic volumepertaining to an atom was assumed to be different accordingto the way in which the atom was combined. A means wasthus furnished, in certain cases, of checking the constitutionwhich had been deduced, in the first place, by chemicalmethods only. (Compare p. 252.)

liCAnnalen. 41, 86, 169; 50, 71; Fogg. Ann. 63, 283; Annalen.94, 257; 95, 121, 307; 96, r, 153, 303 ; Supplementband 5, 323, etc.


This branch, which was investigated by Kopp with greatskill and success, was then followed up further by many in-vestigators. Researches into molecular volumes continueup to the present day, and the results obtained are discussedand turned to account in the same way that they were byKopp.184 But other properties of substances were alsoexamined, and were considered in the same way in connec-tion with composition and constitution.

This was the case especially as regards the refraction oflight by liquids and gases. Since the refractive index of asubstance is dependent upon the wave-length of the light aswell as upon the temperature, it is not itself employed forthe purpose of comparison. It is true that an endeavourwas at first made to render the refractive indices independ-ent of dispersion, by adopting as basis those for a particularwave-length. Thus Landolt at first employed, in his investi-gations, the indices for the C line of incandescent hydrogen.Briihl, on the other hand, employing Cauchy's formula, andafter determining the refractive index for several wave-lengths, calculated a coefficient which was independent ofwave-length and held for waves of infinite length.185

The next endeavour was directed towards obtaining resultsindependent of the temperature, by employing, for the re-fractive power, the expression discovered by Laplace186—

—3-~ [;* = refractive index, d= density]. It soon appeared,

however, that this does not satisfy the required condition ofbeing independent of temperature ; and besides, on the aban-donment of the emission theory of light, it had lost all physical

184 Compare, amongst others, Pierre, Annalen. 56, 139; 64, 158;80, 125 ; 92, 6; Buff, ibid. Supplementband 4, 129; Ramsay, Berichte.12, 1024; Thorpe, Journ. Chem, Soc. 37, 141, 327; Lossen, Annalen.214, 138 ; Elsasser, ibid. 218, 302 ; R. SchifT, ibid. 220, 71, etc. 1MAnna-len. 200, 166. In a subsequent paper (Annalen. 235, 1) Briihl discards therefraction coefficient derived from Cauchy's formula and readopts theformer one. 186 Mecanique celeste. 4, 232.

LECT. xv.] HISTORY OF CHEMlSTftY 331

importance. Gladstone and Dale 1S7 now showed empirically

that the expression ^-^~- fulfilled this condition, in many cases

at least. Landolt adopts the product of this value andthe molecular weight (i.e., the refraction equivalent) as thebasis of his extensive investigations,188 and finds that it isdependent on the constitution (the influence of chemicalconstitution is ascertained, but is not followed up). Hethus succeeds in calculating the refraction equivalents ofthe elementary atoms of carbon, hydrogen, and oxygen, aridin deducing from these, again, the values pertaining to theindividual compounds. These frequently showed closeagreement with the observed values. Landolt, however,confined his observations to the fatty organic compounds.These observations were further extended, first by Haagen,189

and then by Gladstone,100 who determined the refraction ofmany inorganic compounds and the refraction equivalentsof almost all the elements.

7l2 „ jIn the meantime another value, ,--r .•-, was theoretically

deduced as refraction constant by H. A. Lorentz101 and by L.Lorenz102 in two ways that were independent of each other ;and this value was employed especially by Landoltl9B and byhis pupil Briihl. They give the name molecular refraction tothe product obtained by multiplying this value by the molec-ular weight; and Briihl investigated this property in thecases of strongly refracting substances, and of aromaticcompounds in particular.194 He arrives at the conclusionthat the atomic refraction of multivalent elements is vari-able, and that that of carbon, for instance, is distinctlygreater when double or triple carbon linkings (or unsatu-rated carbon valencies, as he calls them) occur in the com-

187 Phil. Trans. 1858, 887 ; 1863, 317. 188 Pogg. Ann. 117, 353 ,* 122545; 123,59$. 189 Ibid. 131, 117. m Proc. Roy. Soc. 16, 439;18, 49 ; 31, 327. m Wiedem. Ann. 9, 641. 192 Ibid. 11, 70. m Berichte.15, 1031. 194 Annalen. 200, 139; 203, I, 255, 363 ; 211, 121, 371-

332 HISTORY OF CHEMISTRY [user. xv.

pound. He determines the amount of the increase for oneethylene linking and for one acetylene linking, and thenagain calculates the molecular refractions ; and in this wayhe frequently arrives at numbers which coincide with theobserved values. Later investigations of Nasini and Bern-heimer195 and of Kanonnikoff196 have only partially con-firmed the conclusions of Briihl; but the latter still hopesto be able to get rid of the exceptions.197 J. Thomsen hasshown, however, that many of the values found by Bruhlcan also be calculated without the assumption of double ortriple carbon linking.198 These investigations attain a specialimportance from the fact that, according to the conclusionsof Exner,199 the molecular refractions furnish, at the sametime, the u true molecular volumes."

I cannot here enter more particularly into a discussionof other investigations which are designed to show, in asimilar manner, a connection between physical and chemicalproperties ; and I shall content myself by drawing attentionto individual ones. Thus there are the investigations whichdemonstrate a relation between the lowering of the freezingpoints of solutions and the molecular weights of the sub-stances in solution (Coppet200 and Raoult201), and which areconnected with similar earlier experiments ;202 the researchof G. Wiedemann on molecular magnetism ;20S and theinvestigations on the transpiration of gases by Graham,204

by O. E. Meyer,205 and by Maxwell,200 and on the transpira-tion of vapours by Lothar Meyer.207 There still remain tobe mentioned, the fundamental investigations of Biot upon

195 Beiblatter zu Wiedem. Ann. 7, 528 ; Accad. dei Lincei [3] 18, 1% etc,196Berichte. 14, 1697; i*> 3047; J. pr. Chem. [2] 31, 321; 32, 497.197Annalen. 235, I. 198Berichte. 19, 2837. m Monatshefte, 6, 249.200 Ann. Chim. [4] 23, 366; 25, 502; 26, 98. ^Comptes Rendus,94« 1517; 95» 187, 1030; Ann. Chim. [5] 28, 133; [6] 2, 99, 115; 4,401 ;8, 289, 317. 202Blagden, Phil. Trans. 1788, 277 ; Rtldorff, Fogg. Ann.114, 63; 116,55; I45» 599- Mro6 'g. Ann. 126, 1 ; 135, 177. " *m PhilTrans. 1846, 573 ; 1849, 349. mPogg. Ann. 125, 586 ; 127, 2$3, 353,m Phil. Trans. 1866, 249. 207 Wiedem. Ann. 7, 497 ; 13, 1.


the rotation of the plane of polarisation,208 and the researchesconnected with them, by Landolt209 and others; and alsothe experiments of Perkin on the electro-magnetic rotationof the plane of polarisation.210

Finally, I must refer in a few words to relations whichhave been discovered between crystalline form and chemicalcomposition, a consequence of which may be a considerableexpansion of the idea of isomorphism. The credit of havingdiscovered these relations belongs to Groth ;211 and his viewshave been extensively confirmed by means of the numerousresearches by himself and his pupils. Groth follows outthe changes of the axial ratios which take place upon theentrance of substituting groups, and in this way arrives atdefinite laws. He gave the name morphotropy to thephenomena, and caused experiments to be made in orderto determine the morphotropic influence of definite substitu-tions. The morphotropic effect of chlorine, bromine, andiodine, for example, proved to be analogous to that ofhydrogen ; and hence these elements have been designatedisomorphotropic.212 It was then announced by Hintze213

that isomorphism might be regarded as a special case ofmorphotropy; a point to which Groth had, however,already directed attention.

208 Ann. Chim. [3] 59, 206. k209 Das optische DrehimgsvermSgen organi-scherSubstanzen, 1879. m J. pr. Chem. [2] 31, 481; 32, 523. 211Pogg •Ann. 141, 31 ; Berichte. 3, 449; compare, however, Laurent, ComptesRendus. 15, 350; 20, 357; Me'thode de chimie. 156; E. 129. 212Hintze,Pogg. Ann. Erganzungsband 6, 195. 213 Habilitationsschrift. Bonn 1884.

L E C T U R E XVI.

THE DOCTRINE OF PHASES—VAN DER WAALS'S EQUATION—THEORY OFSOLUTION —ELECTROLYTIC DISSOCIATION — ELECTRO-CHEMISTRY—ATTAINMENT OF HIGH TEMPERATURES—LOW TEMPERATURES—THE NEW ELEMENTS IN THE ATMOSPHERE—THE CHEMISTRY OFNITROGEN — TRANSITION TEMPERATURE — STEREO-CHEMISTRY—RACEMISM—SYNTHESES IN THE SUGAR AND URIC ACID GROUPS—IODOSO-COMPOUNDS—TERPENES AND PERFUMES—NEW NOMEN-CLATURE.

WHEN we look back upon the development of chemistryduring the last fifteen or twenty years, we. find that it isdistinguished by the constantly increasing prominence ofphysical or, as many call it, general chemistry, which fromsmall beginnings has advanced to the position of a scienceof the first rank. Contributions to this end have, natur-ally, been made in particular by eminent scientists, suchas Horstmann, Gibbs, van der Waals, and van Jt Hoff, whohave devoted themselves to this department exclusivelyand, by their ideas and discoveries, have brought about itsadvancement. On the other hand,- however, it cannot bedenied that this advancement does not coincide fortuitouslywith the appearance of Ostwald's great Text-book of GeneralChemistry, but that the latter, in which the attempt is forthe first time successfully made to give a complete repre-sentation of what has been accomplished up to the presentin this department, aroused and stimulated the tendencytowards investigation in an altogether exceptional manner.Further, the establishment by Ostwald and van Jt HofT ofthe Zeitschrift fiir Physikalische Ckemie} in which all themore important investigators in this department are activeas collaborators, has done a great deal to advance the sub-ject ; so that this publication must be placed side by side

S34

LECT. xvi.] HISTORY OF CHEMISTRY 335

with the best journals representing our science and belooked upon as of equal value with them.

In now passing on to the subject itself, I begin, in thefirst place, with the law of mass action, already mentionedon p. 315, which is apparently destined to play a constantlyincreasing part. Some of the numerous applications of thelaw of mass action which have been made, may be mentionedhere.

The investigation by Hautefeuille,1 and the later andmore extended investigation by Lemoine,2 into the forma-tion of hydriodic acid from its constituent elements, arousedmuch interest. Lemoine believed he had verified the factthat the temperature of decomposition of hydriodic acid wasinfluenced by pressure—^a fact which would have been incontradiction to theory. Consequently, at the instigationof V, Meyer, the experiments were resumed by Bodenstein.8

The latter found, in the first instance, a still more consider-able deviation from the theory regarding the non-dependenceof the equilibrium upon the pressure ; and it was only in alater investigation,4 when the source of the earlier errorhad been recognised and avoided, that complete agreementbetween experiment and theory was established.

The dissociation of gases presented frequent opportunitiesfor the application of the theory, as, for example, in the break-ing up of N2O4,

5 the decomposition of the compound formedby the action of hydrochloric acid gas on methyl ether,6 thedissociation of carbonic anhydride into oxygen and carbonicoxide/ and so on.

The law of mass action has also been turned frequentlyto account in the investigations connected with electrolyticdissociation ; but I shall not here enter upon the considera-tion of the latter subject because it is treated of separatelyfurther on.

1 Comptes Rendus. 64, 608. 2Ann. Chim. [5] 12, 145. 32.physik. Chem. 13, 56. 4 Ibid. 22, I. 5 Natanson, Wiedem.Ann. 24, 454; 27, 606. 6 Friedel, Bull. Soc, Chim. [2] 24, 241.7 Z. physik. Chem. 2, 782,

336 HISTORY OF CHEMISTRY [LECT. XVI.

On the other hand, some other investigations relating tochemical equilibrium will be dealt with more particularly here.

These chemical studies received a new direction and afresh stimulus from the theory of phases, due to Gibbs.8

The phase rule developed by him, and proved both by himand afterwards by van der Waals,0 is to the following effect:—Complete equilibrium can only exist when the number ofphases present exceeds the number of components by one.

By phases, are understood homogeneous portions of asystem. Each state of aggregation represents at least onephase. In the solid or the liquid state, two or more differentphases may exist; a gas, however complex, can only formone phase.

By components are understood all those chemical ele-ments taking part in the equilibrium, whose quantities-aresubject to independent variation.10 Ammonium chloride,for example, has only one component, it being a matter ofindifference whether we choose nitrogen, hydrogen, orchlorine. If excess of ammonia or of hydrochloric acidis added, there are then two independent components.Calcium carbonate, above* its dissociation temperature, hastwo independent components, calcium and carbon; for thecomposition of the solid phases—calcium carbonate andcalcium oxide—cannot be determined by the amount ofcalcium alone. Hence complete heterogen eous equilibrium isestablished in the case of ammonium chloride with two phases,and in the case of calcium carbonate with three phases.

Complete equilibrium is a condition which depends onlyon the temperature, and is mostly definable by a certainvalue of the pressure.

If there are « + 2 phases and only n components, equili-brium is only possible at singular points ; that is to say, atsome definite temperature (multiple point, transition or

8 Trans. Connecticut Acad. 3, 108 and 343 (1876) ; German translationby W. Ostwald, Leipzig 1892. 9 Rec Trav. China. 6, 265, communicatedby Roo2eboom. 101 follow here the exposition by Planck (see ArticleThermochemie, in Ladenburg's Handworterbuch. der Chemie. xi, 636)*


transformation temperature). If there are just as manyphases as there are components, the equilibrium is incom-plete ; that is, to each temperature there corresponds a;series of pressures.

This phase rule has found numerous applications, asthe work of Roozeboom,11 in particular, shows. Rooze-boom studied the connection of the states of aggregation,the equilibrium between water and sulphurous anhydride,the hydrates of ferric chloride, etc. The phase rule canalso be applied to dissociation phenomena, to the reciprocaltransformation of allotropic modifications of elements, andso forth.12

More important perhaps than the phase rule (the sig-nificance of which is exaggerated by many) are van derWaals's theories of corresponding conditions,13 and van 'tHofFs theory of solution.14

Van der Waals makes a distinct advance by substitutingfor the gas equation,

deduced from the laws of Boyle-Mariotte and of Henry-Gay-Lussac, the expression,

in which a and b are constants which depend upon the.cohesion of the gases and the not altogether negligiblevolume of the molecules. (According to van der Waals b isto be considered as representing four" times the volume ofthe molecules.)

This equation not only represents the behaviour of gases(and especially of compressed gases) much more satis-factorily than the original equation, but it is also capable of

11Z. physik. Chem. 2, 449, 5*3; 4, 31 ; 5, 198; 10, 477; Rec. Trav.Chlm. 4 etseq. l2 Compare the summaries by Meyerlioffer Leipzig 1893,and by Bancroft, Ithaca, New York, 1897. 18Die Continuitat des gas-formigen und fliissigen Zustandes, Leipzig 1881. ' 14Lois de l'equilibrechimique dans Tetat dilou^ ou dissous. Stockholm 1886. Abstracted, Z.ph)sik. Chem. I, 481.

Y

338 HISTORY OF CHEMISTRY [LKCT. XVI.

application to liquids. Moreover, since the constants a andb can be determined in a simple manner from the criticaldata (volume, pressure, and temperature) or from the be-haviour of the gases under high pressure, van der Waals'sequation furnishes a mode of giving expression to the entirebehaviour of all homogeneous liquid and gaseous substanceswith respect to changes of pressure, temperature, andvolume; and, on this account, it may be regarded as offundamental significance. Its accuracy has been proved by-Young 15 in particular.

The theory of solution is based upon conceptions thathave arisen from the well-known experiments of Pfeffer,16

which latter only became possible after the discovery byTraube 1T of semiperraeable membranes.

In explaining osmotic pressure as the result of the im-pacts of the dissolved molecules upon the walls of the vessel,van ?t Hofif arrives at a comparison between substances inthe dissolved condition and in the state of gas. The lawsof Boyle-Mariotte, and of Henry-Gay-Lussac, as well as thefundamental hypothesis of Avogadro, can now be applieddirectly to solutions ; so that this branch, which has hithertobeen one of the most obscure in the whole subject ofchemistry, at once becomes fully accessible to investigation.As a consequence, important results, which are capable ofbeing turned to account throughout the whole range ofchemistry, are immediately obtained.

The important relations subsisting between the depressionof freezing point, the diminution of vapour pressure, and theelevation of boiling point on the one hand, and the molecularweight of the dissolved substance on the other (which wereascertained experimentally and formulated by Raou l t u

16 Phil. Mag. [s] 33, I53T; 347 505. 16Osmotische Untewuchungen,Leipzig, 1877. 17 Archiv. f. Anat, u. Thys. 1867, S7. 18Ann. Chim/[6]fc, 66, 99; 8, 2$% 317; 20, 297 ; Compteg Rendus. 87, 167; Z, physik.Chetn. 9, 343, etc. The literature of the predecessors of Raoult is veryfully given in Ostwald's Lehrbuch der AHgemeincn Chcmie, SecondEditionT 1, 705 and 741.

LICCT. xvi.] HISTORY OF CHEMISTRY 339

in particular), now attain their theoretical significancefor the first time. As the outcome of this, and also inconsequence of improvements and simplifications thatRaoult's methods of molecular weight determination under-went,19 these methods very soon obtained a footing ; andtheir results, especially those from the depression of thefreezing point, are considered to be just as accurate as thosefrom the vapour density.

Raoult had already pointed out, however, that aqueoussolutions of salts, of bases, and of acids, in particular, did notagree with his rules ; but always yielded results that weretoo low, and only attained to a value from one half to one-third of that which had to be regarded as the normalnumber. All explanation of this anomaly was at first want-ing, so that the general applicability of van Jt Hoff Js theoryappeared to be placed in doubt. The difficulty was got ridof in the same way as in the case of the abnormal vapourdensities (compare p. 304).

Arrhenius dealt with this matter by exactly the samemethod that Cannizzaro, Kekule, and Kopp had adoptedin solving the other difficulty. His theory, advanced in1887,20 adopts as actually existent that condition whichmust be assumed to exist in order to arrive at an agreementbetween the theory of van \ Hoff and the numbers furnishedby Raoult's rules. He draws attention to the fact that it isin the cases of solutions of those substances which areelectrolytes and break up, under the influence of theelectrical current, into their ions, that numbers are obtainedwhich do not agree with theory. He now assumes that theionisation does not merely take place as a result of the

la Compare especially, Beckmann, Z. physik. Chem. 2, 638 ; 4, 532 ;8, 223; 18,473, etc. *Z. physik. Chem. 1, 631. Clausis (Pogg.Ann. IOI, 338) and Helmholtz (Weidem. Ann. 11, 737) must be mentionedas predecessors of Arrhenius. Planck (Z. physik. Chern. 1, 577) alsoclearly stated the idea of the dissociation of salts in aqueous solutionsimultaneously with Arrhenius.

340 HISTORY OF CHEMISTRY [LF.CT. XVI.

passage of the current, but that it occurs during the dis-solution; and that the latter is thus accompanied by a moreor less complete (electrolytic) dissociation, the extent ofwhich depends principally upon the degree of dilution. Anumber of methods for determining the extent of this dis-sociation very soon presented themselves, as was pointedout by Arrhenius himself,21 and also by Planck,2- Ostwald/2a

and others ; and (what is very important) these methodsgive results that agree with one another.

The hypothesis of Arrhenius found a great manyopponents—indeed it could hardly have been expected thatit would be otherwise. The assumption that an aqueoussolution of common salt contains free sodium and chlorineions (which, however, are nothing but electrically chargedatoms that behave like free molecules) was certain to meetwith opposition from chemists, since it stood in contradic-tion to observation and thus included something of a meta-physical nature. Besides, the explanation of many reactionsthat had formerly appeared simple was rendered much moredifficult; as, for example, the decomposition of water by thealkali metals,24 since in this reaction no combination withoxygen and, on the other hand, no displacement of hydro-gen ions by sodium ions could be assumed. But of whatconsequence are considerations of this kind in face of thegreat advantages which the theory of electrolytic dissocia-tion affords? A large number of otherwise inexplicablefacts are satisfactorily explained by means of it. The so-called lawof thermo-neutrality, of Hess,"25 which has beenconfirmed, in part at least, by the well-known investigationsof Thomsen 26 and of Berthelot,27 is in complete accord withthe ionisation theory, and so are the exceptions to this lawwhich must necessarily exist in cases of incomplete dis-

21 Z. pbysik. Chera. 2, 491. n Wxedetn. Ann. 34, 139. '^Z. physik.Chem. 2, 36 and 270. ' Compare however Ostwald, Lehrbuch, SecondEdition, 2, 989. m Fogg* Ann. $2t 97. 'M Thermochemische Unter**mchungen* I, 63. ^ Ann. Chim. [$] 6, 325-


sociation ; whereas, without this theory the facts concernedconstitute an incomprehensible puzzle.28

It is similar with the identity of the heat of neutralisa-tion of one and the same acid by means of different, bases,and of that of one and the same base by means of differentacids ; also with the law of Oudemans29 and Landolt30 (inaccordance with which the salts of optically active alkaloidsand of optically active acids exhibit the same rotation insolutions of equivalent concentration), with the magneticrotatory power,31 and with theatomic magnetism.32 Further,the principle in accordance with which the spectra of dilutesolutions of different salts with similarly coloured ions areidentical,33 and that according to which the molecular re-fractive power of the salts present in aqueous solution isan additive property,34 are explained, in the same way. Butprobably the most important fact of this kind is that of theproportionality that exists between electrolytic conductivityand avidity in the case of acids,35 with which may be coupledthe proof, furnished by Arrhenius,30 that the extent of thedissociation calculated from the electrolytic conductivityleads to very nearly the same results as that calculatedfrom the depression of the freezing point. In these circum-stances we cannot be in doubt as to whether the hypothesisof Arrhenius is warranted.

This ionisation theory, as it is now commonly called,leads us directly to electro-chemistry, which has madeadvances that were undreamt of twenty years ago, and hasnow developed into a separate branch of science that con-stantly leads to new scientific and practical results. Theenthusiasm with which the discovery of the galvanic currentand of the voltaic pile was welcomed, as sketched in Lecture

>M Compare L. Meyer, Z. physik. Chem. i, 134. ^ Wiedem. Beibl. 9, 635.30 Berichte, 6, 1073. 31 Jahn, Wiedem. Ann. 43,280. & E. Wiedemann,in Ladenburg's HandwSrterbuch. 7, 31. &* Ostwald, Z. physik. Chem. 9,579. u Gladstone, Proc. Roy. Soc. 16, 439 ; Kanonnikof, J. pr. Chem.[2] 31, 339- m Arrhenius, Bihang Svensk. Handlingar, 8, No. 13, 1884 ;Ostwald, J. pr, Chem. [2] 30, 93. :m Z, physik. Ghem. j , 631 ; 2, 49T,

342 HISTORY OF CHEMISTRY [I.FXT. XVI.

V., was, as we now know, perfectly justified. And evenalthough disillusionment followed the great discoveries ofRitter, Davy, Berzelius, and Faraday, and although thisbranch remained unproductive for decades, still the opinionhas been verified of those who believed that untold treasureslay here which should one day be disclosed.

The modern subject of electro-chemistry forms a con-tinuation to those older discoveries, and to the importantinvestigations of Hittorf and of Kohlrausch (already mentionedon p. 326) which were now for the first time fully understood ;and it leads to successive new discoveries.

In this connection, accumulators may first of all be men-tioned here, since they have come into very general use,and without them it would scarcely be possible to employelectricity to advantage. Their introduction is the outcomeof the discovery of polarisation by Ritter,87 and of the veryexhaustive researches of Plante, which extend as far backas the year 1859.88 Plante constructed very powerfulexamples of the so-called secondary batteries; and these wereafterwards improved upon in important particulars by Faure.SC)

The devising by Lippmann of the capillary electrometer,40

which depends upon the change produced in the surface ten-sion of mercury by polarisation, is also worthy of mention.

The theory of the voltaic pile, for which we are in-debted to Nernst,41 is very important. It is founded uponthe theory of diffusion, which was advanced by Nernst him-self, and upon the idea of solution pressure deduced fromvan *t HofFs theory of solution. Nernst also developed thetheory of concentration cells in the same way, and in doingso arrived at the same conclusions that Helmholtz42 hadalready reached by thermodynamical investigation.

These matters must, however, be disposed of here by

•t7 Voigt's Magazin. 6 (1803), 105 ; compare also Gautherot, Sue, Hist.du Galvanisme. 2, 209. M Comptes Rendus, 49, 402; 50, 640; Re-cherches sur lMlectricite, Paris 1879. m German Patent, 18 81. *° Pogg.Ann. 149, 546 (1873). « Z. physik. Chem. 2, 613 ; 4,129. *» fcrlin.Akad, Ber. 187^ 71^.


merely alluding to them, since they really belong more tothe domain of physics than to that of chemistry.

Turning now to subjects that concern us more immedi-ately, we shall here first consider the progress that hasbeen made in analytical chemistry by the application ofelectrolysis. The subject of electrolysis is a very old one,and so early as 1800 Cruickshank predicted that it wouldbe turned to account in this way.43 It was qualitativeanalysis, however, that alone derived any benefit from itat first.44 Magnus afterwards drew attention to the factthat quantitative analysis—that is, the separation of themetals—must be possible by means of electrolysis ;45 andexperiments in the same direction were made moreover byGibbs46 and by Luckow.47 Classen,48 Miller and Kiliani,*9

Smith,60 Vortmann,51 and others, afterwards introduced themanifold applications of electrolysis to quantitative analysis ;and Classen devised the form of apparatus by which the ex-periments are generally carried out. The great importanceof attention to the potential difference in these experimentswas first recognised by Kiliani.52

The applications of electrolysis to metallurgy are probablystill more important. After the researches of Davy, alreadyfully sketched (p. 70), it was especially those of Bunsen53

(published by the latter partly alone and partly in con-junction with Matthiessen) that brought about any notable

43 Nicholson's Journal (quarto), 4, 254. u Compare, amongst others,Davy, Phil. Trans. 1807, 1 ; 1808, I; Becquerel, Me"m. de l'Acad. 10,284; Fischer, Gilb. Ann. 42, 92 ; Gaultier de Claubry, Journ. Pharm.Chim. [3] 17, 125; JNikles, Jahresbericht 1862, 610; Becquerel, Ann.Chim. [2] 43, 380. ^ Pogg. Ann. 102, 1. 46 Z. anal. Chem. 3, 334.47 Dingl. Polyt. Journ. 177, 23r ; 178, 42. ^ Handbuch der Elektrolyse ;Berichte. 27, 163 and 2060. 49 Lehrbuch der analytischen Chemie,Second Edition, Munich 1891. ^ Journ. Amer. Chem. Soc. 16, 93,420; 17, 612, 652 ; Elektrochem. Zeitsch. 1, i85 and 290, 313 ; Z. anorg.Chem. 4, 96, 267, 273; 5, 197 ; 6, 40, 43. 51 Elektrochem. Zeitsch. 1,138; Monatshefte. 14, 536. &2 Berg-und Htittenmannische Zeitschrift.1883. M Annalen. 82, 137; Pogg. Ann. 91,619; 92, 648; Annalen,94, 107, etc,

344 HISTORY OF CHEMISTRY [LECT. xvr.

advancement. Electrolysis first found a technical applica-tion upon the discovery of electrotyping by Jacobi andSpencer in 1839, an art which depends, however, upon anobservation made by De la Rive in 1836.

The technical production of metals by electrolysis onlybecame possible after the discovery, in 1872, of the dyna-mo-electrical machine, which was employed immediatelythereafter (in the North-German Refinery at Hamburg) toremove copper from solutions. Other metals, such as zinc,magnesium, lead, silver, gold, etc., were also producedelectrically afterwards. An operation of especial importancewas the electrolytic production of aluminium, a metal whichBunsen first prepared by this method.54 The technicalprocess of Heroult55 is different, however, from that ofBunsen, inasmuch as it is not a fused double chloride ofthe metal that is electrolysed, but aluminium oxide.

This is the place to refer to the great scientific andpractical results that Moissan obtained as the outcome ofhis experiments with the electric furnace.56 Specially worthyof mention in this connection are the preparation of arti-ficial diamonds ; the production of calcium carbide (whichhad, however, been discovered long before by Wohler57)and of many other carbides; the preparation, in-a state ofpurity, of chromium and of other difficultly fusible metals,etc.. The first preparation of carborundum, which is alsofrequently attributed to Moissan, is due rather to Acheson/'8

an American. Attention must be drawn to the facts thatin many of these experiments electricity is only employedas a'means of attaining high temperatures (30000 to 40000),and that the results can also be obtained in other ways,since the same high temperatures can, of recent years, bereached by means of chemical reactions. An entirely newbranch- of thermo-industry has thus arisen, by means of

« Pogg. Ann. 92, 648. m German Patent, December 1887. m LeFour jfelectrique, Paris 1897 ; Comptes Rendus. 115, 1031 ; 1x6, 2i#, 1429 j117, 425, 679 ; 118, 320, 501, etc. m Annalen. 124, 220. m Comparealso Schutzenberger, Comptes Rendus. 114, 1089.


which great advances have already been made, and arestill to be expected, in metallurgy. Of an earlier date isthe employment of the oxy-hydrogen blowpipe in the melt-ing and working of platinum/59 and so is the combustionof carbon and other elements (such as silicion, sulphur,phosphorus, etc.) in air or oxygen at high temperatures^for the purpose of attaining still higher temperatures ; as,for example, in t h e blast furnace, or in the ingeniousBessemer process. The development of these methods byGoldschmidt,00 and their application to the production. ofmetals such as chromium, manganese, iron, and nickel,, freefrom carbon, and of a large number of alloys, are newhowever.

I may here recall the interesting results obtained, partlyby Victor Meyer01 and partly by Crafts,62 by the applica-?tion of the method of vapour density determination devisedby the former.03 I regard as worthy of mention the proofthat the molecule of iodine, T2, breaks up at high tempera-,tures into single atoms 64 (compare p. 300); and also thefacts that the beginning, at least, of a similar dissociationhas been ascertained in the case of bromine ;65 that themolecule, of arsenic, As4, similarly splits into two ; thatpotassium iodide even at high temperatures corresponds tothe formula KI, and cuprous chloride to the formulaCu2Cl2,etc.

If the attainment of high temperatures has thus been ofservice for the purposes of our science and of technology, solikewise the endeavours, on the other hand, to obtain lowtemperatures have led to great advances, and to results ofaltogether unforeseen importance. A long time has elapsedsince the discovery of the connection between the states of

59 Hare, Phil. Mag. [3] 31, 356 ; further Deville and Debray, Ann.phim. [3] 56, 385. 6° Annalen. 301, 19 ; Z. f. Elektrochemie, 4, 494.61 Berichte. 13, 1010. 62 Comptes Rendus. 90, 183; 92, 39; Berichte.13, 851. 63 Berichte. II, 1867 and 1946; 12, 609 and 68r, etc.u Comptes Fenduc. 90, \Q3 ; 92, 39; Berichte. 13, 85r. ^ L anger andVictor Meyer, Pyro/hemische Untersuchungen, Braunschweig T 8S.5*.

346 HISTORY OF CHEMISTRY [LECT. xvr.

physical aggregation and the critical temperature. Thissubject has been referred to already at p. 322, at whichplace the results of Pictet, of Cailletet, and of Wroblewskyon the liquefaction of the so-called permanent gases arealso stated. Of especial importance were the detailed inves-tigations of Wroblewsky and Olszewsky, who first obtainedquantities of oxygen and nitrogen in the liquid state, andparticularly described many of their properties.66 Themode of measuring temperatures by determining thepotential of thermo-electric currents, which is now largelyemployed, also originated with them.67 In the experimentsthat have been carried out latterly, however, on the liquefac-tion of air and of other gases, Pictet's method has beenabandoned again, and another method has been employedwhich is more nearly related to that of Cailletet. But, thelatter method has been converted into a dynamical or con-tinuous one, in which adiabatic expansion of highlycompressed gases has been utilised in effecting the necessarylowering of temperature. Thus Dewar,68 in his experimentsupon the production of liquid air, liquefied, by its ownexpansion, air which was under a pressure of 100 atmos-pheres and was cooled by solid carbonic anhydride; whereasthe recent technical method consists in cooling exclusivelyby expansion, and the effect of the latter is turned to accountin a very ingenious manner by the employment of a self-intensive apparatus. Linde69 in Germany, and Hampson70

in England almost at the same time constructed technicallyefficient forms of apparatus, based upon this method, for theproduction of liquid air.

Liquid air has not as yet, however, found any technicalapplication upon the large scale. Nearly pure oxygen isobtained from it very cheaply, and the attempt has been

86 Wiedem. Ann. 20, 243 and 860; Wien. Akad. Ber. 1885, 91 (2), 667 ;Monatshefte. 9,, 1067. 67 Compare Holborn and Wien, Wiedem. Ann.59, 220 ; and Ladenburg and Kriigel, Berichte. 32, 1818. m Journ. RoyalInstitution, 1878; 1883-1885; 1892-1899. 69 Z. d. Vereins 4eutscfcerJngenleiire, 39, ir$7t 7° British. Patent, April 189 .

LRCT. xvr.] HISTORY OF CHEMISTRY 347

made to apply it in the technology of explosives, or to theproduction of high temperatures, but no ultimate pronounce-ment can be made with respect to this. Of far greaterimportance are the results that liquid air has achieved inscientific investigation.

In the first place, it must be mentioned that Dewar, byits aid, has succeeded in liquefying hydrogen,71 and inobtaining air, oxygen, and hydrogen in the solid state ; andthat in doing so he has achieved almost everything thatcan be done in this direction. Dewar is at present engagedin trying to reach still lower temperatures by the aid ofliquid hydrogen boiling under low pressure, in order toapproach as nearly as possible to the absolute zero.72

It is also noteworthy that ozone, which was obtained inthe liquid state by Hautefeuille and Chappuis in 1882 bythe aid of liquid ethylene,73 can easily be prepared in anapproximately pure condition by the use of liquid air, sothat Troost was able to determine its boiling point74 andLadenburg its density.75 The latter determination is ofespecial importance, since the molecular formula O3, deducedfrom it, constitutes one of the most emphatic arguments infavour of the whole molecular theory; and this formula,which till then had only been supported by Soret's experi-ments,70 could not be regarded as finally settled.

But the results that have been furnished by this agencywith respect to the discovery of new elements are almostof greater consequence.

When Lord Rayleigh compared the relative density ofatmospheric nitrogen with that of nitrogen prepared fromammonia and other nitrogen compounds, he found a differ-ence (in the third decimal place) which could not possiblybe ascribed to an experimental error.77 He therefore resolved

71 Olszewski was the first, however,* who prepared liquid hydrogen.Comptes Rendus. 101, 238. w Proc. Roy. Soc. 64, 227; Ann.Chim. [7] 17, 5- 73 Comptes Rendus. 94, 1249. 74 Ibid. 126,1751. 75Berichte. 31, 2508, 2830; 32, 221. 7(J Annalen. 138, 45 jSupplementband 5, 148. ^ Nature, 46, 5i2t

348 HISTORY OF CHEMISTRY [LECT. XVT.

upon a minute investigation in order to find out the sub-stance that was mixed with atmospheric nitrogen. Thisinvestigation he then carried out along, with Ramsay, andit led to the discovery of argon, an element of which it isvery difficult to obtain any compounds.78 The molecularweight, deduced from the density, gave, the number 39.92/°and since by Kundt's method (compare p. 300) themonatomiccharacter of the gaseous molecules was indicated, its atomicweight would be represented by the same number. Thequestion as to the position of this element in the periodicsystem is thereby rendered an extremely difficult one, sinceit falls near that of potassium and yet is beyond it.

Ramsay took up the problem from a very general pointof view. It appeared to him highly probable that argonwas a member of a wjiole group of elements, of which grouphe hoped to find additional members associated with nitrogen.It was thus that he came to investigate, amongst other things,the gases evolved from cleveite by heating with sulphuricacid, which Hillebrandt had considered to be nitrogen,80 andthis led him to the discovery of helium. The brightest linein the spectrum of this gas, D3 (Dx and D2 are the sodiumlines), had been observed a long time previously by Lockyerin the spectrum of the sun's photosphere.81 Helium, whoseatomic weight 4 was deduced" from the density of the gasand from the rate of propagation of sound in it, was ananalogue of argon in every respect; and it was thus clear toRamsay that there must be another element which, withatomic weight about 20, should be placed before sodium, inthe same way that helium comes before lithium, and argonprobably before potassium, although the atomic weight ofargon has been found, in the meantime, somewhat higherthan that of potassium.82 A similar thing applies to tellurium,

. 7S Rayleigh and Ramsay* Proc Roy. Soc. 57, 265 ; 2. physik. Chem.16, 344; Phil. Trans. 1895 (A), 187; Berthelot, Comptes Rendus. 120,58c ; 129, 7f. **• Berichte. 31, 3121. m Bull. U.S. Geological Survey;78, 43. m Nature, 53, 319, ?2 Berichte. 31, 31 it.


the atomic weight of which, according to the most recentdeterminations, is greater83 than that of iodine.84

Ramsay now represents the further development of thesubject85 as if the investigation, carried out with his utmostenergy and effort, had remained unproductive, and as if anaccident only had led him on to his further discoveries.There is in reality, however, no such accident in question,for the investigation of the residue from the evaporation ofliquid air was only a link in the chain which, althoughperhaps unknown to himself, represented the course of hisideas. In this way he discovered krypton, the molecularweight of which was ascertained in a preliminary manner tobe 45, but was subsequently fixed at 82.86 In the case ofkrypton, the ratio of the specific heats has also been ascer-tained to be 1.66, so that this gas is also a monatomicelement, the position of which in the periodic system is stillundetermined.

As regards other discoveries, Ramsay found, by thesystematic fractionation of argon 8T (which he condensed bymeans" of liquid air), two new elements—neon, with atomicweight 19.9, which is clearly to be placed therefore betweenhelium and argon and before sodium ; and xenon, the densityof which was first found to be 65 (H = 2), but afterwards 128,The so-called metargon proved on more minute investiga-tion to be carbonic oxide.

Even although all doubt as to the individuality and theelementary nature of these gases is not yet removed,88 stillthese investigations are unquestionably amongst the mostsuccessful that have been carried out during the last twentyyears. Liquid air served not merely as starting material forthe investigations, but Ramsay also employed it, or at leastthe liquid oxygen obtained by its aid, in an ingenious

83 Brauner. Journ. Chem. Soc. 6% 549 ; Kothner, Anrialen. 319, 1.84 Compare, however,- Rydberg, Z. anorg..Chem. 14, 66, 8B Berichte, 31,3116. 86 Z;"physik. Chem. 38, 683. The conflicting results of Ladenburg(Berlin. Akad. Ber. 1900) require explanation. 87 Berichte. 31, 3117,88 Compare Brauner, Ibid. 32, 708.

350 HISTORY OF CHEMISTRY [user. xvi.

manner for the purpose of separating the various newelements.

The question as to the position of these " elements " inthe periodic system has been much discussed, and up to thepresent it is not finally solved. On the other hand, we maynow say that even if our views respecting the connectionbetween the properties of the elements and their atomicweights should be modified on account of these newly dis-covered facts, still the periodic law has rendered excellentservice as an invaluable guide in this obscure region.

Although such unexpected discoveries were thus made,still they will not exercise any considerable influence uponchemistry as a whole, since all these elements apparentlyresemble argon, and probably do not enter into many com-pounds. Hence it may be said that these interestinginvestigations will probably not prove of great significanceas regards their consequences, and that in this respect theywill fail short of other researches which have not excitedthe interest of such wide circles.

I merely recall here the isolation of fluorine by Moissanin 1886,89 and the discovery of nickel carbonyl and alliedcompounds by Mond in 1890,90 and pass on to consider moreparticularly the investigation of the chemistry of nitrogen,which has made great advances in recent years.

The discovery of hydroxylamine, by Lossen, falls underreview here, although, of course, it took place at a muchearlier date (in 1865).01 It has not been referred topreviously, however, since its importance only came to berecognised gradually, a result to which Victor Meyer'sresearches on the oximes02 and their stereo-isomerism9S

materially contributed.

89 Comptes Rendus. 103,102 and 256. m Journ. Chem. Soc 57, 749 j59, 604. fll Zeitschrift fur Chemie. 8, 551; Annalen, Supplementband 6, 220 j160, 242 ; 161, 347» etc. n Meyer and Janny, Berichte. 15, 1324 ; Janny,Ibid. 15, 2778; 16, 170; Meyer, Ibid 16, 822 ; Petraczek, Ibid. 16, 823,etc* m H. Goldschmidt, Berichte. 16, 2176; Auwers and Meyer, Ibid,21, 784, 3SIO J 22» 537, etc


The preparation of phenylhydrazine, by Emil Fischer,94

also deserves mention here. It must be looked upon as ofparticular importance, on account of its leading to theclearing up of the sugar group.95 Following upon this thereare the valuable researches of Curtius, who discoveredhydrazine in 1889,^ and hydrazoic acid in 1890.97 Theutilisation of these two substances has already led to numerousinvestigations, and will lead to others. As worthy of mention,I also refer to the researches of Thiele,98 who (amongst otherthings) found out a convenient and technically practicablemethod for the manufacture of hydrazine; and to those ofRaschig," who cleared up the nitrogen-sulphonic acids, andin doing so discovered the method now employed for theproduction of hydroxylamine.

It does not seem to me that this is the place to entermore fully into this subject, since I am really giving ahistorical sketch, in which only those things that are ofgeneral importance can be prominently brought forward.

I may thus recall here a discovery of HellriegePs whichmarks an epoch in chemistry and agriculture.100 Accordingto Hellriegel, leguminous plants, and lupins in particular,possess the power of assimilating, with the aid of lowerorganisms, the nitrogen of the air. In this connection thefact must not be passed by without mention that Berthelothad previously asserted the assimilation of free nitrogen.101

An observation which is to a certain extent of anopposite character is the proof furnished by Buchner thatfermentation is possible even without living organisms, bymeans of the liquid expressed from yeast (zymase).102

More particular consideration may be given to a re-search by van Jt Hoff, in which the idea and the significance

04 Berichte. 8, 589; compare also Strecker and Roemer, Ibid. 4, 784;and Zeitscbrift fur Chemie. 14, 481. m Berichte. 17, 579. ^Curtiusand Jay, J. pr. Chem. [2] 39, 27. 97 Curtius, Berichte. 23, 3023.l>8 Annalen. 270, 1 ; 273, 133 ; Berichte. 26, 259S and 2645, etc*09 Annalen. 241, 161. 10° Hellriegel and Wilfahrt, Biederm. Centr. 18,179. 101 Comptes Rendus. id6, 569. 102 Berichte. 30, 117, 1110, 2668, etc*

352 HISTORY-:OF CHEMISTRY [LBCT. XVI.

of the transition; temperature are clearly stated.103 Van' tHofF is led to the idea by the comparison of chemicalreactions with the transitions from one of the states ofphysical aggregation to the others; but the same con-ception may be arrived at by the aid of the phase rule.

Since the observations of St Claire Deville (see p. 302),the phenomena of dissociation have been regarded andtreated as analogous to those of evaporation. Van }t HofFnow shows that there are reactions which are comparablewith the process of fusion, and inwhich a fixed temperaturemarks the line of separation between two chemically differentconditions. This fixed temperature he designates the transi-tion temperature j and he demonstrates the accuracy of hisidea in the cases of the formation of double salts (astrakanite),of the preparation of elements in allotropic modifications(sulphur), and of the splitting of racemic substances (sodiumammonium racemate).

He afterwards treated this subject in a much moredetailed manner in an important monograph104 "On theFormation and Decomposition of Double Salts,7' in whichhe explains the theory of the matter, and describes themethods for experimentally determining the transitiontemperature.

These investigations have found very important applica-tions in relation to the deposition of salts from ocean water,103

and in explaining the splitting of racemic compounds byPasteur's methods.

This leads us directly to the subject of stereo-chemistry,which has already, been discussed in Lecture XIII. (see p.268),. but which has acquired so much importance of latethat I must return to it here.106

Attention has already been called to the fact that allcompounds with asymmetric carbon atoms do not possess

m Van 't Hoff and Deventer, Berichte. 19, 2142.' 104 German Editionby.Dr Paul, Leipzig 1897. 105 Berlin: Akad. Ber. 1897, 1898,1899,1900,1901,1902. X06 Compare vanf t Hoff: Die Lagerung der Atome hn Raume,Second Edition, -Braunschweig 1894.


optical activity. It was possible to show, however, thatfacts of this description only constitute apparent exceptionssince it could be proved in very many instances that thecompounds concerned are " racemic" (that is, that, likeracemic acid, they can be split into their enantiomorphousconstituents, or that they are mixtures of such mirror-images), or that they are meso-compounds (that is, thatthey behave like meso-tartaric acid, which, it is true, isonly possible in the case of substances with symmetricalstructural formulae).

The theory of the asymmetric carbon atom was tested,in the first instance, in isolated cases only, and, amongstothers, in the splitting by means of fungi of a series ofalcohols, which Le Bel succeeded in effecting ; l o r and in thesplitting of synthetic coniine,108 which was of importanceinasmuch as it was the preparation, for the first time, of anactive base. The theory was subjected to a systematicexamination by Emil Fischer, in carrying out bis well-known syntheses in the sugar group.109

It is simply astonishing that the theory stood the testof this experimentuni cruets, and that the sagacity of Fischerenabled him to fix the configurations of the individualhexoses110 without encountering any contradictions in doingso, especially when we take into consideration the recentexperiments of Walden,111 in accordance with which it ispossible, by means of simple chemical reactions, to pass atordinary temperatures from an active substance to itsenantiomorph.

As regards the application of the theory of asymmetriccarbon atoms to molecules with doubly linked carbon atoms,—a matter that van ?t Hoff had already mentioned, but onewhich had met with less attention,-—very special notice

107 Comptes Rendus. 87, 213 ; 89, 312 ; Bull. Soc. China. [3] 7, 551 ;Comptes Rendus. 92, 532. 108 Ladenburg, Berichte. 19, 2578 ; Annalen.247, 83. m Berichte. 23, 2114 ; 27» 3189. 110 Ibid. 24, 1836 and 2683*111 Berichte. 28, 2766 j 29, 133 ; 30, 2795 and 3146*

Z


was called to it by Wislicenus,112 who, moreover, had him-self given the first impulse to stereo-chemical conceptionsby his earlier and extended investigations of lactic acid.118

The researches of Wislicenus and his pupils114 havecertainly supplied most valuable contributions towards theclearing up of these remarkable cases of isomerism—an endtowards which (after the discovery of fumaric115 and maleic116

acids) a great many chemists, and even Kekule himself,117

had aspired in vain. In this domain, however, there arestill many unexplained contradictions, as Michael118 andAnschiitz119 in particular have shown. On the other handit must be admitted, that by van't Hoff s theory an extremelyplausible explanation of the products arising from the oxida-tion of fumaric and of maleic acids is rendered possible.120

The applications of the doctrine of asymmetric carbonatoms to substances containing rings are also important andinteresting. The first principles were laid down by van 'tHoff;121 but their significance was only fully recognisedwhen Baeyer published his extended investigations uponhydrogenised aromatic compounds, and, in particular, uponthe hydrophthalic acids,122

While Baeyer's intention in these investigations was todiscover weaknesses in the theory, and even to modifyit, his labours led instead to a further confirmation of it.Besides this, the credit is due to him of having advancedthe so-called tension theory,123 which has already proved ofservice in some cases.

112 Uber die raumliche Anordnung der Atome in organischen Molekulentmd ihre Bestimmung in geometrisch-isomeren ungesattigten Verbindimgen,Leipzig 1887. 113 Annalen. 125, 41 ; 128, 1; 133, 257 ; 146, 145 ; 166, 3 ;and especially 167, 345- 114 Ibid. 246, 53 ; 248, 1, 281; 250, 224 ; 272,1 j274, 99* 115 Pfaff, in Berzelius' Jahresbericht 1828, 216. m Pelouze,Annalen. II, 263. m Ibid. Supplementband 2, III; Zeitschrift furChemie. 10, 654. 118 J. pr. Chem. [2] 38; 43; 46 ; 52, etc. 118 Annalen.254, 168. m Kekule and Anschiitz, Berichte. 13, 2150; 14, 713.121 La chimie dans Tespace. 122 Annalen. 245, 103 ; 251, 257 ; 256, 1 ;258, I and 145 ; 266, 169 ; 269, 145 ; 276, 255. *••» Berichte, 18, 2278.

LECT. xvr.] HISTORY OF CHEMISTRY 355

Emphasis must be laid upon the fact that the importantconsequences which the theory of the asymmetric carbonatom brought forth, gave a spur to the more and morecomplete application of stereo-chemical considerations. Inthis connection the numerous researches may be mentionedwhich deal with the non-occurrence of certain reactions, andexplain this on stereo-chemical grounds.124 Amongst theseinvestigations the best known are those of Victor Meyer onthe formation of esters.125 The asymmetry of the nitrogenatom may also be mentioned in this connection.

The researches of Hantzsch and Werner126 were offundamental significance with respect to the last-namedsubject, and they were capable of explaining the isomerismamongst oximes, which was already familiar at that time.Hantzsch afterwards extended the views respecting thismatter, and turned them to account in explaining the iso-meric hydrazones127 and diazo-compounds.128 It is true thatit was only geometrical isomerism in the case of nitrogenousorganic compounds that was proved by these investigations.Le Bel129 and Ladenburg130 endeavoured to prove thatasymmetric nitrogen can further produce or influenceoptical activity. The investigations of both have, how-ever, been attacked,131 but both have been able to establish

ltWHofmann, Ibid. 17, 1915 ; and 18,1825 ; Jacobson, Ibid. 22, 1219; 25,99a ; 26, 681 and 699, etc. ; Pinner, Ibid. 23, 2917 ; Kuster and Stallberg,Annalen. 278, 207. V2® Berichte. 27, 510, 1580, 3143 ; 28, Ref. 301 and 916 ;29, 830, etc. m Ibid. 23, 11 ; Werner, Raumliche Anordnung der Atomein stickstoffhaltigen Molekiilen, 1890. Compare further, the previouslypublished researches of Willgerodt, J. pr. Chem. 37, 449 ; Burch andMarsh, Journ. Chem. Soc. 55, 656; and especially van 't Hoff, Ansichlen*uber die organische Chemie, Braunschweig 1878-81. m Fehrlin,Berichte. 23, 1574; Krause, Ibid. 23, 3617 ; Hantzsch and Kraft, Ibid. 24,3511 ; Marckwald, Ibid. 25, 31CO. vm Ibid. 27, 1702, 1726, 1857, 2099,2968,3527; 28,741, 1124, 1734, etc. 129 Comptes Rendus. 112, 724.130 Berlin. Akad. Der. 1892, 1067; Berichte. 26,854; 27, 853 and 859.131 Marckwald and Droste-Huelshoff, Tbid. 32, 560; Wolffenstein, Ibid.29, 1956.


the accuracy of their results.132 Pope and Peachey sub-sequently prepared optically active compounds of sulphur 13:J

and of tin.134

Another subject that was much discussed was the signi-ficance of racemism, about which a clear understanding onlybecame possible upon the introduction of the conception ofthe transition temperature, and upon the recognition of theanalogy between racemic substances and double salts. Themost important method of splitting racemic substances—thatby means of optically active substances—remained a standingenigma as long as the existence of partially racemic sub-stances was denied.135 Every difficulty was removed, how-ever, after Ladenburg had shown that such substances dowithout doubt exist,136 and after a transition temperaturehad been recognised in their case also.137

Furthermore, the much debated question as to how atruly racemic substance (inactive by intra-molecular com-pensation) can be distinguished from the mixture of theactive components, may now be looked upon as practicallysettled.138

It is beyond doubt that the founding and developmentof stereo-chemistry (a name which originated with VictorMeyer139) is the most important thing that was accomplishedin organic chemistry during the last two decades of thenineteenth century. Stereo-chemistry possesses a signi-ficance for this period similar to that which the foundationand introduction of the theory of aromatic compounds pos-sessed for the twenty years preceding. There are besides,

ia>i Le Bel, Comptes Rendus. 129, 548 ; Ladenburg, Berichte. 29, 2706 ;34, 3416; compare further Wedekind, Ibid. 32, 511 and 722; Pope andPeachey, Journ. Chem. Soc. 75, 1127; 79* 828. m Ibid. 77* 1072,m Proc. Chem. Soc. x6, 42. m E. Fischer, Berichte. 27, 3226 ; Landolt,Das Optische Drehungsverm8gen, Second Edition, Braunschweig, 1898, 85.136 Ladenburg and Herz, Berichte* 31, 937; Ladenburg and Doctor, Ibid.31,1969. m Ladenburg and Doctor, Ibid. 32, 50* m Roozeboom, Z.physik. Chem. 28, 494; Berichte. 32, 537 ; Ladenburg, Journ. Chem. Soc.75, 465; Berichte. 32, 864; Pope and Peachey, Journ* Chem* Soc. 75,I in . m Berichte. 23, $68,

LRCT. xvi.] HISTORY OF CHEMISTRY 357

however, other important investigations in organic chemistrywhich require to be mentioned.

A matter of general importance was the introduction ofthe idea of tautomerism or desmotropy, which was broughtforward by Laar,140 in 1885, on the strength of some experi-mental observations and remarks by Zincke.141 Laar appliesthe term tautomeric to a compound when two or morestructural formulae can be advanced in explanation of itsinteractions. A very well-known example is furnished byaceto-acetic ether, which reacts, sometimes as if it should berepresented by the ketone formula CH3.CO.CH2.COOC2Hr>,and sometimes as if it should be represented by the enolformula CHrC(OH) : CH.COOC2H5. There are numerousinvestigations dealing with substances of this kind, of whichthere are a large number. Some of the best known of theseinvestigations are those of Claisen,142 of W. Wislicenus,143

and of Knorr.144 Opinions are still widely divergent, withregard to the questions as to whether a desmotropic sub-stance is to be considered as a mixture of two or morecompounds (Laar), or whether the forms are continuouslypassing into one another by means of oscillations (Kekule)or shifting linkings (Knorr145), or finally whether one formis stable under certain conditions while another is stableunder different conditions.

The systematic and, theoretically, almost completedexamination of the sugar group has already been referredto (p. 351)- The uric acid group, which so long resistedelucidation and synthesis, is now completely cleared up,146

and this is chiefly due to Emil Fischer's synthetical investi-gations.147

140 Berichte. 18, 648 ; 19, 730; compare also Butlerow, Annalen. 189,76. 141 Berichte. 17, 3030. 142 Aonalen. 291, 25. 14:} Ibid. 291, 147.144 Ibid. 293, 70. m Ibid. 279, 188. 14(J Compare further, Grimaux,Ann. Chim. [5] 11, 356 ; and 17, 276 ; Horbaczewsky, Monatshefte. 3, 796 ;6, 356 ; 8, 201 ; Behrend and Roosen, Annaien. 251, 235 ; W. Traube,Berichte.33,1371 and 3035. 147 Berichte. 30, 549, $$9, 1839, 1846, 2220,2226, 2400, 3009 ; 31,104, 431, 542,1980, 2546, 2550, 2619, 2622 ; 32, 435.


The hydrogenised aromatic compounds have likewisebeen referred to already (p. 354), but the terpenes have notbeen mentioned. The latter formerly constituted one of themost confusecl sections of organic chemistry, whereas Wallachhas now succeeded in systematising them by his extendedand careful researches.148 But the most important thingabout them, the elucidation of their constitution, is stillwanting; for, in spite of some fortunate attempts byBaeyer,14'-'which led to the synthesis of substances resembling terpenes,no one has yet succeeded in making this clear.

The discovery of the iodo-, iodoso-, and iodonium-com-pounds, for which we are indebted to Willgerodt150 and toVictor Meyer,151 is also important, and these compoundssupply new knowledge concerning the nature of iodine. Thediscovery of antipyrine by Knorr152 was of great importancein medicine, and through it the pyrazol group153 came tobe simultaneously explored.

The preparation of the so-called substantive azo-dyes hasbecome of technical importance;154 and of much greaterconsequence is the manufacture of ammonia soda, and thatof synthetic indigo, according to a method discovered byHeumann.155 The latter is probably the most strikingexample of the eminent service which a close union oftechnical practice with science is capable of rendering. Thedetailed history of this subject, which has now been placedat our disposal in the exposition by Brunck,150 presents anabundance of interesting and instructive matter. In thisconnection it may be pointed out specially that the manu-facture of synthetic indigo has brought to maturity a newmethod of obtaining sulphuric acid,157 which may, perhaps,displace the.old-established method. That catalytic pro

148 Annalen. 225-320 ($2 papers). U9 Ibid. 278, 288 ; Berichte. 26, 232.150 J. pr. Chenx [2] 33, 154 ; Berichte. 25, 349*> ; and 26, 1802. m Ibid.25, 2632; 26, 1354; 27, 1592 ; 28, Ref. 80. 152 Ibid. 17, Ref. 148 and149. Also Ibid. 17, 2032, etc. m Annalen. 279, 188 ; 293,1. 1M GermanPatent No. 32958, 1884. m Berichte. 23, 3043, 3431. "* Ibid. 33,Sonde'rheft, lxxii w Ibid* 34, 4069, •*-


cesses have been called into requisition both in the pre-paration of the anthranilic acid necessary for the indigomanufacture, and in the combination of sulphurous anhydrideand oxygen, has directed the attention of chemists anew tothis subject, which was dealt with so frequently at an earlierperiod.158

Great advances have likewise been made in the prepara-tion of artificial perfumes. Vanilline has already been referredto. The manufacture of piperonal159 (heliotropine), andespecially the synthesis of ionone by Tiemann and Kriiger,160

must be mentioned in this connection.This account of the most recent phases in the develop-

ment of our science must not be concluded, however, withoutreference being made to the valuable, although unfinished,researches which were carried out under the direction ofFriedel, and which aimed at the introduction of a newnomenclature into organic chemistry.161 Although it hasnot yet been possible to extend the system to substancescontaining rings, still there is much that is good and valuablein the principles that have been advanced.

With the foregoing observations I may be permitted toconclude these lectures. I shall be gratified if the mattersthat have been discussed should prove to be of value inconveying an outline of the history of our science; and, inany case, I hope that the lectures may serve as a stimulusto independent study. There are few things which operatemore advantageously in the latter direction than a surveyof the past. We recognise that progress is only possiblewith the united activity of many workers ; we realise thateven the smallest contribution is not useless ; and we areled to exercise our own "small capacities in the hope thatthey also may increase, by a drop, the tide of generalknowledge.

im Ostwald. Elekirochem. Zeitsch.,7, 995- 159 Anrialen. 152,,25.160 Berichte. 26, 2675 ; 28, 1754. 161 Cotapare the report by TiBerkhte.26,1595. 3 d

I N D E X O F A U T H O R S ' N A M E S

A BNEY, 319• ^ Acheson, 344Ampere, 63, 93, 131, 138Anderson, 283, 285Andrews, 200, 321Angstrom, 317Anschlitz, 354Aristotle, 4Aronheim, 280Arrhenius, 339-341Avogadro, 61-63, 93, 103, 105, no,

133, 192, 195, 201, 2ir, 299, 338Awdejeff, 312

%T*AEYER, 276, 279, 280, 282,284, 286, 289, 290, 293, 294,

296, 354) 35SBamberger, 282Bayen, 13Beaume, 36Becher, 5, 7Becquerel, 320Beilstein, 271, 273, 274Bergman, 10, 32, 36, 37, 40, 49, 50,

86Bernhejmer, 332Bertagnini, 292Berthelot, 116, 242-245, 249, 262,

282, 284, 288, 290, 293, 307, 315,324, 325, 340, 351

Berthier, 319Berthollet, 32, 35, 37-46, 48, 5?,

76-78, 86, 88, 89, 96, i n , 113, i124, 153, 314 !

Berthollet (younger), 78860

Berzelius, 34, 45, 50, 66, 69, 71, 74,76, 81, 84-95, 97-102, 104-106,108-110, 112-115, 118-121, 123,126, I3I-I33, 135, 136, 138, 142,149, 158, 162, 164, 168-172, 174,175, 178, 182, 188-190, 207, 223-225, 227-230, 298, 325, 342

Bessemer, 345Beudant, 96, 97Bineau, 148, 192, 300Biot, 332Black, 11Blagden, 22Bodenstein, 335Boerhaave, 8, 20Boullay, 120, 122, 125, 133, 138Boyle, 9, 27, 59, 337, 338Braun, 326Brauner, 313Brewster, 317Brodie, 198-200Briihl, 330-332Briining, 258, 260Bru^natelli, 70Brunck, 358Buchner, 351Buckle, 1Buckton, 228, 239, 248Buff, H. L., 247-249Bunsen, 126, 127, 305, 313, 316,

317, 327, 328, 343, 344Butlerow, 240, 256, 264

Q A D E T , 126, 127Cagniard de la Tour, 320

Cahours? 2I2? 227, 28^ 30;

INDEX OF AUTHORS' NAMES 361

Cailletet, 322, 346Cannizzaro, 298-300, 304, 339Carius, 268, 269Carlisle, 69Caro, 279, 282, 287Cauchy, 330Cavendish, 11, 12, 19, 22, 25,

76Chancel, 186, 210, 262Chappuis, 347Chevreul, 115Chiozza, 212, 213* 215, 293Ciamician, 319Claisen, 294, 357Clark, 118, 152, 158Classen, 343Clausius, 201, 301Clement, 81Cleve, 311Cloez, 247Coppet, 332Couper, 253-255, 260, 269, 308Courtois, 81Crafts, 167, 210, 228, 291, 300,Crookes, 318Cruickshank, 70, 343Curtius, 351

28,

345

I M *> 3 3 T

J^ Dalton, 45, 48, 49, 53-6i, 63-66, 91, 93, 101, 102, 112, 175

Daniell, 326D'Arcet, 77Darwin, 2Davy, 70-86, i o r , 119, 131, 138,

158, 161, 325,342, 343Debray, 303De la Rive, 344De Luc, 22Democritus, 47Demole, 267Dessaignes, 268Derille, 299, 301-307, 320, 352Dewar, 283, 34^, 347Do"bereiner, 114, 122, 311Draper, 327, 328Drion, 320Duiong, 83, 84, 95, 96, 99, 104,

106, 119, 158, T6O, 162, 29SDumas, 101-105, n o , 122-125, 127,

128, 1 ^ 2 - I J B , 140-146, 148-151,

157, 162-168, 170, 176, 178-180,182, 189, 202, 206, 211, 216, 217,225, 226, 229, 232, 240, 299, 311,327

Duppa, 257, 264, 289Dusart, 291

T ? B E L M E N , 320Emmerling, 286

Empedocles, 4, 5Engelhard, 258Erienmeyer, 256, 263, 264, 280,

289, 290Erman, 89Esson, 315Exner, 332

! p A R A D A Y , 106, 117, 118, 141,! X 199 ,281 ,322 ,325 ,342• Favre, 197, 323: Festing, 3191 Fischer, E., 280, 351, 353, 357; Fischer, E. G., 52, 53! Fischer, O., 280i Fittig, 270, 271, 276, 280-282, 290,I 295: Foucault, 317i Fourcroy, 32, 58, 89

Franchimont, 282I Frankland, 203, 204, 225-227, 230-; 235, 242, 249, 262, 264, 290, 2gr,

308Fraunhofer, 317, 318, 328Freund, 263Friedel, 167, 210, 228, 262, 263,

290, 291, 320, 359Fritzsche, 287Fuchs, 96

p A L V A N I , 69^ Gaudin, 319Gautier, 270Gay-Lussac, 45, 58-62, 74, 76-79,

81, 82, 84, 91-93, 96, 101, 104,105, 107, 113, 1x4, 119, 121, 122,124, 125, 140, 196, 320, 337, 338

Get-er, 6Gehlen, c,6Geoffroy, 36, 86

3G2 INDEX OF AUTHORS' NAMES

Gerhardt, no, i n , 143, 149, 161,167, 171, 175, 180-194, 196, 197,202-205, 210-221, 223, 225, 229,230, 233, 235, 240-242, 249, 252,253, 256, 285, 293, 298

Gerichten, 286Gerland, 257Geuther, 260Gibbs, 304, 334, 336, 343Gladstone, 301, 331Glan, 319Glaser, 281Glauber, 156Gmelin, 106, 107, 144, 146, 150,

174, 175, 187-189, 228, 235, 311Goldschmidt, 345Goldschmiedt, 282Grabe, 276, 278-281, 285, 287Graham, 152-155, 157, 207, 208,

332Greiff, 296Griess, 275, 277, 280Griffin, 158Grimaux, 290Groth, 333Grove, 301Guldberg, 314, 315Guyton de Morveau, 8, 9, 32, 89

TAAGEN, 331x Hall, Sir James, 319

Hampson, 346Hantzsch, 355Harcourt, Vernon, 315Hausinann, 319Hautefeuille, 320, 335, 347Hauy, 96Hellriegel, 351Helmholtz, 27, 301, 304, 326, 342H^nnell, 132, 133, 206Henry, 6r, 79, 337, 338Heroult, 344Herschel, Sir J., 317Hess, 323, 340Heumann, 358Higgins, 54Hillebrandt, 348Hintze, 333Hisinger, 69, 71Hittorf, 319, 326, 342H b k , 2.95

Hoff, van 't, 268, 269, 334, 337,339, 3SI-354

Hofmann, 185, 203, 204, 212, 233,239, 247, 248, 266, 270, 279, 280,285, 286, 289, 292, 310

Hoogewerff, 285Hooke, 7, 21Hooker, 282Horbaczewsky, 290Horsford, 246Horstmann, 304, 316, 334Hiiber, 285Hilbner, 277, 29 sHiifner, 289Humboldt, 58Hunt, 291

TNGENHOUSZ, 327-*• Isambert, 303

TACOBI, 344J Jahn, 326Jelett, 316Joule, 301

V"ANE, 209, 227, 293-L*- Kanonnikoff, 332Kant, 47Kay, 243Kekule", 221-223, 233, 240, 241,

249-253, 255, 256, 258, 260, 261,269-276, 278-280, 282, 289, 291,294, 304, 308, 339, 354, 3S7

Kiliani, 343Kirchhoff, 316-318Kirwan, 12, 13, 19, 28Klaproth, 44, 112Knorr, 357, 35$Koch, 319Ktfnigs, 284-286Ktaer, 277, 283Kohlrausch, 326, 342Kolbe, 116, 136, 172, 182, 203, 220,

223-231, 233-239. 249, 255, 257-259, 261-264, 269, 280, 289, 291,292

Kopp, 4, 96, 216, 252, 253, 304,329, 330, 339

$59


Kruss, 319Kundt, 3co, 348

T AAR, 357*-* Ladenburg, 277, 286, 290,

295, 347, 355, 356Landolt, 330, 331, 333, 341Laplace, 22, 26, 28, 29, 323, 325,

330Laurent, i n , 128, 136, 138, 142-

150, 161-164, 167, 169, 176, 179.180, 185, 187, 191-197, 202-205,209, 213, 217, 232, 233, 235, 240,266

Lautemann, 258Lavoisier, 4, 11, 13, 16, 17, 20-29,

31-36, 49, 50, 53, 58, 71, 77, 80-83, 89, 101, 108, 111-113, 119,124, 126, 138, 323, 325

Le Bel, 268, 353, 355Leblanc, 229Lecoq de Boisbaudran, 313, 319Lemery, 7, 12, 111Lemoine, 335Lenssen, 311Leucippus, 47Lewkowitsch, 268Lieben, 291, 294Liebermann, 279, 281Liebig, 109, no, 112, 114, 115,117,

125, 126, 128, 131-138, 14O) J42,149, 154-162, 164, 170, 175, 178-180, 186, 188, 190, 202, 205-207,2ir, 224, 226-229, 235, 240, 292

Limpricht, 221, 287, 289Linde, 346Lippmann, 342Lockyer, 348L5wig, 227Lommel, 328Lorentz, H. A., 331Lorenz, L., 331Lossen, 350Lourenco, 247Luckow, 343Lucretius, 47

V f ACQUER, 9, 10, 13I V X Madrell, 258Magnus, 131, 223, 343Malaguti, 142, 162, 171, 233Marcet, 224

, Marignac, 250, 306, 310I Mariotte, 59, 337, 338; Matthiessen, 343I Maxwell, 301, 304, 332i Mayer, J. R., 301} Mayow, 7, 21

Melsens, 171, 224Mendelejeff, 103, 311-313, 321Mendius, 221, 291Menschutkin, 315Meusnier, 58Meyer, Lothar, 103, 311, 332Meyer, O. E., 332Meyer, V., 287, 300, 335, 345, 35°,

355, 356, 358Michael, 354Miller, W. A., 326Miller, W. v., 343Millon, 199Mitscherlich, A., 319Mitscherlich, E., 95-98, 106, 139,

158, 181, 182, 207, 224, 235, 275,299, 301, 319

Moissan, 344, 350Mond, 350MorkownikofF, 264Mousson, 317Miiller, H., 292

AJASINI, 332Natanson, 279

Natterer, 322Naumann, 303, 304Nernst, 342Neumann, 96Newlands, 103, 311, 312Newton, 8Nicholson, 69Nilson, 313

f^DLING, 223, 228, 233, 242,w 243, 245, 249

j Olszewsky, 346! Ostermeyer, 2811 Ostwald, 315, 334, 34°

Oudemans, 341

! pASTEUR, 267, 268, 3$?i + Peacfrey, 356

364 INDEX OF AUTHORS' NAMES

Pean de Saint Gilles, 315Pebal, 30sPeligot, 135, 142Pelouze, 291, 292Perkin, 257, 261, 279, 289, 292,

294, 333Persoz, J., 280Peter mann, 277Petit, 95, 96, 99, 104, 106, 298Pettenkofer, 311Pfaundler, 304Piccard, 282Pictet, 322, 346Piria, 229, 291Planck, 340PJante, 342Pliny, 5, 6Plucker, 319Pontin, 76Pope, 356Priestley, 11, 12, 16-19, 24> 25* 28

Pringsheim, 328Proust, 42-46, 48, 112Prout, 102, 311

X> AMSAY, 284, 347-349AV- Rankine, 301Raoult, 332, 338, 339Raschig, 351Rayleigh, Lord, 347Regnault, 96, 136-138, 142, 143,

162, 165, 216, 228, 298, 301, 302Reich, 318Reichenbach, 271Reimer, 290, 292Rey, Jean, 7, 21Richter, J. B., 45, 48, 50-52, 64,

153Richter, Th., 318Ritter, 73, 342Robinson, 305Rochleder, 269, 285Roozeboom, 337Roscoe, 46, 310, 327, 328Rose, H., 137, 170, 298Rosenstiehl, 279Rossi, 291

CAUSSURE, 113kJ Scheele, 10, IT, 13, 16, 19, 20,

24, 78,80, n r , 115, 327hjel, 2x6

Schiff, H., 311Schiff, R., 278Schmitt, 280Schonbein, 200Schorlemmer, 265Schweizer, 227Seebeck, 76Seguin, 58Senarmont, 320Serullas, 224Silbermann, 197, 323Silva, 290Simon, 70Simpson, 292Skraup, 284, 285Smith, Angus, 48, 50Smith, E. F., 343Soret, 200, 347Spencer, 344Stadeler, 263Stahl, 5-8, 11, 14, 20Stas, 103Stewart, Balfour, 317Strecker, 246, 257, 289Swan, 317

p , 2cox Talbot, 317

Than, 306Thenard, 74, 76-79, 81, 113, 114,

119, 121, 127Thenard, Paul, 212Thiele, 3SiThilorier, 322Thomsen, 278, 315, 324, 325, 332t

34OThomson, James, 301Thomson, Thomas, 48, 5 3» 64, 92,

102Thorpe, 308Tiemann, 290, 359Tillet, 13Tollens, 27 r, 290Troost, 299, 307, 347Tschermak, 310Tyndall, 317

TTLRICH, 25Su Uslar, 22t


"TTALKNTINE, Basil, 123v Van Dorp, 285Van Helm on t, 11Vauquelin, 44, 58, 112Verguin, 279Vierordt, 319Volhard, 289Volta, 69, 72Vortmann, 343

WAAGE, 314, 315Waals, van der, 334, 336-338

Wachendorff, 296Walden, 353Wallach, 358Walter, 166Wanklyn, 228, 235, 305Warburg, 300Watt, 28Weidel, 284, 286Weltzien, 311WenzeJ, 50Werner, 355Wertheim, 285, 289Wiedemann, G., 316, 332

Wilhelmy, 315Willgerodt, 358Williamson, 111, 196, 201, 202,

204-215, 221, 223, 226, 229, 233,235, 237, 240-243, 249

" Windier, S. C. H.," 179W inkier, 292, 313Wischnegradzky, 264, 286Wislicenus, 265, 292, 354Wislicenus, W., 357Wohler, 109, 116, 125, 126, 131,

140, 167, 170, 227, 238, 344Wolf, 321Wollaston, 48, 49, 64-66, 102, 106,

174, 187, 317Woskresensky, 278Wroblewsky, 322, 346Wurtz, 199, 204, 210, 211, 214,

217, 223, 233, 242, 245-249, 257-259, 263, 264, 269, 271, 290, 291,294, 3O7, 3io

Z , 133, 134Zimmermann, 313

Zincke, 290Zinin, 289

I N D E X O F S U B J E C T S

A BSOLUTE boiling point, 320 |" ^ Accumulators, 342 :Acetamide, 228Acetic acid, 120, 141, 164, 171, 208, I

etc., 226 etc., 233, 235 etc., :254 !

acid, formulae, 190, 191ether, 208 •

Aceto-acetic ether, syntheses by !means of, 264 etc., 291 ;

Acetone, 209, 236, 238 jAcetonic acid, 263, 264Acetonitrile, 250Acetyl, 137, 171

theory, 138Acetylene, 269Acichlorides, discovery, 212Acid amides, 212Acidifying principle, 25, 26, 82Acids, dibasic, 183, 194 etc., 210,

212 etc., 229, 238ideas of Davy and of Dulong,

83, 84, 161Lavoisier's views concerning",

25, 31, 33, 78, 108monobasic, 183, 195polybasic, 185, 190, 194 etc.,

202, 242, 292polybasic, existence admitted by

Kolbe, 238tribasic, 239views of Liebig, 157 etc., i6r

Acridine, 287Acrylic aldehyde, 294Active mass, 314Additions, kinds distinguished by

Gerhardt, 182Affinity, 85 etc., 314, 315

Berthollet's views, 37 etc.degree, 254

366

Affinity, elective, 254Guldberg and Wazige's investi-

gations, 314 etc.tables, 36, 41

Alanine, 257synthesis, 256, 289

Alcohol, formulae, 205, 254radicals, 203synthesis, 116, 288views of Berzelius concerning,

132views of Dumas concerning,

141viewsof Gay-Lussacconcerning,

122views of Liebig concerning,

133 etc." Alcoholic hydrogen," 261Alcohols, 236

conversion into acids (Kolbe'sviews), 236

new class predicted by Kolbe,237, 262, 264

polyatomic, 245 etc.polyatomic, theory of, 242tertiary, 264

Aldehyde, 228, 236 etcAldehydene, 136Aldehydes, 295Aldehydines, 295Aldol, 294Alizarine, 279, 281Alkalies, decomposition, 67, 74 etc.

relations of caustic and mild10, II

Alkaloids, relations to pyridine,285, 286

Alkyl bases, synthesis, 292, 293oxybenzoic acids, 275

Allantoin, synthesis, 290

INDEX OF SUBJECTS 367

Allotropy,.H9Alloxantlne, synthesis, 290Allyl alcohol, 253Aluminium ethyl, 227Amid-acids, 185, 195, 215, 221Amide, 137

theory, 138Amides, 185, 195

of dibasic acids, 2 r 3Amido-acids, 257Amines, 211 etc., 214, 217Ammonia, discovery, 11

soda, 358theory, 138

Ammonium, 130amalgam, 76carbamate, vapour density, 300carbamate, dissociation, 307carbonate, dissociation, 303chloride, dissociation, 305, 306hydrosulphide, dissociation,

303, 307theory, 131, 138

Amyl alcohol, 268ether, 208

Amylene hydrate, 263, 264Anhydrides, 195, 295

dibasic, 215mixed, 210, 213of dibasic acids, 213, 215of monobasic acids, 212, 213,295

Anilid-acids, 185, 195Anilides, 185, 195Aniline dyes, 279Anisic acid, 229Anthracene, 281, 295

synthesis, 289, 293Anthranilic acid, 359Anthraquinone, 281, 282Antimony pentachloride, vapour

density, 301Antipyrine, 358Aqua regia, IIArgon, 348, 349Aromatic compounds, 270

compounds, hydrogeiiised, 354compounds, isomerism in, 273?

etc.compounds, oriho-, meta-, and

para-, 276 etc., 295compounds, position of substi-

tuted atoms or groups, 276etc.

Aromatic compounds, theory of,271, 273 etc., 277,279

hydrocarbons, oxidation intoacids, 275

Arsenic, vapour density, 299, 345Ascending series, methods of, 291Aspartic acid, 268Assimilation in plants, relation of

light to, 327 etc.Asymmetric carbon atom theory,

268 etc., 352 etc.nitrogen, 355 etc.

Atom, 49, 65, 101, 102, 104, 142,150, 190, 192 etc., 230, 240,297

and equivalent, 190Atomic magnetism, 341

refraction, 331theory, 47 etc., 53, 106, no,

175 etc., 201weight, 174weights, 54, 56 etc., 99, 100

etc., 175, 187 etc., 298 etc.weights, "new," 241weights, views of Cannizzaro,

298 etc.Atomicities, metals possessing seve-

ral, 242Atomicity, 240, 254, 282, 307

and basicity, distinction, 162,256, 259

Atoms, determination of number incompounds, 56, 64, 91 etc.

migration of, 2266wandering of, 266

Avidity, 316Avogadro's hypothesis, 61 etc., I03

etc, no, 133, 192, 195,201,211, 299

Azo-compounds, 275dyes, 280dyes, substantive, 358

"D ARIUM sulphate, formulae, 218*-* Barred formulae (Berzelius),

9°.Baryta, discovery, 10Bases, polyatomic, 242Basicity and atomicity, distinction

162law of, 183, 184, 220of acids, 157

Benzamide, 125

368 INDEX OF SUBJECTS

Benzene, 271, 272 etc., 283, 2S5,287

Kekule's formula, 272, 278ortho-dicarbonic acid (Phthalic

acid), 285prism formula, 278sulphonic acid, 222, 223synthesis, 284, 288, 293

Benzoic acid, 125, 142, 147, 170,257, 271, 273

ether, 125, 134Benzoyl, 125, 126, 170

acetic ether, 291chloride, 125, 170compounds, 125sulphuric acid, 182

Benzyl alcohol, 274chloride, 274

Beryllium, atomic weight, 3 r2Bismuth, nitrate, 242

oxide, 242Bitter almond oil, 140, 147

almond oil, Liebig and Wohleron, 109, 125

Boiling point, elevation, 338points of liquids, 329

Butyric acid, 263ether, 264

Butyro-lactic ether, 258

( ACODYL, 126, 127, 221, 228,231, 232

oxide, 127Cacodylic acid, 127Cadmium, vapour density, 299Caesium, discovery, 318Calcium carbide, 344

carbonate, crystallised, artificial,319

carbonate, dissociation, 303Calx, 6

reduction by hydrogen, 12Capillary electrometer, 342Carbazol, 287Carbon, in all organic compounds,

i nquadrivalence, 250 etc., 272

Carbonic acid, 243, 261anhydride, 11, 25, 28anhydride, dissociation, 302anhydride, isotherms, 321anhydride, liquefaction, 322ethers, 265

Carbonic oxide, 26y, 270oxide, discovery, 1xoxide, dissociation, 302

Carborundum, 344Carbylamines, 270Catalytic action, 207, 358Chemical difference (Bridie), 19K,

200equilibrium, 336mass, 37 etc., 314system of Berzelius, 85 etc.system of Kolbe, 230 etc., 235

etc.volume, 315

Chloral, 140, 141, 147hydrate, dissociation, 307

Chlorides of dibasic acids, 215Chlorine, chemical nature, 79 etc.,

discovery, 10('hlorlactic ether, 257Chlormethyl-sulphonii* arid, 224Chloroform, r4O, 147, 250Chlorophyll, 327Chlorosul phonic acid, 214ChlorotolueneH, 274Chlorpicrin, 250Chlorpropionic ether, 258Chlorpropionyl chloride, 258Choline synthesis, 290Chromyl chloride, 170Chrysene (Naphthalene- phenari-

threne), 2S2Oinnamie aldehyde, synthesis, 203Citric acid, ir, 154, 242

acid, synthesis, 290Classification of the elements, 310Collidine, synthesis, 289, 2 4Combination, a consequence «f de-

composition, 19Hfixed or varying proportion's, 35,

40 etc.Combining weight, 103,106,174 etc.Combustion, views respecting, 5

etc., $1Lavoisier's theory of, t$

Complete equilibrium, 336Components, 336Concentration cdls, 343Condensation, synthews hy% 293 etc.Condensations, internal, 294 etc,Coniine, splitting of synthetic, 353

synthesis, 290


Conjugated compounds, 226radicals, 220 etc., 228, 230

Constitution of compounds, 225, 329Continuity of liquid and gaseous

states, 322Copula, 171 etc., 184, 225Copulas, theory of, 227, 233Corresponding conditions, van der

Waals's theories, 337Coupled compounds, 171, 184, 185

compounds, basicity of, 183,184

compounds, Gerhardt's firstviews, 180, 182 etc.

compounds, Gerhardt's laterviews, 183

Coupling, Frankland's views, 230etc.

Creatine, synthesis, 289Cresols, 2 74Critical pressure, 321

temperature, 320, 321, 346Crotonic acid, synthesis, 289

aldehyde, synthesis, 294Cumarine, synthesis, 292, 295Cuprous chloride, formula, 345Cyanic acid, 116, 117, 147, 154, 255Cyanogen, 124 etc., 147

compounds, 124, 125Cyanuric acid, 155, 259

T)APHNETINE, 295Densities of gases and va-

pours, 104 etc.Dephlogisticated air, 18Desmotropy, 357Diamines, 295, 296Diamonds, artificial, 344Diazo-compounds, 275Dibasic lactates, 258, 260

lactic ether, 258Dibromo-benzenes, 277

pyridine, 286Dichlorrnethyl-sulphonic acid, 224Diethyl (Butane), synthesis, 290Dimethyl benzene, 271, 275

carbinol, 238, 262(Ethane), synthesis, 290identical with ethyl hydride, 265

Dimethylamine, 262Dimorphism, 106, 117Dioxy-anthraquinone (Alizarine),

279

Diphenyl, synthesis, 293Diphenylene methane, 282Dissociation, 301

analogous to evaporation, 352Deville's experiments, 301electrolytic, 340 etc.of gase?, 335theory of, 301 etc., 304

Disulphetholic acid, 248Disulphobenzolic acid, 239Disulphometholic acid, 239Dithionic acid, 243Divisibility of elementary molecules,

198, 200 etc.Double atoms, 90, 175

salts, 352, 356Dualism, 85, 108, 109, 119, 169

etc., 173Dualistic theory, 88 etc.Dyads, Laurent's, 192, 195Dynamo-electrical machine, 344

T7KA-ALUMINIUM (Scandium),•^ 313Eka-boron (Gallium), 313Eka-silicon (Germanium), 313Electric furnace, 344Electro-chemical theory of Berzelius,

74, 85 etc., 108 etc., 121, 168etc., 172, 173, 230

chemical theory of Davy, 72 etc.chemistry, 341 etc.

Electrolysis, 325, 343Electrolytic conductivity, 341

dissociation, 340 etc.production of metals, 344, 345

Electro typing, 344Element, definition, 27Elements, classification according to

valencies, 310of Becher, 5of Empedocles, 4, 5

Enantiomorph forms, 267, 353Endothermic and exothermic re-

actions, 324, 325Enol formula, 357Equilibrium, chemical, 336Equivalence, 50 etc., 139Equivalent, 49, 64, 101 etc., 139,

142, 150, 151, 174 etc., 187,188, 190 etc., 196, 231, 240297, 307

2 A

370 INDEX O F SUBJECTS

Equivalent weight, 307Equivalents, 53, 65, 66, 107, 151,

193Gmelin's, 175, 188Gerhardt's, 241

Ester formation, limit, 315formation rate, 315

Ethenyl-toluylene-diamine, 295xylene-diamine, 295

Ether, 204 etc., 210, 233formulae, 205, 208, 254views of Berzelius concerning,

132views of Dumas concerning, 134views of Gay-Lussac concern-

ing, 122views of Liebig concerning, 133

etc., 179Williamson's experiments, 204

Etherification, theories of, 206 etc.Etherin, 124 etc., 135, J37

theory, 122 etc., 137, 206theory attacked by Liebig, 132

Ethers, mixed, 206, 208, 2ioEthionic acid, 131Ethyl, 133, 137, 209, 226, 227, 229

alcohol, 237amyl ether, 208benzene, 271carbinol, 262methyl carbonate, 210methyl oxalate, 210pyridines, 284sulphonic acid, 258sulphuric acid, 184, 206 etc.,

223, 224theory, 138, 226

Ethylamine, 217, 247, 262a substituted ammonia, 204

Ethylene, 247, 248, 269, 295oxide, 246, 247, 295sulphocyanide, 248sulphurous acid, 248synthesis, 288

•RARADAY'S law, 106, 325A Felspars, artificial, 320

Tschermak's research on, 310Fermentation without living organ-

isms, 351Ferric oxide, 242

sulphate, 194

Ferricum, 194Ferrosum, 194Ferrous oxide, 242

sulphate, 193, 194Fire, an element, 4, 8Fixed air, II, 24Fluoranthene (Idry]), 282Fluorene (Diphenylene methane)

. 2 8 2

Fluorine, isolation, 350Formic acid, 120, 235, 236, 292

acid, synthesis, 116, 288ether, tribasic (Kay), 243

Formulae, barred, 90constitutional, 253, 255empirical, 186equivalent, 193 etc.four-volume, 189graphic^ 260, 265molecular, 194structural, 265synoptic, 186two-volume, 192

Fuchsine, 279Fraunhofer lines, 317 etc.Freezing points, depression, 332,

. 338, 339Fulminic acid, 117, 154Fumaric acid, 354Furfuran, 287

p A L L I C acid, ir^ Gallium, 313, 318Galvanic current, decompositions by

aid of, 68 etc.Gaseous volumes, law of, 58 etc.,

91 etc., 108, 196Gases, dissociation of, 335

liquefaction of, 322, 346 etc.transpiration of, 332

Germanium, 313Glucoses, 268Glyeerie acid, 259Glycerine, Berthelot's researches,

242 etc.discovery, 115synthesis, 290

Glycocoll, 257synthesis, 289

Glycol, 246 etc., 255, 257, 259Glycollic acid, 246, 257 etc.


Gycols, 242, 248Guanidine, synthesis, 2S9

T-TEAT, Lavoisier's views con-cerning, 26 etc.

Heats of formation, 324of neutralisation, 341

Heliotropine, 359Helium, 348Heterologous series, 217Homologous compounds, 216

compounds, regular differencesin boiling points, 216

series, 216Homologues of marsh gas, 203Hydracids, 82Hydrazine, 351Hydrazoic acid, 3$IHydriodic acid, formation from j

elements, 335Hydrobenzarnide, 147, 148Hydrochloric acid, discovery, 11

acid, dissociation, 302Hydrocyanic acid, 11, 78, 124, 147,

255,292Hydrofluoric acid, discovery, 11Hydrogen, 11, 12

acids (or hydracids), 82, 119,158, 161

identity with phlogiston, 12presence in metals suspected,

76 etc.Ilydrophthalic acids, 354Hydroquinone, 278Hydroxylamine, 350, 35rHypochlorous anhydride, 195Hypothesis of Avogadro, 61 etc.,

103 etc., no, 133, 192, 195,201, 211, 299

of Prout, 102, 311

TMIDRS, 195Increase of weight during com-

bustion, 7, 2 1, 24Indigo, 290, 296, 358, 359Indium, atomic weight, 312

discovery, 318Indol, 286, 287, 295Inflammable air, 12Internal condensation, 294 etc.

oxidation, 296

Iodine, discovery, 81vapour density, 300, 345

Iodo-, iodoso-, and iodonium com-pounds, 358

lonisation,. 339 etc.Ionone, 359fsatine, 295, 296fsethionic acid, 131, 222, 224, 258Isobutyric acid, 263

ether, 264Isologous series, 216Isomers in benzene series, 273 etc.

in benzene series, Korner's rule,277

in pyridine series, 284Isomeric amyl alcohols, 263

propyl alcohols, 262, 263triphenyl-phosphine oxides, 309valerianic acids, 263

Isomerism, 117 etc., 262, 265, 288early observations, 117in the benzene series, 273 etc.physical, 268

Isomorphism, 95 etc., 106, 140Isonitriles, 270Isophthalic acid, 276

TAHRESBERICHTE, 102

TTETONE formula, 357x Ketones, 236, 295mixed, 209

Kirchhoff's law, 317Kolbe's reaction, 292Krypton, 349

T ACTAMETHAN, 258J~/ Lactic acid, 11, 246, 2

354 .257 etc.,

acid, discussion regarding itsconstitution, 256 etc.

acid, formulae, 256, 2*57acid, views as to its basicity,

256, 257Lactone acids, 295Lactones, 295Lactyl chloride, 258Law of chemical mass action, 315,

335of Dulong and Petit, 95 etc.,

106, 298of Oudemans and Landolt, 341


Law of selective absorption, 317of the even number of atoms,

192, 202of ther mo-neutrality, 34°

Leucic acid, 262Leucine, synthesis, 289Light, chemical effects of, 326 etc.Linking of carbon atoms (Kekule),

250 etc., 272

TV/TAGNETIC rotatory power, 3411VX Malaguti's ethers, 171, 233Maleic acid, 354Malic acid, 11, 154

acid, synthesis, 289Malonic ether, 291Mandelic acid, 268, 292Manganese, 170

discovery, 10peroxide, 170

Manganic acid, 170Marsh gas, 250

gas, synthesis, 288gas, type, 250

Matter, indestructibility of, 15, 22Mauve'ine, 279Mercaptans, discovery, 134Mercury ethyl, 227

fulminate, 250vapour density, 299

Mesitylene, 276synthesis, 293

Meso-compounds, 353Metal-ammonia and metal-ammo-

nium compounds, 310Metalepsy, 142Metals, existence as such in salts,

159polybasicity, 242

Metameric substances, 118, 119Metaphosphoric acid, 154, 158, 243Metargon, 349Methyl (Ethane), 203, 226, 227

alcohol, 237aldehyde, 236benzene, 271, 273benzenes, 283carbinol, 262chloride, 250cyanide, 226ethyl ether, 20$, 206, 210

Methyl propyl-phenunthrene, 282pyridines, 283, 284sulphonic acid, 224, 239toluene, 271

Methylamine, 262Methylene, 135, 147Minerals, syntheses, 319, 32oMixed anhydrides, 210, 213

ethers, 206, 208ketones, 209types, 221 etc.

Mixtures, 35, 45, 107Molecular compounds, 308, 309

magnetism, 332physics, 329 etc.refraction, 331, 332.volumes, 329, 330, 332weights, 187 etc., 196,209,299,

339Molecule, 192 etc., 230, 240, 297,

298chemical, 201, 203, 210, etc.physical, 211

Molecules, 53, 61 etc., 195, 196Monochloracetic acid, 229Morphotropy, 333Multiple point, 336

proportions, law of, 48, 53 etc.,91, 108, 115, 175

Muriaticum^ 80, 81Mustard oil, synthesis, 289

XTAPHTHALRNK. X43, 276,x " 280, 281, 283, 2S5, 287, 29s

isomeric derivatives, 281mono-sulphonic acids, 281phenanthrene, 282

«-NaphthoI, 280Nascent state, 196Neon, 349Neurine, 247, 290Neutrality, law of, 50Nickel carbonyl, discovery, 3$oNitric acid, 195, 236, 242, 243

acid, composition, 25oxide, discovery, 11

Nitriles, 195, 270conversion into acids, 226, 292

Nitro-aeria] spirit, 21Nitro-benzene, xttiNitrogen, assimilation, 351


Nitrogen, equivalent of, 148sulphonic acids, 351

Nomenclature of Berzelius, 89new system of chemical, 32 etc.,

109Notation of Berzelius, 90Nucleus theory of Laurent, 143 etc.,

167, 174, 180, 202

(~\\L of the Dutch chemists, 136,

Olefiant gas, 122 etc.Organic acids, Lie big's researches,

154analysis, 28 etc., 113 etc.analysis of nitrogenous sub-

stances, 114chemistry, new nomenclature,

35?chemistry, separate treatment,

108, i n etc.compounds, classification by

Gerhardt, 215 etc.compounds, constitution of,

117, 121, 288compounds, derivatives of in-

organic compounds, 235Organo-metallic compounds, 227,

232Ortho-condensation, 296

(a£) pyridine dicarbonic acid,285

Orthotoluidine, 279Orthrin, 126Osmotic pressure, 338Othyl, 209Oxalic acid, 11, 122, 226, 243, 255,

259Oxamide, 185, 214Oxanilide, 185Oxidation of substituted pyridines,

n • 2H

Oximes, 350Oxindol, synthesis, 296Oxybenzoic acid, 257Oxycumarines, 295Oxygen, assumed presence in hydro-

chloric acid, 76, 78 etc.discovery, 11, 16, 24

Oxy-isobutyric acid, 264Oxypropionic acid, 257

Ozone, density, 347discovery, 200

pARA - OXYBENZOIC acid,277

Perfumes, artificial, 359Periodic law, 103, 311 etc.Perkin's reaction, 292Phase rule, 336 etc., 352Phases, theory of, 336 etc.Phenanthrene, 281Phenol aldehydes, synthesis, 292

dyes, 280Phenols, synthesis, 291Phenyl, 229

hydrazine, 351sulphonic acid, 239

Phenylene diamines, 277Phlogistians, chemical knowledge

of, 10Phlogisticated air, 18Phlogiston theory, 5 etc., 12 etc.Phosgene, 170Phosphoric acid, 236, 244

acids, Graham's investigations,152 etc.

acids, supposed isomerism, 118,152

anhydride, 25Phosphorus, acids of, 243

pentabromide, vapour density,301

pentachloride, vapour density,3d

pentachloride, dissociation, 305,3O7

vapour density, 106, 299Photo-chemical induction, 328Phthalei'nes, 280, 295Phthalic acid, 238, 276

anhydride, 282Picene, 283Picoline, synthesis, 289, 294Picric acid, 221Piperidine, 285

synthesis, 290Piperonal, 359Polybasic acids, 185, 190, 194 etc.,

202, 242, 292acids, theory of, 151, 154 etc.

Polyethylene alcohol?, 247


Polarity (Brodie), 198 etc.Polymerism, 118Potassamide, 76, 77Potassium, discovery, 67, 74 etc-

ethyl, 228iodide, formula, 345

Principle of Hess, 323of maximum work, 325

Probability theory (Maxwell's), 304Proin, 126Propionic acid, 257, 258

acid, synthesis, 235ether, 258

Propyl aldehyde, 253pyridines, 284, 286

Propylene, 269glycol, 257 m

Prout's hypothesis, 102, 311Purpurine, 281Pyrazol group, 358Pyrene, 283Pyridine, 283 etc.

carbonic acid from oxidation ofnicotine, 285

dicarbonic acid, 286series, isomerism in, 284series, position of substituted

atoms or groups, 284synthesis, 284tricarbonic acid, 286

Pyrophosphoric acid, 153, 158, 243Pyrrol, 286, 287

QUINIZARINE, 281Vw Quinoline, 283 etc., 295

from decomposition of al-kaloids, 285

synthesis, 284, 285, 296Quinone, 278, 279

formulae for, 278Quinones, Grabe's examination of,

278 etc.

T> ACEMIC acid, n8 r 267 etc.acid, Pasteur's modes of de-

composing, 267 etc.acid, synthesis, 289 .and partially racemic sub-

stances, 356compounds, splitting of, 352,

353

Racemism, 356Radical, 31, 33, 124, 126 etc., 167,

169, 217,225compound, 109Liebig's definition, 128theory, no, 121, 130 etc., 174,

176 etc., 230Radicals, 186, 217

binary, 170conjugated, 220 etc., 228, 230containing metals, 221, 231,

235, 242different, recognised by Kolbe,

229isolation, 226polyatomic, 240, 247with basicity greater than one,

214 ,221Refraction equivalents, 331Refractive index, 330 etc.Relations between electrical and

chemical forces, 325between optical and chemical

properties, 326by weight in chemical changes,

11, 21Replacement, 139Residues, theory of, 180 etc., 293Respiration, 29 etc.Retene (Methyl - propyl - phenan-

threne), 282Rosaniline, 280, 295Rosolicacid, 280, 295Rotation of plane of polarisation,

333of plane of polarisation, electro-

magnetic, 333Rubidium, discovery, 318Rubies, artificial, 320

C A L I C Y L I C acid, 229u Salts, 242

amphid, 84, 120haloid, 84, 120neutral, 161views regarding, 120

Saiylic acid, 271, 273Saturating capacity (Frankland) ,

23T, 242Scandium, 313, 318Secondary batteries, 342Semi-permeable membranes, 338

INDKX OF SUBJECTS 375

St!.'u|uioxi».!e>5 242 Sulphurous arid, discovery nSllkk arid, «jriiv;mviv;, 310 anhydride, dissociation, 302Silirnis, tiliyl, 2z¥, Sulphury! chloride, 214

fluoride, i!i -romy, ti Superoxides, yoi«.'|*l,irrjj;fiit «f vatl i»n l*y, 167 Synthesis by condensation, 294

Sodium. • IrvMveiy. (17, 74 vw. , of aromatic hydrocarbons, 290,riliyl, 2JH ^ | 291

5v*h:'i--n. v m': IMt1* theoiy, 337- | of hydrocarbons, 290,*3M ' of minerals, 319

P* J »*' .M? •. of organic compounds, 116,W v a*a* 1 4« :iii df liquid , 252, ; 288'etc.

• » !.;»'> :i rrARTAR emetic, 159* .«J i *,?!'» ; Tartaric acid, 154, 156, 159,!»< <% }v \ 190

,-}*'«'f»»» *K,ily i*» }ld v\\. } acid, formula, 202A if''y , ff?» >,i 11 jiu\ ;pf ju'i«l, inactive, 267 etc.«H ''it } ami , i"1 iuhi, iiomctism, 118, 267

, /,/•'»/ < hmwu , J«<'!thftli< f* , Jij ' acid, left rotating, 267 etc.Si • 1 1 « htihi tiy, | i v ac id, lijjht rotating, 267 etc.^irirn ii<« «v, it). JI»H, 35,% %1;f ' Taunnti, i.ynthej-is, 289Sh'i * UitU, 2i2 Taut(imcri:ttn, 357hf «H <!**ry 5? t in Tellurium* atomic weight, 313, 348,SuUt $',*•«# * 349Nu! Stun- J, tiiiiif ti'.is 2n|t 214,217 * ethyl, 227h«h l»M,ii 1, ib,*Mfr, i '», 1(2 ttr.t i Tvmpcraturtfi, high, attainment of,

l' 5 «u.f ." ,\ ,**$* a*' , 344 i tr-c ,ntly 4 'tftv I"*I*» ft, 140 ' low, attainment of, 345 etc.«1 r.irl < n ly ? i*ik 00, H17 Ti.rr|4itlKdic jt« id, 275, 277

S'jnwir uuit2>8 1 Itntu th lomfHirainon, 1,-aurent's,SWR* »;ro j», 5M, 553, 3S7 Jf)ii son4u |ii H «JH i rj*l« a2«», 23 1 Teri m"., 26S, 35HS«l|»} i. l«* 4>i«! iHf, T«-tta-i»h«n«»l (Fuifuran), 2H7• J*J *4t» *f jr<ili1ui:»»' »•! .1 l»on TiMliiuin, »li:i< ovrry, 318

(K»4irj til Thnmal dlect, 335I i fUt «<t«! » 3 '/ Theimo fhenshtry, 323 etc,^MI| I • J Jh, t\t I*1*, |H.* 1 re.t*iiu'h»'si of Thotuiti'ti, 315,Sa'i'J'rfrti/*ic «tii(!. P3» 2*i» 2/2, 324

3 H*\ Thi'*« 'iic* arid, 24!V \\ b Th i f f

' A»W« irl if»2 v 1 ( i il'iii 1 hbft!«!ti, 2H71 fii)! t 4| L if nriiJ) Tin, op f<'>illv<t« live contpounds,356

» »• s«|4ir#115* itlnr ifiirt- T"lufii«*, 271 • 273l< ss 1 < " Tolnylif* .iiiil, 175

11| 'it ** ?.' ty, i*»ff 2' I 2 /I Tr.u*t.f ».itioii lftupcratuns S37

l»»*w »» I 12 l«j% 2 ^ 2f ». 1 r,iti'itt<m temperature., 336, 337,

« iv#t . iKt 1 . i ' \ 18' l -| *ir,nr jii.itmn of uajen, 33234 . j>; *t 1 <»t v#«j*<*tii , 352

^rc» I' , n f»« ;r , T»ii*lt, I;«*lrtMt ri'-i. 311• >»!, N 1? I < n t 1:: lihU^t tc iu uid, 221, 22$, 22<j»

l

•A 376 INDEX OF SUBJECTS

Trichloracetic acid, analogy withacetic acid, 164, 171

acid, synthesis, 116Trichlormethyi-sulphonic acid, 224Trimethylamine, 262Trimethyl benzene, 276

carbinol, 238, 262Types, condensed, 22 r, 247 — .,

Dumas' theory of, 163, 165,174, 176

Gerhardt's theory of, 211, 217,220 etc., 232, 233

mechanical, 165mixed, 221 etc. .molecular, 165 &tc.

Typical hydrogen, 259 etc.Tyrosine, synthesis, 290

TTMBELLIFERONE, 295^ Unitary system, 162, 167 etc.Unsaturated acids, 270

compounds, 269 etc., 308Uranium, atomic weight doubled,

313Urea, synthesis, 116, 288Uric acid, 11

group, 357synthesis, 290

T7ALENCY, 233, 240, 248, 250etc., 254, 267, 288, 307 etc.,325

Valency, constant or variable,307 etc.

Van der Waals's equation, 337• Vanilline, synthesis, 290, 359Vapour densities, 105, 339

densities, abnormal, 299 etc.,304 etc., 339

densit'es, ratio of, 300pressure, diminution, 338

Vapours, transpiration, 332Vital force, 115

"V^TATER, composition, 22, 28dissociation, 302, 305

in combination, 90presence in oxygen acids

doubted, 158 etc.presence in many organic com-

pounds doubted, 180regarded as type by William-

son, 209supposed conversion into earth,

22, 23

VYLENE, 271, 275•*x Xenon, 349

ethyl, discovery, 227vapour density, 299

Zymase, 351

PRINTED BY OLIVER AND BOYB, EDINBURGH.

by dr a. laden burg translathi) from tiir second …

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