isis volume 64 issue 3 1973 james maclachlan - a test of an imaginary experiment of galileo's

A Test of an "Imaginary" Experiment of Galileo'sAuthor(s): James MacLachlanSource: Isis, Vol. 64, No. 3 (Sep., 1973), pp. 374-379Published by: The University of Chicago Press on behalf of The History of Science SocietyStable URL: http://www.jstor.org/stable/229724 .Accessed: 09/05/2014 19:58

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A Test of an "4Imaginary Experiment of Galileos

By James MacLachlan*

I

NUMEROUS EXPERIENCES ARE INTRODUCED in Galileo's Two New Sciences (1638) to support arguments about the nature of various phenomena.

While Ernst Mach (in the last century) saw appeals to observation and experiment as evidence of Galileo's modern spirit, Alexandre Koyre (from 1937 onward) expressed grave doubts that Galileo had actually performed many of the experiments he described.' Such a divergence of opinion continues to pervade discussions about the role of experimentation in the work of Galileo and in the foundations of physics in the seventeenth century.2

Now, there are appeals to at least three kinds of experiment in Galileo's writings: real experiments, imaginary experiments, and thought-experiments. Whereas a real experiment is one that Galileo actually performed, an imaginary experiment is one that he could have performed but did not, perhaps because he was so convinced that he already knew how it would turn out. Then there are genuine thought-experiments (called Gedankenexperimente by Mach) that cannot be performed, either for logical reasons or for lack of adequate equipment.3 In the work of Galileo (as in that of Newton, Einstein, and Heisenberg) there are clear examples of thought-experiments

Received May 1972: revised/accepted Aug. 1972. *Institute for the History and Philosophy of

Science and Technology, University of Toronto, Toronto 2B, Ontario, Canada.

' Ernst Mach, The Science of Mechanics. A Critical and Historical Account of its Develop- ment, trans. T. J. McCormack (6 eds.; La Salle: Open Court, 1893, 1902, 1907, 1919, 1942, 1960), Ch. II, Sec. 1. Mach's discussion of Galileo was somewhat modified in successive editions. Alexandre Koyre, Etudes galileennes (Paris:Hermann, 1939), esp. Pts. II, III; and Metaphysics and Measurement. Essays in Scienti- fic Revolution (Cambridge, Mass.: Harvard Uni- versity Press, 1968), Chs. I-IV.

2 Differing views of Galileo are expressed by Rene Dugas, Mechanics in the Seventeenth Century. From the Scholastic Antecedents to Classical Thought, trans. F. Jacquot (New York: Capital Book Co., 1958), pp. 80-87, and E. J.

Dijksterhuis, The Mechanization of the World Picture, trans. C. Dikshoorn (Oxford: Clarendon Press, 1961), pp. 333-345. Deeper studies may be found in Galileo, Man of Science, ed. E. McMullin (New York:Basic Books, 1967) in the essays by Dominique Dubarle, "Galileo's Methodology of Natural Science," pp. 295-314, and Thomas B. Settle, "Galileo's Use of Experiment as a Tool of Investigation," pp. 315-337.

3 The motion to be expected of the moon if gravity were "switched off" is an example of a logically impossible thought-experiment. Drop- ping a stone and a feather onto the surface of the moon was a thought-experiment until men were able to land on the moon. I believe that the literature of the history of science would be well served if the term thought-experiment were reserved for such unperformable experiments and clearly distinguished from merely unper- formed imaginary experiments.

374


TEST OF AN "IMAGINARY" GALILEAN EXPERIMEN E 375

that are widely acknowledged to have played an important role in the development of certain ideas.4

Yet, contention remains about which of Galileo's other experiments were imaginary and which were real. In fact, some of Galileo's experiments are less real than others. On the one hand, his claim that the period of a pendulum is entirely independent of amplitude can easily be demonstrated not to be true for amplitudes greater than 30 degrees.5 On the other hand, although Koyre considered Galileo's inclined-plane experiment to be "completely worthless,"6 Thomas Settle has shown that it was likely a real experiment. He tested the experiment according to Galileo's description and found there to be no difficulty in supposing Galileo to have had sufficient resources to attain the results he described.7

Did Galileo engage deeply in the direct interrogation of nature, or was he more con- cerned with shifting science from the shoulders of Aristotle to those of Plato? Con- tinuing disagreement over this issue should mean that the examination of particular experimental claims by Galileo can contribute to our proper understanding of the part played by experience in the development of his ideas. To that end, here is the report of a test of another Galilean experiment identified as imaginary by Koyre.8

II

In a passage in the First Day of the Two New Sciences Salviati described a procedure (see Fig. 1, in Sec. V) involving water and wine :9

. . . I filled with water a glass ball that had an opening as narrow as a straw stem ... and turned it over with its mouth downward. However, neither the water, although very heavy and suited to falling through air, nor the air, although very light and much inclined to rise in water, will agree, the former to falling out of the hole [of the ball], the latter to rising upon entering [therein]; but remain, both of them, stubborn and perverse [in their places]. On the contrary, as soon as we shall present to that hole a vessel con- taining red wine, which is only imperceptibly less heavy than water, we shall see it immediately rise slowly in red streaks through the water; and the water, with the same slowness, descend through the wine, without in the least mixing together, until finally the ball would be completely full of wine, and all the water would fall to the bottom of the vessel. Now, what should one say, and what arguments should be appealed to, except that there is between water and air an incompatibility that I do not understand, but which, perhaps....10

4T. S. Kuhn, "A Function for Thought Experiments," in L'aventure de l'esprit. Melanges Alexandre Koyre (Paris:Hermann, 1964), pp. 307-334.

5 Le Opere di Galileo Galilei, Edizione Nazionale, ed. A. Favaro (Florence:Tipografia Barbera, 1890-1909), Vol. VIII, p. 139; in Two New Sciences, trans. H. Crew and A. de Salvio (New York: Macmillan, 1914 and numerous reprints), pp. 95-96; cf. Marin Mersenne, Les nouvelles pensees de Galil6e (Paris: Guenon, 1639), pp. 72-73.

6 Koyre, Metaphysics and Measurement, p. 94. 7 T. Settle, "An Experiment in the History of

Science," Science, 1961, 133:19-23. 8 A. Koyre, "Le De Motu Gravium de Galilee,

De l'experience imaginaire et de son abus," Revue d'Histoire des Sciences, 1960, 13:197-245. Reprinted in Koyre, Metaphysics and Measure- ment, pp. 44-88; trans. by R. E. W. Maddison, with the title "Galileo's Treatise De Motu Gravium, The Use and Abuse of Imaginary Experiment."

9 Galileo, Opere, Vol. VIII, pp. 115-116; Two New Sciences (Crew and de Salvio), p. 71.

10 To provide a close comparison with the comments of Prof. Koyre, this passage has been translated from his French translation in Rev. Hist. Sci., pp. 240-241. The Galileo passage in Maddison's translation (Koyre, Metaphysics and Measurement, p. 83) was made directly from the Italian.


376 JAMES MACLACHLAN

Water is so much more dense than air that one may be surprised at the "unwilling- ness" of water to fall out of the hole when the globe of water is inverted in the air. On the other hand, water seems to have no difficulty in falling out of the globe into wine, although the two liquids have very similar densities. Galileo seems to have observed a real phenomenon, although he lacked a satisfactory explanation of it.

III

Did Galileo really perform this experiment? Had he actually observed the inter- change of water and wine that he described? Koyre thought not. In 1960 he published the foregoing description by Galileo, and then commented:

I confess that I share Salviati's perplexity. It is, indeed, difficult to put forward an explanation of the astonishing experiment he has just reported; particularly, because, if we repeated it exactly as described, we should see the wine rise in the glass globe (filled with water), and water fall into the vessel (full of wine); but we should not see the water and the wine simply replacing each other; we should see the formation of a mixture.11

What is the conclusion? Do we have to admit that red wines of the seventeenth century had properties no longer possessed by the wines of today-properties that made them, like oil, immiscible with water? Or can we. suppose that Galileo, who undoubtedly never mixed water with his wine (for wine to him was "the incarnation of the light of the sun"), had never made the experiment; but, having heard of it, reconstructed it in his imagination, accepting the complete and essential incompatbility of water with wine as an indubitable fact ?-Personally, I feel that the latter supposition is the right one.12

That water and wine should mix together immediately is an assertion that Koyre might have derived from ordinary experience or from a priori conviction. Since he did not say whether he had performed the experiment, I decided to test what Galileo had described.

In the late summer of 1971 I filled an after-shave bottle with water and inverted it over a goblet of red wine. A piece of drinking straw sealed in the mouth of the bottle dipped beneath the surface of the wine. For more than an hour I watched in fascina- tion as a perfectly clear layer of water formed at the bottom of the goblet and became deeper and deeper!

As Galileo had described, a thin red streamer wafted up through the water in the bottle and occupied a progressively redder and larger region at the top of the bottle. A light shining through the goblet made possible the detection of a streamer of water descending through the wine to form the layer at the bottom. After about two hours the bottle above had become a quite uniform red, and the layer of red left at the top of the goblet began to descend, ultimately making the liquid in the goblet a uniform pink.

The bottle that was first filled with water was about 8 centimeters tall, 6 centimeters wide, and about 2 centimeters thick. The diameter of the straw was about 4 millimeters, The maximum volume of clear water that appeared at the bottom of the goblet was just over 60 milliliters, about three-fifths of the total volume of the wine in the goblet.

I have repeated the experiment a number of times with the following variations: (1) Since the opening of the bottle was only about 6 millimeters wide, it was applied

11 Koyre's footnote at this point will be reported and discussed in Sec. IV.

P.KyRev, Metaphysics and Measurement, p. 84; cf. Rev. Hist. Sci., p. 241.


TEST OF AN "IMAGINARY" GALILEAN EXPERIMENT 377

directly to the surface of the wine. (2) Several varieties of red wine were used (though, fortunately, none from the seventeenth century). (3) The bottle and goblet were replaced with a Florence flask and a beaker, using again a piece of drinking straw for the outlet from the flask. In all variations essentially the same results were observed, with the clear layer of water beneath the wine occupying from 40 to 60 per cent of the original wine volume. In some cases the water in the bottle became mixed up with wine more quickly than in the first trial, but the pronounced clear layer beneath the wine always appeared in the lower vessel. With a tube narrower than the drinking straw the exchange of liquids was minimal, and the trial was terminated.

IV

Professor Koyre proposed another variation: Results more nearly in agreement with Salviati's assertion would be obtained by having

two openings, instead of one, in the glass flask, and fitting a straw, or a narrow tube, to each in such a manner that one (A) is directed to the interior of the flask, and the other (B) to the exterior. We should then see a streak of wine streaming from the tube A towards the top of the flask, and a streak of water streaming to the bottom of the vessel, with the result that the wine would collect at the top, and the water at the bottom. Unfortunately, even in this case, there would be mixing. Furthermore, Salviati pro- vides one orifice only to his flask, not two; nor does he provide any straw.13

Koyr6 seems to have thought that the water and wine needed separate tubes in order to be exchanged with less mixing (see Fig. 2). As a matter of fact, using two tubes changes the experiment drastically and quite destroys the contrast Galileo had estab- lished. For, if a flask of water with two tubes in its mouth is inverted in air, then water and air will exchange places much more readily than the water and wine do. Yet Galileo had appealed to this experiment to demonstrate an incompatibility between water and air that does not exist between water and wine. Besides, I have shown that a single opening is sufficient, and even that no straw is needed to conduct the liquids.

Nevertheless, the two-tube experiment was worth trying. It was rather more difficult to perform because one of the tubes had to be clamped to prevent the water from flowing out (and air in) while the bottle was being inverted and the tubes submerged beneath the wine. Once that was done, the transfer of liquids followed essentially the same course as before-except that the whole operation occurred much more quickly. Only about 15 minutes were needed to arrive at a result that had taken 90 minutes using only one tube (or none, when the narrow mouth of the bottle was applied to the wine surface).

v

For the experiment to proceed as Galileo described it, the water must fall through the wine gently enough that no turbulence is produced to cause the water and wine to mix. At the same time, the water must traverse its path in a time short enough to fore- stall mixing by molecular diffusion. The results of my test and an elementary applica- tion of hydrodynamic principles suggest that the size of the hole in the globe of water is a critical factor in determining the nature of the downward flow of water.

'3 Koyre, Metaphysics and Measurement, p. 84n; cf. Rev. Hist. Sci., p. 241n.


378 JAMES MACLACHLAN

water ~ ~ ~ ~ ~ wae

Figure 1. Galileo's experiment. Figure 2. Koyr6's experiment.

At the water-wine interface (in the hole) there is a tendency to instability as a result of the greater density of the water. For a small enough hole (perhaps 2 millimeters) surface-tensi'on effects may be sufficient to inhibit the flow of water and wine. For a large enough hole (upward of 10 millimeters) the instability is so great that the flow of water is turbulent (nonlaminar) and considerable mixing will occur.114 Evidently Galileo's "1straw stem" and my 4- to 6-millimeter diameters are optimal for the occurrence of a smooth, laminar flow of the water.

For a hole of the optimum size, then, the instability at the water-wine interface results in the water "dropping" through the hole, displacing the wine, and setting up a convective flow in the wine. The wine is extruded into the water container above, in a thin jet. The "wafting" of the wine upward through the bottle suggests a degree of instability resulting from a higher linear speed than that of the water streaming downward, indicating that the water occupies a larger fraction of the hole than the wine. A

14 For a hole that large, of course, water will flow out quite readily when the bottle is held

mouth downward in air, so that the conditions of the Galilean experiment are again not realized.


TEST OF AN "IMAGINARY" GALILEAN EXPERIMENT 379

more detailed account of this phenomenon would require a rather complex analysis in hydrodynamics, which has not yet been undertaken. 15

When a narrow-mouthed vessel full of water is inverted in air the water does not fall out. Galileo attributed this phenomenon to an "incompatibility" between water and air. Now, when the same vessel is inverted with its mouth beneath the surface of wine, the two liquids exchange places, not exhibiting that kind of incompatibility. However, since the manner of exchange of the two liquids made no contribution to Galileo's argument, I believe that he would have been satisfied if the water and wine had merely interpenetrated gradually. But they did not; and Galileo's detailed description of the striking behavior of the wine and water convinces me that he did indeed see what I have seen. However imaginary this experiment may have been for Koyre, it was cer- tainly a real experiment for Galileo.

15 1 am grateful to Prof. C. 0. Hines of the University of Toronto and Dr. W. R. Peltier of the University of Colorado for discussions of the hydrodynamic principles involved in this ex-

periment. This research has also benefited from the kind encouragement of Prof. Stillman Drake of the University of Toronto.


Article Contentsp.374p.375p.376p.377p.378p.379

Issue Table of ContentsIsis, Vol. 64, No. 3 (Sep., 1973), pp. 287-440Front Matter [pp.287-289]Galileo's Experimental Confirmation of Horizontal Inertia: Unpublished Manuscripts (Galileo Gleanings XXII) [pp.291-305]Lavoisier's Theory of Acidity [pp.306-325]Science and Culture in the American Middle West [pp.326-340]Limits of the Recapitulation Theory: Carl Friedrich Kielmeyer's Critique of the Presumed Parallelism of Earth History, Ontogeny, and the Present Order of Organisms [pp.341-350]Prophatius Judaeus and the Toledan Tables [pp.351-355]The Decline of Cartesianism in Mechanics: The Leibnizian-Cartesian Debates [pp.356-373]A Test of an "Imaginary" Experiment of Galileo's [pp.374-379]loge: Lancelot Law Whyte, 1896-1972 [pp.380-381]Notes & CorrespondenceThe Provenance of John Dee's Manuscript of the De Superficierum Divisionibus of Machometus Bagdedinus [pp.382-383]The Affirmation of Empty Space by an Eleventh-Century Muctazilite [pp.384-385]Letters to the Editor [pp.385-386]

Correction: Archives of the Royal Society of Great Britain in Facsimile. Minutes of Manager's Meetings, 1799-1900. Volumes I-II: 1799-1802 [p.386]News [pp.387-388]Book ReviewsBiochemistry [pp.389-390]Birkenmajer [pp.390-393]Lyell [pp.394-395]

History of Scienceuntitled [pp.395-398]

Philosophy of Scienceuntitled [pp.398-399]untitled [pp.399-400]

Scientific Institutionsuntitled [p.401]

Social Relations of Scienceuntitled [pp.401-402]

Mathematicsuntitled [pp.402-404]untitled [pp.404-405]

Biological Sciencesuntitled [pp.405-406]untitled [pp.406-408]

Medicineuntitled [pp.408-409]

Technologyuntitled [pp.409-410]untitled [pp.410-411]untitled [pp.411-412]

Classical Antiquityuntitled [pp.412-413]

The Far Eastuntitled [pp.413-416]untitled [pp.417-418]

Renaissance & Reformationuntitled [p.419]untitled [pp.419-420]untitled [p.420]

Seventeenth & Eighteenth Centuriesuntitled [pp.420-421]untitled [pp.421-422]untitled [pp.422-424]untitled [pp.424-425]untitled [pp.425-426]

Nineteenth & Twentieth Centuriesuntitled [pp.426-427]untitled [p.427]untitled [pp.427-429]untitled [pp.429-430]untitled [pp.430-431]untitled [pp.431-432]

Contemporary Sciencesuntitled [pp.432-433]untitled [pp.433-435]untitled [p.435]untitled [pp.435-436]untitled [pp.436-437]

Back Matter [pp.438-440]

isis volume 64 issue 3 1973 james maclachlan - a test of an imaginary experiment of galileo's

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