soddy (1917) complexity elements

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The Complexity of the Chemical ElementsAuthor(s): Frederick SoddySource: The Scientific Monthly, Vol. 5, No. 5 (Nov., 1917), pp. 451-462Published by: American Association for the Advancement of Science

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COMPLEXITY OF CHEMICAJ4 ELEMENTS 451

THE COMPLEXITY OF THE CHEMICALELEMENTS1

By Professor FREDERICK SODDY, M.A., F.R.S.

T E elementsfthechemistrenowknownobe complexin three differentenses. 'In the first sense the com-

plexity s one thatconcerns he generalnature ofmatter,nd

therefore f all the elements n common o greateror less de-gree. It followsfromthe relations betweenmatter and elec-tricitywhichhave developedgraduallyduring hepast centuryas the resultofexperimentsmade and theoriesbornwithin hefour walls of this institution. Associated initiallywith thenames of Davy and Faraday, theyhave only nthesedayscometo full fruition's the resultoftheverybrilliant lucidation fthe real natureof electricity y your distinguished rofessor

ofphysics, ir JosephThomson. Such an advance, developingslowlyand fitfully, ith ong intervalsof apparentstagnation,needs to be reviewedfromgenerationto generation,disen-tangledfromthe undergrowthhat obscures t, and its clearconclusions rivenhome. This complexityf the chemicalele-ments s a consequence fthe condition hatneitherfreeelec-tricitynor free matter can be studied alone, except in veryspecial phenomena. Our experimental nowledge fmatter nquantity s necessarily onfinedo the complexofmatter andelectricitywhich constituteshe materialworld. This applieseven to the free elementsof the chemist,which in realityare no morefreethenthantheyare in theircompounds. Thedifferences merely hat,whereas nthe attertheelements recombinedwith otherelements,n the so-calledfreestate theyare combinedwithelectricity. shall touchbutbriefly n thisfirst; spect, as in principle t is now fairly well understood.But its consistentnd detailed pplication othestudyofchem-

ical character s still acking.The second sense inwhichtheelements, r some ofthem tleast, are knownnow to be complexhas, in sharp contrast othe first,developedsuddenlyand startlingly romthe recog-nition n radioactivechanges,of differentadio-elements, on-separablebychemicalmeans,nowcalled isotopes. The naturalcorollary f this is thatthe chemicalelement epresents ather

I Lecture beforethe Royal Institutionof Great Britain.








452HTHE SCIENTIFIC MONTHLY

a type of element, hemembers f the type being only chem-ically alike. Alike theyare in mostof those propertieswhichwere studiedprior to the last decadeof last century nd whichare due, as we now think, o the outer shells of the atom,soalike that all the criteria,hitherto elied uponby the chemistas beingthe most nfallible nd searching,woulddeclarethemto be identical. The apparent identity oes even deeper intothe region reached by X-ray spectrum nalysis which fails todistinguish etween hem. The differences foundonly n thatinnermost egionof all,'thenucleusof the atom,of whichradio-

activephenomena irstmade us aware.But, thoughthesephenomenapointed the way, and easilyshowed to be different hat the chemist and spectroscopistwould have decided obe identical,t did more. It showedthatalthough hefiner nd newer criteria, elied uponby the chem-ist in his analysis of matter,must of necessityfail in thesecases, beingultimatelylectricaln character, etthe differenceshould be obvious n thatmost studiedand distinctive harac-teristic fall-the criterion ywhichDaltonfirst istinguishedthe differentinds of atoms-the atomicweight. Those whohave devotedthemselves o the exact determinationf theseweightshave now confirmed he differencen two separatecases, which, n absence of whatperhaps theymightregardaspreconceivednotions, heywere unableto discoverforthem-

selves. This is the experimental evelopmento which wishmoreespeciallyto directyourattention. It indicatesthatthechemicalanalysis of matter s, even within ts own province,

superficial atherthanultimate, nd that thereare indefinitelymore distinctelements han the ninety-two ossible types ofelement ccommodated ythe presentperiodicsystem.

The third enseinwhich heelements re known o be com-plex is thatwhich, n the form of philosophical peculations,has come down to us from the ancients,which inspiredthelabors of the alchemists f theMiddleAges, and which n theformof Prout's hypothesis as reappearedin scientific hem-istry. It is the sensethat denies to naturetheright o be com-

plex,and from he'earliest imes, aithoutstripping nowledge,has underlain he beliefthat all the elementsmust be builtupof the same primordial tuff. The facts of radioactivephe-nomenahave shown hatall theradio-elementsre indeedmadeup of lead and helium,and this has definitely emovedthequestionfromthe region'of pure speculation. We know thathelium s certainly material' onstituent f the elements ntheProutiansense,and it wouldbeharmless, fprobably ruit-








COMPLEXITY OF CHEMICAL ELEMENTS 453

less, to anticipate he day of fullerknowledge y atom buildingand unbuildingon paper. Apart altogetherfrom this, how-ever, the existenceof isotopes,the generalization oncerningthePeriodic Law thathas arisen from he studyof radioactivechange on the one hand and the spectraof X-rayson theother,and experiments n the scattering f a-particlesby matter,dogive us for the first ime a definite onception s to what con-'stitutes he difference etween one element nd another. Wecan say how gold would result from ead or mercury, venthough the controlof the processes necessaryto effect he

change still eludes us. The nuclear atom proposed by SirErnest Rutherford,ven though,dmittedly,t is only generaland incomplete eginning o a complete heory fatomicstruc-ture, enormously implifies he correlation f a large numberof diversefacts. This and what survivesofthe old electronictheoryof matter, n so far as it attemptedto explain theperiodic aw,'willtherefore e briefly eferred o in conclusion.

THE FREE ELEMENT A COMPOUND OF MATTER AND ELECTRICITY

AlthoughDavy and Faraday were the contemporaries fDalton, t mustbe rememberedhat t took chemists ifty earsto put the atomictheoryon a definitend unassailablebasis,so that neither fthese nvestigators ad the benefit ftheveryclear viewwe holdto-day. Davywas theoriginator f the firstelectrochemicalheory f chemical ombination,nd Faraday'sdictum., the forcesof chemical ffinitynd electricityre oneand thesame, t is safetosay, nspires ll themodern ttempts

to reducechemical haracter o a science n thesense of some-thing hat can be measuredqua:ntitatively,s well as expressedqualitatively. Faraday's work on the laws of electrolysis ndthe discovery hat followedfrom t, when the atomictheorycame tobefully eveloped,hat all monovalent tomsor radiclescarrythe same charge,that divalentatoms carrytwice thischargeand'so on,can be regarded o-day s a simple extensionof the law of multipleproportions romcompoundsbetweenmatter and matterto compounds. etweenmatter. nd elec-tricity. Long beforethe electric hargehad been isolated,. .rthe properties f electricity ivorcedfrommatterdiscovered,the same law of multipleproportionswhichled, without nypossibilityof escape, to an atomictheoryof matter, ed, asHelmholtzpointedout in his well-known araday lecturetotheChemical ociety nthis theater n 1881,to an atomic heoryof electricity.

The workofHittorf n themigration f ions,thebold and








454 THE SCIENTIFIC MONTHLY

upsetting onclusion f Arrhenius hat in solutionmany ofthecompoundshitherto egarded as most stable exist dissociatedinto ions,the realizationthat mostof the reactionsthat takeplace instantaneously,nd are utilizedfor the identificationfelementsn chemical nalysis, re reactions f ions rather hanof the element n question,made very familiarto chemists heenormousdifference etweentheproperties f the elements nthe charged and in the electrically eutral tate.

More slowly appreciated,and not yet perhaps sufficientlyemphasized,was the unparalleled ntensityf thesechargesin

comparisonwith anythingthat electrical science can show,whichcan be expressed ritely ythestatementhat thechargeon a milligram fhydrogen ons would raise the potentialofthe world 100,000 volts. Or, if we consideranotheraspect,and calculatehowmanyfreehydrogenonsyoucouldforce ntoa bottlewithout ursting t, provided, fcourse, hatyoucoulddo so withoutdischarging he ions, you wouldfind hat,werethe bottleof the strongest teel,the breech of a gun,forex-ample, twould burst,byreasonofthemutualrepulsion f thecharges,beforeas much was put in as would, n the form ofhydrogen as, showthe spectrum f the element n a vacuumtube.

Then came the fundamentaldvances in our knowledge fthe natureof electricity,ts isolationas the electron, r atomofnegativeelectricity,he great extension ftheconception fions to explainthe conduction f electricityhrough ases, thetheoretical easoning, ue inparttoHeaviside,thattheelectron

mustpossess inertia nversely roportional o the diameterofthesphereonwhich t is concentratedyreason of the electro-magneticprinciplesdiscoveredby Faraday, leadingto the all-embracingmonism that all mass may be of electro-magneticorigin,

This put the coping-stone o the conclusionthat the ele-mentsas we apprehendthemin ordinarymatter are alwayscompounds. In the free ' state they are compoundsof theelement n multiple tomicproportionswith theelectron. Theions,which re thereal chemically ncombinedtomsof matter,can nomoreexistfree n quantity han can theelectrons.

The compoundmaybe individual s between he atomandtheelectron, r itmaybe statistical, ffectinghetotal numbermerely of the oppositecharges, and the elementpresumablywill be an insulatoror conductorof electricity ccordingly.Analogously,with compounds, he formerconditionappliesto unionized ompounds uchas are met with n thedomainof









organic chemistry, r ionized, as in the important lasses ofinorganiccompounds, he acids, bases and salts. Just as the

chemisthas long regardedthe union of hydrogen nd chlorineas precededby the decomposition f the hydrogen nd chlorinemolecule, o he should now further egard the union itself asa decomposition f the hydrogen tom nto the positive on andthe negative electron, nd a combination fthe latterwith thechlorine tom.

One of the barriersto theproper understandingnd quan-titative development f chemical characterfromthis basis is,perhaps, heconventionaldea derivedfrom lectrostatics,hatopposite electriccharges neutralizeone another. In atomicelectricity r chemistry, hough the equality of the oppositecharges s a necessary ondition orexistence, here s'no suchthing s neutralization,rtheelectrically eutral tate. Everyatom being the seat of distinctopposite charges, intenselylocalized, the state of electric neutrality an apply only to aremotepoint outside it, remote n comparisonwith its owndiameter. We are gettingback to the conception f Berzelius,

with some possibilityof understanding t, that the atom ofhydrogen, or example,may be strongly lectro-positive,ndthat of chlorine trongly lectro-negative, ith regard to oneanother, nd yeteach maybe electrically eutral n the molarsense. Some day it may be possibleto map the electricfieldsurroundingachoftheninety-two ossibletypesofatom,overdistances comparablewith the atomic diameter. Then thestudyof chemical haracterwouldbecomea science n Kelvin'ssense,of something hat could be reducedto a number. Butthe mathematical onceptions nd methodsof attack used inelectrostatics or macroscopicdistances are ill-suitedfor thepurposesof chemistry, hich will have to developmethodsofits own.

We have to face an apparent paradox thatthe greatertheaffinityhat binds togetherthe material and electricalcon-stituents f the atom,the less is its combining ower in thechemical ense. In otherwords,the chemical ffinitys in in.

verseratio to the affinityf matterfor electrons. The heliumatomsoffer very imple nd instructivease. Helium s non-valentand in the zero family, ossessing absolutelyno powerof chemical combination hat can be detected. Yet we knowthe atompossessestwo electrons, or in radioactivechangeitis expelledwithout hemas the a-particle. The dischargeofelectricity hrough t and positive-ray nalysis show that theelectrons, r certainly ne of them, re detachableby electric









agencies, althoughnot by chemicalagencies. One would ex-pect helium to act as a diad, forminghelides analogous to

oxides.Professor Armstrongfor long advocated the view that

these inertgases reallyare endowedwith such strong hemicalaffinitieshattheyare compounds hat have neverbeendecom-posed. They certainly ave such strong ffinitiesorelectronsthat the atom,the complexof the + ion and electrons, an notbe decomposed hemically. Yet, in this case,wheretheaffinityof the matterforthe electron s at a maximum, he chemical

combining oweris absent.These gases seem to furnish he nearest standardwe haveto electricneutralityn the atomic ense. The negativechargeof the electronsexactly' atisfies the positive charge of thematter,and the atomic complexis chemically, ecause elec-trically,neutral. In the case of the electro-positivelements,hydrogennd thealkali-metals,ne electronmorethansatisfiesthe positive chargeon the ion, and'so long as the equalityofopposite charges s not altered, he electron ries to get away.

In the case of the electro-negativelements, uch as thehalo-gens, the negative charge, though equal presumablyto thepositive, s not sufficiento neutralizethe atom. Hence thesegroupsshow strongmutualaffinity,ne havingmoreand theother less negative electricity han would make the systematomicallyneutral ike helium. The electron xplainswell themerelynumerical spect of valency. But chemicalcombiningpoweritselfseems to requirethe idea that equal and opposite

chargesin the atomic sense are only exactlyequivalent n thecase ofthe inertgases. None of these deas are nownew,buttheirconsistentpplication o thestudyof chemical ompoundsseems curiously o hang fire, s though somethingwere stilllacking.

It is so difficultor the chemist onsistentlyo realize thatchemical affinitys due to a dissociating s well as to a com-biningtendency nd is a differentialffect. There is onlyoneaffinity,robably, nd it is thesame as thatbetweenoppositely

charged spheres. But,atomiccharges beingenormous nd thedistancesover which heyoperate n chemicalphenomena eingminute, hisaffinitys colossal,evenin comparisonwith chem-ical standards. Whatthe chemist ecognizes s affinitys dueto relatively lightdifferencesetweenthe magnitudeof theuniversal tendency f the electron o combinewithmatter nthe case ofthe differenttoms. Over all is the necessary on-ditionthatthe opposite chargesshould be equivalent,but this









being satisfied,he individualatomsdisplaythe tendencies n-herent n their tructure,ometo lose,others o;gain electrons,in order,as we believefromSir JosephThomson'steaching,to accommodate he number f electrons n the outermost ingto some definite umber. Chemical affinityeeds that someshall lose as wellas othersgain. Chemicalunion s always pre-ceded by a dissociation. The tendency' o combine,only,isspecific o any particular -atom,but the energyand drivingpower of combination s-the universalattraction f the + forthe- change,of matterfor the electron.

THE ELECTRICAL THEORY OF MATTER

Another arrier hatundoubtedlyxiststo the better ppre-ciation of the modernpoint of view, even amongthosemostwilling o learn, s the confusionhatexistsbetween heearlierand the present ttempt o explainthe relationbetweenmatterand electricity. We know negative electricity part frommatter as the electron. We know positive electricity partfrom the electron, he'hydrogen on and the radiant heliumatomor a-particleofradioactive hangeforexample, nd it ismatter n the'free or electrically ncombined ondition. In-deed, if you want to findmatter free and uncombined, hesimpleelementary articleof matter nthesense ofcomplexitybeing discussed,youwillgo, paradoxically, owhat thechemistterms compound ather han to that whichhe terms he freeelement. If this compounds ionizedcompletelyt constitutesthe nearest pproachtomattern the free tate. Thus all acids

owe their common cidic quality to really freehydrogen, hehydrogen on, a particle more different rom the hydrogenatom than the atom s from hehydrogenmolecule.

Positive electricitys thus emphaticallynot the mere ab-sence of electricity,nd any electricaltheoryof matter pur-porting o explainmatter n termsof electricity oes so by thepalpable sophistryfcallingtwo fundamentallyifferenthingsbythe samename. The dualismremainswhetheryou speak ofmatterand electricity,r of positive and negative electricity,and the chemistwould do well to stick to his conceptionofmatter,until the physicisthas got a new name for positiveelectricity hichwill notconfuse t with the onlykind of elec-tricity hat can existapart frommatter.

On the other hand, the theory of the electro-magneticoriginof mass or inertia s a true monism. It tries to explainconsistentlywo things-the inertia of the electron and theinertia of matter-by the same cause. The inertia of the









former eingaccounted orby the well-knownlectro-magneticprinciplesof Faraday, by the assumption hat the charge on

theelectrons concentratednto a sphereofappropriate adius;the2,000-fold reater nertiaof thehydrogen on,forexample,can be accountedforby shrinking he sphereto one two-thou-sandthof the electronic adius.

But the electricaldualism remainscompletely nexplained.Call the electron and thehydrogenonH. The facts are thattwoE's repel one anotherwiththe same forceand accordingtothe same law as two H's repeleach other, r as an H attracts

an E. These very remarkableproperties f H and E are notexplained by the explanationof the inertia. Are E and Hmade up of the same stuff r of two differenttuffs? We donot know,and certainlyhave no good reason to assume,thatmatter minus its electrons s made of the same thingas theelectron. We have stillto reckonwith twodifferenthings.

THE CHEMICALELEMENTS NOT NECESSARILYOMOGENEOUSI pass now to thesecond andmost novelsense in whichthe

elements, r someof them at least, are complex. In their dis-covery of new radioactiveelements,M. and Mme. Curie usedradioactivity s a method of chemical analysis preciselyasBunsenand Kirchoff,nd laterSir William Crookes, sed spec-trumanalysis to discover aesiumand rubidium, nd thallium.The new methodyielded t once,fromuraniumminerals, hreenew radio-elements,adium,polonium nd actinium. Accord-ing to the theoryof Sir Ernest Rutherford nd myself, hese

elements are intermediatemembers in a long sequence ofchanges of the parent elementuranium. In a mineral thevariousmembers f theseries mustcoexist n equilibrium, ro-vided nonesucceed n escapingfrom he mineral, n quantitiesinverselyproportional o their respectiverates of change, ordirectly roportional.o their periods of average life. Radiumchanges sufficientlylowly to accumulate in small but pon-derable quantity n a uranium mineral,and so it was shownto be a new memberof the alkaline-earth amilyof elements,

with atomic weight 226.0, occupyinga vacant place in theperiodictable. Poloniumchanges 4,500 timesmore rapidly,and can only xistto the extent f a fewhundredthsf a milli-gramin a ton of uraniummineral. Actinium lso, though tslifeperiod s stillunknown, nd very possibly s quite long, sscarce for anotherreason,that t is notin the mainline of dis-integration, utin a branch eries whichclaimsonlya few percent. of the uraniumatoms disintegrating. In spite of this,









polonium nd actiniumhave .just as much right to be consid-ered new elements,probably, s radiumhas. Poloniumhas

great resemblance n chemicalcharacterboth to bismuth ndtellurium, ut was separated from he first y Mme.Curie andfrom he second by Marckwald. In the position t occupiesasthe last member f the sulphurgroup,bismuth nd telluriumare its neighbors n the periodic able. Actinium esembles herare-earth lements, nd most closely anthanum,but an en-richment f the proportion f actiniumfrom anthanumhasbeen effectedy Giesel. The smallnessofthe quantities lone

preventstheircomplete eparationin the form of pure com-pounds as was done forradium.The three gaseous members, he emanations of radium,

actiniumand thorium,were put in theirproper place in theperiodictable almostas soon as radium was, for,beingchem-ically nertgases, their characterization as simple. Theyarethe last members f the argon family, nd the factthat thereare threeof about the same atomicweightwas probablythefirst ndication, lthough'not clearly appreciated,that more

than one chemicalelement ouldoccupythe same place in theperiodictable.

The extensionof the three disintegrationeries proceededapace; new members were being continually dded, but noother new radio-elements-new, hat is, in possessinga newchemical character-were discovered. The four longest-livedto be added,radio-leadorradium-D, s it is now morepreciselytermed, nd ionium n the uranium eries,and mesothorium-I

and radiothoriumn the thorium eries,couldnot be separatedfrom other constituents always present in the minerals,radium-D from ead, ionium and radiothorium romthorium,and mesothorium-I romradium. An appreciable proportionof the radioactivity f a uraniummineral s due to radium-Dand itsproducts, nd its separationwouldhave been a valuabletechnicalachievement, ut, though many attemptshave beenmade, this has never been accomplished, nd, we know now,probablyneverwill be.

Sevenyears ago itwas thegeneral opinion n the thencom-paratively ndeveloped nowledge f the chemistry f the radio-elements, hat therewas nothing speciallyremarkable n this.The chemists familiarwithmanypairs or groups of elements,theseparationof which s laborious nd difficult,nd the radio-chemist ad not thenfully ppreciated hepower of radioactiveanalysis n detecting very slightchange n the proportions ftwoelements, neor both of whichwere radioactive. The case









is not at all like that of the rare-earth roupof elements, orexample, n whichtheequivalent r atomicweight s used as aguide to the progressof the separation. Here the total dif-ference n the equivalentof the completely eparated elementsis only a very small percentage of the equivalent, and theseparation must already have proceededa long way before tcan be ascertained.

Humannature plays its part in scientificdvances, and thechemist s human ike the rest. My ownviews on the matterdevelopedwith some speed when, n 1910, came across a new

case of this phenomenon. Trying o find utthe chemical har-acter of mesothorium-I, hich had beenkept secret for tech-nical reasons,I found t to have precisely he same chemicalcharacter s radium, discoverywhich was made in the sameyear by Marckwald, nd actuallyfirst ublishedby him. I de-layed my publication omemonths o complete very carefulfractional rystallization f the barium-radium-mesothorium-Ichloride eparatedfrom horianite. Although great numberoffractionations ere performed,nd theradiumwas enriched,withregardto thebarium, everal hundred imes, heratiobe-tween the radium and mesothorium-Iwas, within the verysmallmarginof errorpossible n carefulradioactivemeasure-ments,notaffectedy the process. I felt ustifiedn concludingfromthis case, and its analogy with the several other similarcases thenknown, hat radiumand mesothorium-I ere non-separable by chemicalprocesses, nd had a chemical haracternot merely ike, but identical. It followedthat some of the

commonelementsmight similarlybe mixtures of chemicallyidentical elements. In the cases cited,thenon-separable airsdiffern atomicweightfrom2 to 4 units. Hence the lack ofany regularnumerical elationships etween heatomicweightswouldonthis view follownaturally.2This idea was elaboratedin the Chemical Society'sAnnual Reporton Radioactivity or1910, n theconcludingection umming p the position t thattime. Thiswas, I think, hebeginning f theconception fdif-ferent elements denticalchemically,which later came to be

termed isotopes, thoughit is sometimesattributed o K.Fajans, whose valuable contributionso radioactivity ad notat that date commenced,nd whose first ontributiono thissubject didnotappear till 1913.

In thesix or sevenyearsthat have elapsedthe view has re-ceived completevindication. Really, three distinct ines ofadvance converged o a common onclusion, nd, so far as is

2Trans. Chem.Soc., 1911,XCIX., 72.









possible, hesemaybe disentangled. First,therehas been theexact chemical haracterization rom he new point of view ofeveryone of the membersof the three disintegration eries,with ives overone minute. Secondly, ame thesweepinggen-eralizations n the interpretationf the periodic aw. Lastly,therehas been the first eginnings f our experimental nowl-edge of atomicstructure,whichgot beyond he electronic on-stituents nd at thematerialatom tself.

In pursuanceof the first,AlexanderFleck, at my request,commenced carefulsystematic tudy of the chemical char-

acterof all the radio-elements nown ofwhichour knowledgewas lackingorimperfect,o see whichwereand whichwerenotseparable fromknown chemical lements. Seldom can the re-sultsof so much ong and laboriouschemicalworkbe expressedin so few words. Every one,that it was possibleto examine,was found o be chemicallydenticaleitherwithsomecommonelement or with anotherof the new radio-elements. Of themore important haracterizations,mesothorium-Ilwas foundto be non-separable romactinium, adium-Afrompolonium,

the three B-members and radium-D from lead, the threeC-membersand radium-E from bismuth, actinium-D andthorium-D romthallium. These resultsnaturally ook sometimeto complete, nd becameknownfairly widely to othersworkingn the subject before heywerepublished, hroughA.S. Russell,an.old student,who was thencarryingon his in-vestigations n :radioactivity:n Manchester. Their interpre-tationconstituteshe second ine of advance.

Before that is considered, t may firstbe said that everycase of chemicalnon-separability ut forwardhas stood thetestoftime, nd all themanyskilledworkerswho have pittedtheir chemicalskill against Nature in this quest have merelyconfirmedt. The evidence t thepresentday is toonumerousand detailedto recount. It comesfrom ources, uch as in thetechnical xtraction fmesothorium rommonazite,whereoneprocess is repeated a nearly endless number of times; fromtrials of a very greatvarietyofmethods, s, for example, n

the investigations n radium-D and lead by Paneth and vonHevesy; it is drawnfrom otallynewmethods, s in thebeau-tiful.proofythe same authorsofthe electro-chemicaldentityofthesetwoisotopes; t is at the basis ofthe use ofradioactiveelements s indicators orstudyingheproperties fa commonelement, sotopicwith it, at concentrations oo feeble to beotherwisedealtwith,and from arge numbersof isolated ob-servations, s well as prolonged ystematicresearches. One









of the finest xamplesof the latterkind of work, he Austrianresearches n ionium,will be dealtwith ater. Themostrecent,which ppeared ast month,s byT. W. Richards ndN. F. Hall,who subjected ead fromAustraliancarnotite, ontaininghere-fore radium-D,to over a thousandfractionalcrystallizationsin theform fchloride,without ppreciably ltering he atomicweightor the,8-activity. o that it may be- afely stated thatno one who has ever reallytested his conclusion ow doubts t,and after ll they lonehave a right o an opinion.

This statement f the non-separabilityy chemicalmethods

ofpairs or groupsof elements uffers erhaps frombeing in anegativeform. It looks too much ike a mere negativeresult,a failure,but in reality t is one ofthe most sweepingpositivegeneralizations hat could be made. Ionium wo say is non-separable fromthorium, ut everychemistknows thorium sreadily separated fromeveryother known element. Hence,one now knowsevery detail of the chemistry f the vast ma-jorityof thesenew radio-elementsy proxy, ven when theirlife s tobe measured nminutes rseconds, s completelys if

theywere obtainable, ike thorium s, by the ton. The dif-ference t makes can onlybe appreciatedby thosewho havelived through arlier days, when, n some cases, dealingwiththe separation of radio-constituentsromcomplexminerals,aftereverychemicalseparationone took the separated partsto theelectroscopeo find ut where hedesired onstituent as.

As the evidence ccumulated hat we had to deal herewithsomethingnew and fundamental,he questionnaturally rose

whetherthe spectrumof isotopeswould be the same. Thespectrums known, ike the chemicalcharacter, o be an elec-tronicrather hanmass phenomenon,nd it was tobe expectedthatthe identityhould extendto the spectrum. The questionhas beentested verythoroughly yA. S. Russell and R. Rossiin this country, nd by the Austrian workers at the RadiumInstitutof Vienna, for ionium and thorium, nd by variousworkers or thevarious sotopesof ead. No certaindifferencehas been found, nd it may be concludedthat the spectraof

isotopes are identical. This identityprobablyextendsto theX-ray spectra,Rutherfordnd Andradahavingshownthat thespectrum fthey-rays f radium-B s identicalwith theX-rayspectrum f its isotope, ead.

(To be continued)

soddy (1917) complexity elements

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