VOLUME 33, NO. 6, JUNE, 1956
THE CHEMICAL BASIS FOR PROUT'S ROBERT SIEGFRIED University of Arkansas, Fayetteville, Arkansas
T H E origin of Prout's hypothesis has been an intriguing point of speculation for some time. Prout himself is largely responsible for this, for when he first suggested that hydrogen might be the modernequivalent of the protyle of the ancients he stated without further comment that the idea was "not altogether new." Recent opinion has generally favored Humphry Davy as the most likely source of ideas suggestive of Prout's hypothesis. One bit of evidence available from Prout's DaDers has - -been overlooked in the citing of support for Davy's claim. The contents of Pront's first paper are clearly described by the title "On the relation between the specific gravities of bodies in their gaseous state and the weights of their atom^."^ The paper contains no origi~
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' Presented before the Division of History of Chemistry at the 128th Meeting of the American Chemical Society, Minneapolis, September, 1955. J. C., ( T h e Scientific AchieveSee, for example, GREGORY, ments of Sir Humphry Davy," Oxford University Press, London, 1930, p. 86; and 0. T. BENFEY,J. CHEM.EDUC.,29, 78 (1952). 'This paper was first printed anonymously in Thomson's Annals ojPhilosophy, 6, 321 (1815). It is probably more accessible in the Alembic Club Reprints $20, 25-37, which also contain Prout's second paper of 1816. All references to Prout's papers are to the Alembic Club Reprint.
nal data of his own, but is concerned with the calculations of atomic weights by the application of GayLussac's law of combining volumes t o gas densities as determined by various other workers. I n summarizing the contents of his tables of calculated data, he stated: That all the elementary numbers, hydrogen being considered as 1, are divisible by 4, except carbon, mote, and barytium, and these are divisible by 2, appearing therefore to indicate that they are modified by a higher number than that of unity or hydrogen. Is the other number 16, or oxygen? And are all substances compounded of these two element^?^
There is nothing in Prout's paper itself to indicate why he chose oxygen as the representative for the higher number, or indeed why he felt the need for a higher number a t all. It seems likely that he was here showing a strong influence from Hnmphry Davy, who had on numerous occasions proposed a hydrogen-oxygen dualism based on the results of his electrochemical researches. It was only in a later paper offering a correction to his first, that Prout acknowledged that on the basis of his atomic weight data only one fundamental substance was necessary to explain the numerical simplicity among the atomic weights. 'Alembic Club Reprints #20, p. 37.
264 If the views we have ventured to advance be correot, we may almost consider the rp&rsiiAn of the ancients to be realized in hydrogen; an opinion, by the by, not altogether new.s
If we accept as probable the influence of Davy, we should also seek the experimental basis of his comments which t o Prout in 1816 were "not altogether new." Davy's suggestions respecting the f ~ d & e n t a l nature of hydrogen were not based on any mathematical simplicity discernible among the atomic weights, but on the apparent Presence of in of the undecompounded bodies, and, anal~logy of behavior, on the possible presence of it in others. I n spite of the fact that the chemical evidence favoring the presence of hydrogen in some of the elementary substances was widely acknowledged by other chemists of the time, this chemical basis for Prout's hypothesis has not been adequately explored. It is the purpose of this paper to examine the chemical evidence which made-p&sible Davfs numerous conjectures on the nature of the elemerits, and particularly the evidence the suggestion that hydrogen was the fundamental substance. Davy's career between the years 1808 and 1812 has been called "a period of perple~ities,"~ a phrase equally applicable to the whole of chemistry a t that time. The perplexity arose from the uncertainty of the identification of the elements. Lavoisier had developed the principle of denoting a substance an element only if it resisted serious effort to decompose it. The advantages of such au operational definition were clearly recognized by the chemists following Lavoisier, particularly in contrast with the confusion associated with chemical behavior when explained in terms of earth, air, fire, water, mercury, salt, sulfur, phlogiston, etc. But such an operational definition was not yet felt to be totally adequate or intellectually satisfying. An element might be usefully defined as a substance as yet undecomposed, but it ought to be defined as something undecomposable. Out of the divided loyalty to the new operational concept on the one hand, and the old metaphysical one on the other, arose the perplexities which so plagued Humphry Davy and, to a lesser extent, other leading chemists of the early nineteenth century. These uncertainties would not have been so significant had not there been the accumulation of a considerable amount of evidence that some of the substances presumed t o be simple were possibly compound, with hydrogen the most commonly presumed constituent. The evidence was not clear cut, as is usual with erroneous data, but the science of chemistry was yet too young t o speak with assurance about the true nature of the elements. I n 1812 Davy expressed his views about the elements as follows: We know nothing of the true elements belonging to nature; but as far as we can reason from the relations of the propertiesof
Ibid., p. 40. @Gregory, in his biography of Davy, uses this phrase for the title of Chap. 4, pp. 58-72.
JOURNAL OF CHEMICAL EDUCATION matter, hydrogen is the substanoe which approaches nearest t o what the elements may be supposed to be. It has energetic powers of combination, its particles are highly repulsive as to each other, and attractive of the particles of other matter; it entern into combination in a quantity very much smaller than any other substance, and in this respect it is approached by no known body.'
Though all the properties of hydrogen listed by Davy suggest that hydrogen might be "nearest to what the elements may be supposed to be," this statementwas not mere conjecture. If hydrogen were the fundamental substance, it should be possible to obtain it from such previously undecompouuded bodies as sulfur and phosphorus, for example. ~~d D~~ had found i t there, rn the ~ ~ k~~~t~~ ~ r to ithe ~~~~~l ~ society for 1808, D~~~ that when sulfur was heated in an electric arc, permanent gas was produced which proved t"V , h, h ~ , a ~ , , , , m,~fiap ..JY.VaY.I -1
The existence of hydrogen in sulphur is fully ~roved,and we have no right to consider a substanoe, which can be produced from it in such large quantities, merely as an rtecidental ingredient.~
He obtained by similar means, phosphoretted hydrogen (phosphine) from phosphorus and drew analogous conclusions about the composition of pho~phorus.~ Davy's willingness t o accept this evidence as "proof" of the hydrogen content of sulfur seems rather remarkable, but there are two circumstances which make it seem a more normal reaction. As has been pointed out, Davy carried no fixed concept of what was an element and what was not, and he was thus perfectly willing to believe that sulfur was not if the evidence so indicated. He almost never used the term element, for that implied a final knowledge which the science of chemistry could not rightfully claim. He preferred the more descriptive, but noncommittal term of undecompounded bodies. Secondly, Davy's own evidence was not the first to indicate that sulfur contained hydrogen. Indeed, he had undertaken the experiment because of previous indication of the decomposability of sulfur. I have referred, on a former occasion, to the experiments of Mr. Clayfield and M. Berthollet, jun., which seemed to shew that sulphur, in its common form, contained hydrogen. In considering the analytical powers of the Voltaic apparatus, it occurred to me, that though sulphur, from its being a non-omduotor, could not be expected to yield its elements to electrical attractions and repulsions of the opposite surfaces, yet that the intense heat, connected with the contact of these surfaces, might possibly effect some alteration in it, and tend to separate any elastic matter it might contain."
Clayfield never published his experimental work, but Davy described it in a footnoten in his famous paper,
' "The Collected Works of Sir Humphry Davy," edited by his DAVP, Smith, Elder and Co., London, 1839-40. brother, JOHN Quotation is from Vol. IV, '%lements of Chemical Philosophy," pp. 35859. Ibid., V., p. 163. 9 Ibid., p. 168. loIbid., p. 160. " Ibid,, p. 73.
VOLUME 33, NO. 6, JUNE, 1956
"On the decomposition of the fixed alkalis." This experiment was camed out in Davy's presence in 1799. It consisted of the union of copper and sulfur in such a way as t o collect any gases given off. Both hydrogen sulfide and sulfur dioxide were produced. Davy assumed that the hydrogen had been a part of the sulfur, and the oxygen for the sulfur dioxide he assumed to have been a part of some copper oxide originally present as impurity. Davy later conducted some very similar experiments on the combination of potassium and sulfur, and from this reaction also was able to collect some hydrogen sulfide." Berthollet, jun., whom Davy referred to, was the young son of the more famous Claude Louis Berthollet. ks a result of some experiments involving the reaction between charcoal and sulfur, Berthollet concluded that hydrogen must be a constituent of both the sulfur and the charcoal. The memoir describing this work was reviewed a t considerable length in the Anmles de chimie12 and reported on favorably by MM. Fourcroy, Deyeux, and Vauquelin, a very competent trio. In addition t o these experiments referred to by Davy, there was another investigator reporting hydrogen as a constituent of sulfur. This was F. R. Curaudau, whose first paper on this topic, "Sulfur and its decomposition," appeared in the Annales de chimie" a t about the same time. Curaudau introduced into an iron tube four parts of animal charcoal and two parts of potassium d a t e and heated strongly. The properties of the products were such as t o indicate to Curaudau that sulfur was composed of carbon and hydrogen. At the end of his paper he also promised to reveal soon the elements of phosphorus and iron and something about the alkali metals. The editors of the Annales apparently felt some reservations about the validity of Curaudau's work, for they inserted a note a t the end of the paper stating that neither the experiments nor the conclusions were guaranteed by the editors. Their reason for publishing the paper in spite of their reservations was that it dealt with a problem currently of much concern to the chemists of Europe. The memoir was also submitted to the chemical section of the Institute of France for criticism and review. The report was made by MM. Vauquelin and C. L. Berthollet and appeared in the same volume of the A n n a l e ~ . The ~ ~ critics conclude that,
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. the facts which he presented do not give anything like the decomposition of sulfur, or of its composition by the combination of hydrogen and carbon, and all the products of this operation are explained by the known nature of the substance mixed in the reaction. -
1% F o m c ~ oA. ~ ,F., N. DEYEW,AND L. N. VAOQUELIN, Ann. chim. (Paris), 61, 127 (1807). ~"UBAUDAU, F. R.,Ann. chim. (Paris),67, 72 (1808). Davy nowhere refers to the work of Curaudau, though he generally was aware of the contents of the Anmles. In new of the nature of Curaudrtu's efforts, as described below, Davy may have preferred not to mention them. 1 4 VAUQUELIN. L. N.. A m C. L. BERTHOLLET. Ann. chim. (Paris), 67, 151'(1808).
If M. Curaudau proposes to present to the class of mathematical and physical science the experiments which he announced a t the end of his memoir, that he would make known the elements of phosphorus and iron, we invite him to avail himself of the regular methods of the physical ~ciences.'~
Undaunted, Curaudau did submit the promised memoir which, however, was not printed. I t was reported on to the chemical section of the Institute by M. Deyeux." I n this memoir Curaudau reported the production of phosphorus-containing compounds without using any materials which had phosphorus in them. Curaudau's experiments were repeated in his presence and tests were made on the purity of his starting materials. As a result of the latter, it was found that the animal charcoal he had been using contained 40 per cent calcium phosphate plus a little phosphate of iron! In conclusion, M. Deyeux in his report stated, A f t w whsr has tokrn plncr, s r inn, prrwmc that 11. Curnudau a h wcogniirrs his error, will no leure altvmpt to rrrurn ru the very difl;cdf quwrims whivh he believed he ha*l rrwror airh auccess; but in any case, we invite him to employ greater rigor in the researches which he wishes to submit to the judgment of the chS8."
This severe criticism of Curaudau's sloppy and careless work was well justified, but it is significant that the editors of the Annales were willing t o use so much of their space to the discussion of it, and that the critics were willing t o spend so much time and effort to show that his work was poor. No effort was made by the critics t o belittle the idea that sulfur, phosphoms, or any other such materials might prove t o be compound. The question of what materials were elementary was obviously still an open one, but too difficult for M. Curaudau to be attempting to answer. As late as 1812, Davy was still expressing his uncertainty in the following. The term element is used as synonomous with undecompounded body; but in modern chemistry its application is limited to the results of experiments. The improvements taking place in the methods of examining bodies, are constantly changing the opinions of chemists with respect to their nature, and there is no reason to suppose that any real indestmctihle pn'miple has been yet discovered. Matter may ultimately be found to be the same in essence, differing only in the arrangements of its particles; or two or three simple substances may produce all the varieties of compound bodies. The results of our operations must be considered as offering a t best approximations only to the true howledge of things, and should never be exalted as a standard to estimate the resources of nature.'a
However, this cautious attitude did not prevent him from speculating about the ultimate constitution of matter. He offered frequent conjectures on this subject in his writings of the years between 1808 and 1812. The most elaborate of these appeared near the end of his "Elements of Chemical Philosophy." Ibid., p. 163. DEYEOX,N., Ann. chim. (Paria), 68. 94 (1808). " Ibid., p. 104. '8 DATLVY, op. eit., Vol. IV, p. 132. '6
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JOURNAL OF CHEMICAL EDUCATION
I have already hinted a t the idea that all inflammable matters may be similarly constituted, and may contain hydraen. And on this supposition they may he conceived to owe their powers both with oxygen and chlorine to the attractive of energies of their combined hydrogen."
He then listed several metals and nonmetals suggesting that each might be considered as composed of an unknown basis plus sufficient hydrogen t o just account for its combination with oxygen. For example, sulfur, whose combining weight he-took as 30, might be made Of 24 weight units Of an unknown basis and of hydrogen, the amount of hydrogen equivalent to the oxveen needed for the formation of sulfuric acid. SO.. " Though he continued by giving extensive illustrations of this idea, he concluded by cautioning that ". . .these transient views have been developed merely for the sake of pointing out a promising path of inquiry."* ~~~h of D ~ conjecturing ~ ~ was' based ~ on analogy of behavior of similar substances. His suggestion that the alkali metals might contain hydrogen &s based on the fact that volatile alkali (ammonia) was known to contain hydrogen, and thus by analogy the alkali metals might also contain it. This of course was pure speculation, and he offered no comments concerning the nature of the other ingredient. He specifically denied, however, that the alkali metals were composed of the alkalies (oxides) and hydrogen, a view held for a time by Gay-Lussac and Thenard. The only direct evidence that undecomponnded inflammables contained hydrogen was from those experiments which apparently produced hydrogen from sulfur and phosphorus. I n spite of its erroneous nature, Davy apparently never questioned the experimental validity of his own work. As late as 1818, he wrote of knowing "that minute portions of phosphoretted hydrogen were separated from phosphorus by voltaic electri~ity."~~ The presumed presence of hydrogen in sulfur and phosphorus made possible a revival of interest in a modified phlogiston theory. Davy himself played around with the idea, and while admitting its possibility, he always preferred the anti-phlogistic explanation because of ". . .its beauty and precision, [rather than from] a conviction of its permanency and truth."22
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Ibid., p. 359. I b i d , p. 360. 21 Ibid., Vol. V., p. 370. P' Ibid., p. 89. l9
I n America, however, Davy's "discovery" of hydrogen in sulfur and phosphorus was seized by two eminent chemists as complete vindication of a modified phlogiston theory and some effort was made to revive
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~h~~~ Thomson indicated an awareness of the revived idea of hydrogen as a principle of inflammability in a biographical account of Joseph Priestley pubKOLA
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I should not, for my part, he surprised to see some of the apinions of Dr. Priestley, which was looked upon as most absurdas, for example, that all combustible bodies contain a common principle of inflammability-established in a satisfactory manner.''
Also, John Gorham in 1816 when he was installed as Erving Professor of Chemistry at Harvard was still to say: Hydrogen has been discovered to constitute a part of all bodies, except the metals, and should they be proved to contain it, the doctrine of phlogiston must be renewed in a demonstrable and permanent form.'
Thus we see that the possibility that hydrogen might constitute a part of many or even all the other undecompounded bodies was very much a part of the chemical climate of the time. Against this background, Prout's hypothesis appears as a particularized form of a general idea already very much in the air. This analysis of the origins of Prout's hypothesis is not intended to detract from Prout's own originality, for there are two aspects of his treatment of the idea which are new. I n the form finally presented, hydrogen is suggested as the only simple substance, a view apparently not seriously considered before. Prout, by basing his suggestion on the integral values of the atomic weights, not only was original, but also turned the idea into the context of the atomic theory. Here it proved to be infinitely more fruitful than it ever could have been while enmeshed in the morass of the metaphysical concept of the element. These men were Samuel Lstham Mitchill of Columbia and John Redman Coxe of the University of Pennsylvania. See this author's paper, "An attempt in the United States to resolve the differences between the oxygen and phlogiston theories," Isis, 46, 327 (1955). THOMSON, THOMAS, Annals of Philosophg, 1, 93 (1813). " GORHAM, JOHN, New Engl. J. Med. Surg., 6 , 13, (1817).
SYMPOSIUM ON ANCIENT CHEMISTRY BEING PLANNED TEE Division of History of Chemistry will hold an an-day symposium on the history of ancient chemistry at the fall meeting of the American Chemical Society in Atlantic City. PreColumbian, Ancient, Near-Eastern, and Ancient European chemical developments will be discussed. The chairman of the symposium is Dr. Martin Levey, 229 West Hortter Street, Philadelphia. 19, Pennsylvania. He is affiliated with Pennsylvania. State College at Pottsville. Anyone interested in participating in this symposium should direct inquiries to Dr. Levey.
