Sir Humphry Davy on the Nature of the Diamond

Sir Humphry Davy on the Nature of the DiamondAuthor(s): Robert SiegfriedSource: Isis, Vol. 57, No. 3 (Autumn, 1966), pp. 325-335Published by: The University of Chicago Press on behalf of The History of Science SocietyStable URL: http://www.jstor.org/stable/228364 .

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Sir Humphry Davy on the Nature

of the Diamond*

By Robert Siegfried **

IN OCTOBER, 1813, Sir Humphry Davy embarked at Plymouth for what was to become a leisurely eighteen-month tour of France, Italy, and

adjoining territories then under the domination of Napoleonic France. Sir Humphry's scientific fame was already great enough to obtain the special permission necessary for the trip, whose ostensible purpose was to examine 'the extinct volcanoes in Auvergne and the active ones near Naples. He was looking for evidence to support his very tentative hypothesis that volcanic activity was produced by the reaction of ground water intermittently reach- ing large subterranean deposits of the active metals of the alkaline earths. " Such an assumption," he had written, " would offer a theory for the phe- nomena of volcanoes, the formation of lavas, and the excitement and effects ,of subterranous heat ...."1

In addition to his wife, Davy was accompanied by Michael Faraday and a "portable chemical apparatus," 2 all acquired within the previous year and a half. His marriage in April of 1812, immediately after receiving his knighthood, had not lessened his dedication to experimental inquiry, and the portable apparatus allowed him to maintain his scientific pursuits even while enjoying Lady Davy's company in travel.

Nor was the portable equipment merely an affectation. Shortly after his arrival in Paris in November, 1813, he completed with its help the work necessary to write a paper, " Experiments and Observations on a New Substance which Becomes Violet-Coloured by Heat." 8 A sample of this new substance, iodine as it came to be called, had been given to him by

* This paper was read to the Eighth Annual Meeting of the Midwest Junto, 2 April 1965, in Stillwater, Oklahoma.

** University of Wisconsin. Work for this paper was in part supported by a grant (GS- 393) from the National Science Foundation, whose help is gratefully acknowledged.

1 "Electro-Chemical Researches on the De- composition of the Earths; with Observations on the Metals Obtained from the Alkaline Earths, and on the Amalgam Procured from Ammonia," Philosophical Transactions of the Royal Society, 1808, [98]:333-370, p. 369. All of Davy's works referred to in this paper can

Isis, 1966, VOL. 57, 3, No. 189.

also be found in The Collected Works of Sir Humphry Davy, Bart., edited by his brother John Davy (London: Smith, Elder & Co., 1839- 1840); future reference to this collection will be as follows: Works, Vol. 5, pp. 138-139. This idea of the cause of volcanic action was somewhat more fully developed in a public lecture of 1811. See John Davy, Memoirs of the Life of Sir Humphry Davy, Bart. (London: Longman, Rees, Orme, Brown, Green & Long- man, 1836), Vol. 1, pp. 396-397.

2 John Davy, Memoirs, Vol. 1, p. 456. 3 Phil. Trans., 1814, [104]:74-93; Works, Vol.

5, pp. 437-456.

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ROBERT SIEGFRIED

his most loyal admirer in France, M. Ampere. Davy immediately recognized the similarity of iodine to chlorine and fluorine, both of which he had previously classified with oxygen, and from that observation was enabled rapidly to determine the many specific properties which confirmed its. classification as the " fourth supporter of combustion." 4 The establishment of a whole class of supporters of combustion thoroughly shook the validity of Lavoisier's oxygen-centered system of chemistry. And Davy's work on iodine which confirmed the inadequacy of that system was actually performed in France, much to the discomfort of some of the French chemists. As Davy's first biographer expressed the situation, Davy " had, by a single blow, fatally mutilated the system which was the pride and glory of their nation: it was ungenerous to remind them of his triumph." 5 Particularly, J. L. Gay-Lussac, who had thought the investigation of iodine to be his own project, was much annoyed by Davy's actions. The situation called for more tact than Davy possessed, and he departed from Paris at the end of the year leaving behind some ruffled feelings.

His visit to the extinct volcanoes in Auvergne delayed him hardly at all, though a month's stay in Montpellier allowed him to find further maritime sources of iodine. His arrival in Florence in March, 1814, gave him access to the laboratory of the Accademia del Cimento, where he carried out his experiments on the combustion of the diamond. For this work he used the great burning lens in their natural history cabinet, the same one used in the very first experiments of this sort accomplished in 1694 by the workers of Cosmo III, Grand Duke of Tuscany.6 Although Davy's earliest experiments on the diamond date from 1808, he had never had the opportunity to carry out its combustion. He now embarked upon a full-scale investigation of the composition of that body.

Where Davy had offended the French savants by his lack of tact, he alarmed the custodians of the great lens by " the hasty, and apparently careless manner in which he conducted his experiments." 7 But whatever discomforts Davy's actions created in others, he was still a brilliant experi- mentalist. He was able to arrange the apparatus in a way that the diamond

4 This phrase was used in a letter to his brother John, dated 1 March 1814. Memoirs, Vol. 1, p. 482.

5 John Ayrton Paris, The Life of Sir Hum-

phry Davy, Bart. (London: H. Colburn & R. Bentley, 1831), p. 278. (All page references to this work are to the single-volume edition.) Davy's doubts about the validity of the oxygen theory were first clearly expressed in his paper reporting the decomposition of the fixed alkalies in 1807. Finding these substances to be oxides of the new metals sodium and potassium, he hypothesized that the volatile alkali (ammonia) might also contain oxygen. His experimental efforts to confirm this analogy appearing to be successful, he wrote that " the principle of acidity of the French Nomencla-

ture, might now likewise be called the principle of alkalescence" (" On some New Phe- nomena of Chemical Changes Produced by Electricity, Particularly the Decomposition of the Fixed Alkalies, and the Exhibition of the New Substances which Constitute Their Bases; and on the General Nature of Alkaline Bodies," Phil. Trans., 1808, [98]:1-44, p. 41; Works, Vol. 5, p. 98). The manner in which the oxygen was present in ammonia was never decided, though the possibility that nitrogen was an oxygen-containing compound was later tested, as will be seen.

6 In this and in other general biographical matters I have followed the accounts of John Davy's Memoirs and Paris' Life.

7 Paris, Life, p. 283.

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SIR HUMPHRY DAVY ON THE NATURE OF THE DIAMOND

continued to burn even when withdrawn from the focus of the lens. This discovery made the completion of his work rapid and effective. His related work on the combustion of other carbonaceous material was accomplished in the laboratories of the Accademia dei Lincei in Rome during the fol- lowing month.

Descriptions of these experiments and Davy's conclusions based on them were embodied in a paper which was sent back to London and read to the Royal Society on 23 June 1814. Prior to this time, Davy had persistently expressed the view that there must be a small difference of composition between the diamond and charcoal which accounted for the great differences in their properties. But his own best efforts described in this paper had failed to detect any significant chemical difference, and he wrote, " I am more inclined to adopt the opinion of Mr. Tennant, that the difference depends upon crystallization." 8

This definitive result terminated for Davy his reliance on composition as the exclusive explanation for the properties of bodies. The general view

(it can hardly be called a theory) that substances with different properties must have different compositions was widely held at the time. It became specifically directed toward the identification of the chemical elements fol- lowing Lavoisier's operational definition of that concept. Davy, who had taught himself chemical philosophy from Lavoisier's Traite,9 held this view perhaps more consciously than most of his colleagues. All his most important work from 1806 to 1814 dealt in one way or another with chemical analysis and the problem of the identification of elementary substances.

Although Davy's work on the nature of the diamond is not well known, it is closely related to his famous researches on chlorine and the active metals. His establishment of these bodies as elementary ones greatly clarified related problems of composition. His work on the diamond helped establish the need to consider structure as well as composition in the explanation of the properties of matter. It is intended in this paper to trace the development of Davy's concern with the nature of the diamond from the earliest mention in 1808 to the completion of the Italian work in 1814.

This account also provides a convincing example of Davy the experi- mentalist. The sequence of observation-analogy-experiment, which for Davy typified the ideal of chemical philosophy, is beautifully evident in this story. For all his easy speculations Davy was fundamentally an empirical thinker whose speculations were immediately derived from his laboratory experience and designed to provide specific suggestions for further investigations.

The history of the relationship between diamond and charcoal begins in 1773 when Lavoisier and his co-workers were able to show that the gaseous combustion product of the diamond was the same fixed air produced by the combustion of charcoal.10 No quantitative values were determined how-

8" Some Experiments on the Combustion 9 John Davy, Memoirs, Vol. 1, p. 42. of the Diamond and Other Carbonaceous Sub- 10 J. R. Partington, A History of Chemistry stances," Phil. Trans., 1814, [104]:557-570, p. (London: Macmillan, 1962), Vol. 3, pp. 381- 568; Works, Vol. 5, p. 489. 384.

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ROBERT SIEGFRIED

ever. In 1796, Smithson Tennant found that the quantity of fixed air produced by the combustion of the diamond did " not differ much from that which, according to M. Lavoisier, might be obtained from an equal weight of charcoal." 11 Tennant concluded that the diamond " consists of charcoal, differing from the usual state of that substance only by its crystallized form." 12

In 1797, Guyton de Morveau determined, as a result of his experiments, that charcoal required significantly less oxygen for total combustion than did diamond and inferred that the charcoal was a suboxide of diamond.13 Guyton's work contained many chances for serious experimental error, and its reliability was challenged by 1807 by two Englishmen. In a paper communicated to the Royal Society William Allen and William H. Pepys under- took a study " On the Quantity of Carbon in Carbonic Acid, and on the Nature of the Diamond." Their carefully performed experimental work confirmed the earlier work of Tennant and Lavoisier and they concluded " that the diamond and all carbonaceous substances . . . differ principally from each other in the state of aggregation of their particles." 14

Thus the fact that the diamond was made principally of the same carbonaceous material as charcoal was definitely established before Davy first made any experiments on these materials. Also presented were the two alternative views that the differences between the two substances were to be explained either by differences in the arrangement of their internal parts or by differences in chemical composition. The experimental evidence seemed to favor the structural interpretation. Indeed the paper of Allen and Pepys was sufficiently admired that, as Davy wrote to Pepys, " If you and Allen had been one person, the Council of the Royal Society would have voted to you the Copleian Medal; but it is an indivisible thing and cannot be given to two." 15

In spite of the fact that the best chemical evidence showed no difference in the composition of the two materials, Davy expressed the view that internal structure was not enough, some chemical difference would also be found:

... minute researches upon their chemical relations, when examined by new analytical methods will, I am inclined to believe, show that the great difference in their physical properties does not merely depend upon the difference of the mechanical arrangement of their parts, but likewise upon differences in their intimate chemical nature.16

The new analysical methods Davy had in mind involved the use of

potassium metal. His earlier decomposition of the fixed alkalies had resulted from his appreciation of the unlimited powers of electricity to decompose

11 " On the Nature of the Diamond," Phil. Chimie, 1799, 31:72-112. Trans., 1797, 87:123-127, p. 126. 14 Phil. Trans., 1807, [97]:267-292, p. 292.

12Ibid., p. 124. 15 Letter dated 13 Nov. 1807, quoted by 13 " Du proces-verbal des experiences faites /a Paris, Life, p. 179.

l'Ecole Polytechnique dans les annees V et VI, 16 Bakerian Lecture, Dec., 1808, Phil. Trans., sur la combustion du diamant," Annales, de 1809, [99]:70; Works, Vol. 5, p. 171.

328




materials held together by electrical forces. Although he recognized that the voltaic apparatus could not be used with nonconductors, the intense activity of potassium provided him with a chemical alternative.

Davy's third Bakerian Lecture, delivered in December of 1808, was " An Account of Some New Analytical Researches on the Nature of Certain Bodies, Particularly the Alkalies, Phosphorus, Sulphur, Carbonaceous Mat- ter, and the Acids Hitherto Undecomposed; with Some General Observa- tions on Chemical Theory." 17 The title accurately reflects the nature of the paper, which contains a broad assortment of hastily begun and incom- pletely pursued investigations using the new analytical tools. Not sur- prisingly this paper was followed by another attempting to complete the work begun in the first.

Of concern to us in these accounts is the description of his attempts to analyze the diamond. Since that substance was a nonconductor of electricity, he attempted its decomposition by reacting it with metallic potassium. When these substances were heated in a glass tube, he found that there was no elastic fluid produced and no evident reaction. The diamond was blackened, however, and scales formed, which appeared to Davy "as if they consisted of plumbago covered by the gray oxide of potassium." 18

In another experiment of the same sort, the residual potassium produced less hydrogen when added to water than it should have if there had been no action between it and the diamond. Davy concluded,

The experiments on the diamond render it extremely likely that it contains oxygen; but the quantity must be exceedingly minute, though probably sufficient to render the compound non-conducting ... .19

Fearful perhaps lest the minuteness of the quantity of oxygen appear inadequate to explain the differences between diamond and charcoal, Davy immediately added:

Whoever considers the difference between iron and steel, in which there does not exist more than 1/200 of plumbago, or the difference between the amalgam of ammonium, and mercury, in which the quantity of new matter is not more than 1/12000, or that between the metals and their sub-oxides, some of Which contain less than 1/10 of oxygen, will not be disposed to question the principle, that minute differences in chemical composition may produce great differences in external and physical characters.20

Davy's too convenient " principle " by which he justified the significance of so minute a quantity of oxygen was eventually abandoned because of its evident conflict with the doctrine of definite proportions. The idea that the diamond contained a little oxygen persisted for several years as a working hypothesis. Characteristically, efforts to test it were immediately

17 Phil. Trans., 1809, [99]:39-104; Works, 19 Phil. Trans., 1809, [99]:74; Works, Vol. 5, Vol. 5, pp. 140-204. p. 175.

18 Phil. Trans., 1809, [99]:73; Works, Vol. 5, 20 Phil. Trans., 1809, [99]:74-75; Works, p. 174. Vol. 5, p. 175.

329



ROBERT SIEGFRIED.

forthcoming. In the paper published as an appendix to the Bakerian Lecture for 1808, Davy wrote:

On the idea which I have stated . . . that the diamond may consist of the carbonaceous matter combined with a little oxygen, I exposed charcoal in- tensely ignited, by Voltaic electricity, to nitrogen, conceiving it possible that if this body was an oxide, containing oxygen very intimately combined, it might part with it in small proportions to carbonaceous matter, and give an important result.21

The important result he evidently had in mind was the production of a diamond. Such a result would have confirmed not only his hypothesis on the nature of that substance but also his previously suggested idea that nitrogen contained oxygen.

With similar justification, Davy heated charcoal to incandescence by means of electricity in an atmosphere of oxymuriatic acid (chlorine), which unlike nitrogen was assumed by nearly every one to contain oxygen. " My view in making this experiment," he wrote, " was to ascertain whether some new combination of carbonaceous matter with oxygen might not be formed in the process." 22 There was, of course, no reaction in either experiment.

The lack of reaction between charcoal, with its strong affinity for oxygen, and oxymuriatic acid, which according to Lavoisier's theory contained an abundance of readily available oxygen, greatly puzzled him, and in 1810 led to his systematic investigation of oxymuriatic acid. In his first paper concerned with the properties of that substance, he wrote:

One of the most singular facts that I have observed on this subject . . . is, that charcoal, even when ignited to whiteness in oxymuriatic or muriatic acid gases, by the Voltaic battery, effects no change in them....

This experiment, which I have several times repeated, led me to doubt of the existence of oxygen in that substance, which has been supposed to contain it above all others in a loose and active state; and to make a more vigorous investigation than had been hitherto attempted for its detection.23

Thus Davy's attempt to confirm the possible presence of oxygen in the diamond led him to his famous investigation of the nature of oxymuriatic acid. This research produced the conclusions that it contained no detectable

oxygen and the substance should be considered simple until further evidence

might argue the contrary. With characteristic avoidance of theoretical com-

mitment, he suggested the name chlorine as accurately descriptive of its

color, whatever its ultimate place in theory. " Should it hereafter be dis-

21 "New Analytical Researches on the Na- 23 " Researches on Oxymuriatic Acid, Its Na- ture of Certain Bodies, being an Appendix to ture and Combinations; and on the Elements the Bakerian Lecture for 1808," Phil. Trans., of the Muriatic Acid: With Some Experiments 1809, [99]:450-470, p. 466; Works, Vol. 5, pp. on Sulphur and Phosphorus, Made in the 220-221. Laboratory of the Royal Institution," Phil.

22 Phil. Trans., 1809, [99]:467; Works, Vol. 5, Trans., 1810 [100]:231-257, p. 232; Works, Vol.

p. 222. 5, p. 285.

330




covered to be compound, and even to contain oxygen, this name can imply no error, and cannot necessarily require a change." 24

Acceptance of Davy's judgment with regard to the simple nature of chlorine forced a reevaluation of the oxygen-centered system of Lavoisier. In that system oxygen had played a unique role as the acidic principle and the sole supporter of combustion. Prior to Davy's work on chlorine the similarity of its properties to those of oxygen had been explained by its presumed oxygen content. Now Davy classified it with oxygen as a true analogue in electro-negativity and as a " peculiar acidifying and dissolving principle, forming compounds with combustible bodies." 25

Another paper on chlorine and its compounds described additional evidence in favor of his view that chlorine had not yet been decompounded and that it and oxygen " belong to the same class of bodies." 26 With this paper in 1811 Davy concluded his researches on chlorine, leaving to his brother John the task of defending publicly the new view and its consequences.

The year 1812 marks a turning point in Davy's life and career. In April he was knighted by the Prince Regent, just three days before his marriage. In between he delivered his farewell lecture at the Royal Institution, for at this time he resigned his active association as Professor of Chemistry. But it is to his Elements of Chemical Philosophy,27 published shortly after his marriage, that we must turn for his more considered views on chemical matters. This book, on which he had been working since the previous autumn, provides a kind of summary of his previously accomplished work - the book is clearly dominated by material from his own researches. The work is labeled Volume 1, but no other volumes ever appeared, even as Thomas Thomson foresaw.28

This work, incomplete though it is, provides some of the best evidence of Davy's rather unsystematic approach to science. His willingness to attack problems in isolation with an ad hoc sort of opportunism is well illustrated by two examples. The discoveries relating to the nature of chlorine led him in this work to classify it with oxygen in a new class of " empyreal undecompounded substances." Although oxygen and chlorine were the only elementary supporters of combustion, he specifically disavowed any belief that a " peculiar substance, or form of matter is necessary" for the effects of combustion. Instead he explained combustion as " a general result, of the actions of any substances possessed of strong chemical attractions, or different electrical relations, and that it takes place in all cases in which an intense and violent motion can be conceived to be communicated to the

24" On Some of the Combinations of Oxy- 27 London: Bradford & Inskeep, 1812. This muriatic Gas and Oxygen, and on the Chemi- book appears as Vol. 4 of Works. cal Relations of These Principles, to Inflam- 28 Thomas Thomson in a review printed in mable Bodies," Phil. Trans., 1811, [101]:1-35, Annals of Philosophy (1813, 1:371-375, p. 372), p. 32; Works, Vol. 5, p. 345. stated that " to publish a complete system of

25Phil. Trans., 1810, [100]:243; Works, Vol. chemistry, all the facts contained in which 5, p. 297. shall have been verified by the author, I hold

26" On a Combination of Oxymuriatic Gas to be impossible." It was for this reason that and Oxygen Gas," Phil. Trans., 1811, [101]: Thomson entertained "considerable doubts 155-162, p. 161; Works, Vol. 5, p. 355. whether the work will ever be completed."

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ROBERT SIEGFRIED

corpuscules of bodies." 29 Although here rejecting the idea of a principle of combustion, he later gives an extended discussion of the idea that all inflammable bodies contain hydrogen, in an openly speculative revival of phlogiston. His apparent motivation for this revival is an attempt to reduce the number of elementary bodies necessary for a complete system of chemistry.30 Two other ideas that similarly reduce the required number of elementary bodies conclude his conjectures.

" The same ponderable matter in different electrical states" might constitute substances chemically different, he thought. He cites the parallel case for steam, water, and ice formed of the same ponderable matter but in different relations to temperature.

His other briefly considered suggestion involves structural differences:

That the forms of natural bodies may depend upon different arrangements of the same particles of matter has been a favourite hypothesis advanced in the earliest era of physical research, and often supported by the reasonings of the ablest philosophers. This sublime chemical speculation, [has been] sanc- tioned by the authority of Hooke, Newton, and Boscovich. ....31

But he is noncommital, and clearly he does not adhere strongly to any of these hypotheses.

On the nature of the diamond, he is similarly uncommitted in this book. He describes his own work which suggested that the diamond might have oxygen in it% but adds that the

... quantity, if any, must be very minute, which does not harmonise with the doctrine of definite proportions. If it should be ultimately found that the diamond is merely pure carbon, it will be an argument in favour of the varieties of elementary forms being produced by different aggregations or arrangements of particles of the same matter; for it is scarcely possible to fix upon bodies less analogous than lamp black, and the most perfect and beautiful of the gems.32

From the evidence in Elements of Chemical Philosophy, it is clear that in 1812 Davy had not committed himself to either of the chief alternative

explanations of the diamond problem. But for the first time in public print he allowed the possibility that structure alone might account for the differences between diamond and charcoal.

His work on the fluorine compounds in 1813 seemed to revive his interest in a chemical explanation. In his Elements, Davy had classified the fluoric

principle under the heading " Of Some Substances, the Nature of Which is Not Yet Certainly Known," 33 but in 1813 he wrote, "I was forcibly struck by the analogy between the oxymuriatic and the fluoric compounds, and led to doubt of the justness of my ideas respecting the nature of fluoric

29 Elements, pp. 225-226; Works, Vol. 4, 31 Elements, pp. 488-489; Works, Vol. 4, p. 166. p. 364.

30 For an analysis of this point, see my paper 32Elements, pp. 312-313; Works, Vol. 4, "The Phlogistic Conjectures of Humphry p. 231. Davy," Chymia, 1964, 9:117-124. 33Elements, p. 465; Works, Vol. 4, p. 347.

332




acid." 84 Davy does not explain why, if so forcibly struck, he delayed so long in pursuing the investigation. He also acknowledged his indebtedness to M. Ampere, who pointed out the " analogy between muriatic and fluoric compounds . . . [which] were formed in consequence of my ideas on chlorine." 35

In a pair of papers published in 1813 Davy systematically explored the properties of the fluoric compounds using their analogy with the chlorine compounds as a guide. Although unsuccessful in his efforts to isolate and preserve the fluoric principle (fluorine), he gathered enough evidence to justify his conclusion that the fluoric compounds did indeed contain a simple substance like chlorine. " We have now a triad of supporters of combustion," he wrote his brother in April, 1813.36

At this point, Davy could see larger possibilities than simple analogy. Having determined that the combining weight of fluorine " is less than half of that representing chlorine," 87 he could write:

As the investigation of nature proceeds, it is not improbable, that other more subtile bodies belonging to this class will be discovered, and perhaps some of the characteristic differences of those substances, which apparently give the same products by analysis, may depend upon this circumstance.

The conjecture appears worth hazarding, whether the carbonaceous matter in the diamond may not be united to an extremely light and subtile principle of this kind, which has hitherto escaped detection, but which may be ex- pelled, or newly combined, during its combustion in oxygen. That some chemical difference must exist between the hardest and most beautiful of the gems and charcoal, between a non-conductor and a conductor of electricity, it is scarcely possible, notwithstanding the elaborate experiments that have been made on the subject, to doubt: and it seems reasonable to expect, that a very refined or perfect chemistry will confirm the analogies of nature, and shew that bodies cannot be exactly the same in composition or chemical nature, and yet totally different in all their physical properties.38

This paper was completed just before he left for the continental tour previously described, and the immediacy with which he added iodine to his predicted list of supporters of combustion helped establish his confidence in this new classification. At the conclusion of his first paper on iodine, he wrote:

In my last paper, presented to the Society two months ago, I ventured to suggest that it was probable, that new species of matter, which act with respect to inflammable bodies, like oxygen, chlorine, and fluorine, would be discovered. I had not hoped, at that time, to be able so soon to describe the

84" Some Experiments and Observations on 37 "An Account of Some New Experiments the Substances Produced in Different Chemical on the Fluoric Compounds; with Some Obser- Processes on Fluor Spar," Phil. Trans., 1813, vations on other Objects of Chemical Inquiry," [103]:263-279, p. 264; Works, Vol. 5, p. 409. Phil. Trans., 1814, [104]:62-73, p. 65; Works,

85 Phil. Trans., 1813, [103]:265; Works, Vol. Vol. 5, p. 428. 5, p. 410. 38 Phil. Trans., 1814, [104]:72-73; Works,

36 John Davy, Memoirs, Vol. 1, p. 459. Vol. 5, p. 436.

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ROBERT SIEGFRIED

properties of a body of this kind, which forms an acid with hydrogen, like chlorine and fluorine, and which in some of its combinations resembles oxygen.39

The unexpectedly sudden confirmation of part of his prediction that other supporters of combustion would be discovered evidently greatly heightened his desire to test the other part of his suggestion, namely that a " more subtile body of this class " might be found in the diamond. His brother and biographer described the situation well.

After the discovery of another supporter of combustion in iodine, even before his anticipation had been published, . . . he was strengthened in the probability of the idea [that the carbonaceous matter in diamond might be united to a subtile principle of the oxygen class], and determined, as soon as possible, to subject it to the test of experiment. At Florence he found the facilities required, and he immediately entered upon the inquiry.40

In Sir Humphry's words, the object in these experiments was to examine minutely whether any peculiar matter was separated from the diamond during its combustion, and to determine whether the gas, formed in this process, was precisely the same in its minute chemical nature, as that formed in the combustion of common charcoal.41

Davy was able to detect no peculiar matter during the combustion of the diamond, and within the limits of his ability to determine, the gas formed was the same as that from the combustion of charcoal. Thus after six years of publicly expressed belief that there must be a chemical difference between these two substances, his own carefully obtained evidence forced him to a contrary conclusion.

The only chemical difference perceptible between diamond and the purest charcoal, is that the last contains a minute proportion of hydrogen; but can a quantity of an element, less in some cases than 1/50000 part of the weight of the substance, occasion so great a difference in physical and chemical characters? This is possible, yet it is contrary to analogy, and I am more inclined to adopt the opinion of Mr. Tennant, that the difference depends upon crystallization.42

Even here, when he is for the first time expressing a clear preference for a structural rather than a chemical difference between the two substances, he appears reluctant to abandon totally a possible chemical explanation.

None of Davy's later published researches concerned itself with the problem of the diamond, but there is one bit of clear evidence that he came to

accept without reluctance the idea that the arrangement of the parts of a substance could determine its properties. This comes in a paper written in 1816 primarily to take exception to some theoretical views of Gay-Lussac.

I cannot admit M. Gay Lussac's views on the classification of the unde-

compounded substances, nor can I adopt his ideas respecting their properties

39 Phil. Trans., 1814, [104]:92; Works, Vol. 5, p. 456. 5, p. 479.

40 John Davy, Memoirs, Vol. 2, p. 484. 42 Phil. Trans., 1814, [104]:558; Works, Vol. 41Phil. Trans., 1814, [104]:558; Works, Vol. 5, p. 489.

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as chemical agents. He considers hydrogen as an alkalizing principle, and azote as an acidifying principle. This is an attempt to introduce into chemistry a doctrine of occult qualities, and to refer to some mysterious and inexplicable energy what must depend upon a peculiar corpuscular arrangement. If hydrogen be an alkalizing principle, it is strange that it should form some of the strongest acids by uniting to bodies not in themselves acid; and if azote be an acidifying principle, it is equally strange that it should form nearly nine-tenths of the weight of the volatile alkali. It is impossible to infer what will be the qualities of a compound from the qualities of its constituents; and if M. Gay Lussac's views were correct, the prussic basis of azote and carbon ought to have its acid properties diminished, and not increased, as he has proved them to be, by combination with hydrogen.

When certain properties are found belonging to a compound, we have no right to attribute these properties to any of its elements to the exclusion of the rest, but they must be regarded as the result of combination.43

This statement unambiguously rejects the explanation of chemical properties by " principle," but does not commit him to a structural alternative more definite than " the result of combination." As we have earlier seen in the Elements of Chemical Philosophy, this caution is characteristic of

Davy's empirical temperment. He was willing to consider almost any hypothesis that might be momentarily useful; he was unwilling to commit himself to any. He expressed both sides of this attitude later in his criticism of Gay-Lussac:

The substitution of analogy for fact is the bane of chemical philosophy; the legitimate use of analogy is to connect facts together, and to guide to new

experiments.44

The most striking consequence of tracing the story of Davy's work on the diamond is the manner in which it connects all his major work of the 1808-1814 period. From his earliest attempts to decompose the diamond

by potassium, he was led to hypothesize the presence of a little oxygen. Attempts to synthesize the diamond by oxidating charcoal generated the

fact that charcoal would not react with oxymuriatic acid. Failure to decom-

pose that substance (later named chlorine) created a new class of supporters of combustion, which, confirmed by the work on fluorine, suggested the

possibility of other " more subtile" members of the class that might explain the diamond's differences from charcoal. The discovery of iodine intensified

his interest in testing this hypothesis, which he accomplished in his definitive

work in Florence in 1814. It is hard to escape the judgment that Davy

really did use analogy to connect facts together and as a guide to new

experiments, even as he said he did.45

43 "On the Analogies Between the Unde- compounded Substances, and on the Constitu- 45 For an interpretation of Davy's work on tion of Acids," Journal of Science and the Arts, the diamond differing markedly from that 1816, 1:283-288, pp. 285-286; Works, Vol. 5, given here, see L. Pearce Williams' " Boscovich p. 513. and the British Chemists," in Roger Joseph

44J. Sci. Arts, 1816, 1:288; Works, Vol. 5, Boscovich, edited by Lancelot Law Whyte p. 516. (London: Allen & Unwin, 1961).

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Sir Humphry Davy on the Nature of the Diamond