The Discovery of Potassium and Sodium, and the Problem of the Chemical Elements

The Discovery of Potassium and Sodium, and the Problem of the Chemical ElementsAuthor(s): Robert SiegfriedSource: Isis, Vol. 54, No. 2 (Jun., 1963), pp. 247-258Published by: The University of Chicago Press on behalf of The History of Science SocietyStable URL: http://www.jstor.org/stable/228541 .

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The Discovery of Potassium and

Sodium, and the Problem of

the Chemical Elements

By Robert Siegfried *

THE publication of Lavoisier's Traite Elementaire de Chimie in 1789 marks a convenient date for the beginning of the modern era of

chemistry.1 Lavoisier began this work as an elaboration of an earlier expressed need for a new chemical nomenclature, but in the writing discovered " the impossibility of isolating the Nomenclature from the science and the science from the Nomenclature." 2 Meanwhile the systematic Methode de Nomenclature Chimique 3 had appeared in 1787, and its principles were incorporated in the Traite.

Although the new terminology reflected the oxygen-centered philosophy of Lavoisier, its real utility lay in the systematic expression of composition. Fundamental to this was the list of simple substances in terms of which the composition of all others could be expressed. This list included the total of those substances " that we have not been able to decompose by any means." 4

This definition based the assignment of simplicity on experience and

seemingly avoided the vagueness of the metaphysical elements, earth, air, fire and water. Not only was this definition adopted by most of the systematic chemical writers after Lavoisier, but the word " element" with its ancient connotation of finality was frequently avoided.

In spite of the widespread acceptance of this definition, assignment to the list of simple substances was never final. No matter how many failures to decompose a substance, there was no certainty that it was undecomposable. Newly discovered substances were subject to extended discussion and ex-

periment to determine more certainly their simple or compound nature.

* University of Illinois. This paper is based voisier, William Dawson and Sons, Ltd., and in part upon work made possible by a grant E. Weil, London, 1954. from the National Science Foundation. 2 Traite, vi.

1A. L. Lavoisier, Traite tlementaire de 3 Methode de Nomenclature Chimique, Pro- Chimie, Chez Cuchet, Paris, 1789. Specific refer- posee par MM. de Morveau, Lavoisier, Ber- ences to the Traite in this paper are to the tholet (sic) et de Fourcroy. Chez Cuchet, Paris, edition of 1793, cited by Duveen and Klickstein 1787. as the pirated edition, item 156 in A Biblio- 4 Traite, vol. XVII.

g7aphy of the Works of Antoine Laurent La-

ISIS, 1963, VOL. 54, PART 2, No. 176. 247



ROBERT SIEGFRIED

It is the purpose of this paper to illuminate the status of the concept of the chemical element during the early nineteenth century by a study of the writings about the new metals potassium and sodium. The unusual properties of these metals and the important position of the alkalies in the scheme of contemporary chemistry produced a literature particularly extensive and revealing.

In October 1807, when Humphry Davy passed an electric current through some moist potash, minute globules of potassium burst through the crust of potash and took fire. At the sight of this, Davy " bounded about the room in ecstatic delight."

5 He had good reason to be joyous, for the successful decomposition of the fixed alkalies seemed to justify his recently expressed belief that the power of the voltaic apparatus gave hope of dis-

covering " the true elements of bodies." 6 Whether the new metals were " true elements" or not, became the unspoken question in the voluminous discussion which followed their discovery. The nature of the alkalies from which the new metals were made had been a point of uncertainty ever since Lavoisier had omitted them from his list of simple substances. Though there had been much conjecture concerning their possible composition, " that metals existed in the fixed alkalies seems ... never to have been sus-

pected." 7 Some of Davy's delight must have come from surprise as well as

success. At the time of Davy's decomposition of the fixed alkalies, the generally

accepted system of chemistry was essentially that of Lavoisier. Oxygen, which had played the leading role in the overthrow of the phlogiston theory, had been made the central element in the new system. It combined with certain acidifiable elements (sulfur, phosphorus, carbon, etc.) to produce acids. Metals, to dissolve in acids, had first to be oxidized. " Oxygen is thus the bond of union between the metals and acids; and from this we are led to suppose that oxygen is contained in all substances which have a strong affinity with acids." 8 Continuing this reasoning, Lavoisier hinted that the earths, lime, magnesia, baryta, and alumina might be metallic oxides. Never- theless, he included them in the list of simple substances because they were as yet undecomposed. Rather inconsistently, he failed to include the fixed alkalies, potash and soda, in his table of simple substances even though they also were still undecomposed.9 Evidently he felt their analogy to ammonia

5 Humphry's brother John reported the story Lavoisier had anticipated Davy's discovery of from an account by their cousin Edmund Davy, the composition of the alkalies. See, for ex- who was at the time Humphry's assistant. John ample, M. M. Pattison Muir, A History of Davy (ed.), The Collected Works of Sir Hum- Chemical Theories and Laws, John Wiley and

phry Davy, Smith, Elder and Co., London, Sons, New York, 1907, p. 233; F. J. Moore, A 1839-1840, 9 volumes. Vol. I, p. 109. (Here- History of Chemistry, 3rd edition, McGraw-

after, Works.) Hill Book Co., New York, 1939, p. 98. Muir 6 Works, V, p. 54. Phil. Trans., 97, 54 (1807) . and Moore both state only that Lavoisier 7 Works, V, footnote p. 99. Phil. Trans., 98, thought the fixed alkalies to be oxides of some

42 (1808). unspecified "radical." More recently Henry 8 Traite, vol. I, p. 179. Guerlac has credited Davy with merely ful- 9 Lavoisier's inconsistency in his handling of filling " Lavoisier's prophecy" that the alkalies

the alkaline earths and the fixed alkalies has were metallic oxides. Isis, 52, 204 (1961). led more than one writer into assuming that

248



THE DISCOVERY OF POTASSIUM AND SODIUM

was better than their analogy to the earths, for he stated, "... analogy leads us to suspect that azote is one of the principal constituents of the alkalies in

general, as has been proved in regard to ammonia: but we have only slight presumptions, unconfirmed by decisive experiments, respecting the composition of potash and soda." 10

In spite of this expressed caution, he omitted the fixed alkalies from his list of elements " because they are evidently compound, though we are

ignorant as yet of the nature of the principles which enter into their combination." 'l

Systematic chemical writers after Lavoisier generally used a definition of the chemical element equivalent to that given by Lavoisier, and with it

essentially the same list of simple substances, though many included the fixed alkalies. Like Lavoisier, these chemists strongly suspected them of

being compound. This state of affairs allowed easy speculation concerning their possible composition, and many claims were made to show them to be combinations of the earths with hydrogen, with nitrogen, or with carbon. These schemes, though different in detail, were alike in being highly con-

jectural and based on fuzzy inference from complicated experiments.12 Davy, too, had his speculative thoughts on the composition of the fixed

alkalies, though he fortunately kept them to himself until after he had

actually decomposed them. According to his brother, John, Humphry's strongest suspicion had been that potash,

... might consist of phosphorus, or sulphur united to nitrogen; for, as the volatile alkali was regarded as composed of an extremely light inflammable body, hydrogen united to nitrogen, I conceived that phosphorus and sulphur, as much denser bodies, might produce denser alkaline matter; and as there were no known combinations of these with nitrogen, it was probable that there might be unknown combinations.13

But Davy brought more to the task of decomposing the fixed alkalies than the belief in their compound nature; he had the tool for success. Davy's experimentation with the chemical effects of electricity began immediately upon the publication of the electrical decomposition of water in 1800.14

By 1806 his thinking on this topic was so far advanced that he suggested the identity of chemical affinity with the electrical attraction of oppositely charged components.15 Recognizing that the power of chemical affinity is

o10 Traite, vol. I, p. 170. was able to extract a few ounces of nitrate of 11 Traite, vol. I, p. 195. potash, from which he concluded that potash 12 For one of the more bizarre, though not was a combination of lime and nitrogen. He

atypical conjectures, see J. A. Chaptal, lilemens also believed that soda was a combination of de Chymie, 3rd ed., Chez Deterville, Paris, magnesia and nitrogen. 1796, vol. I, p. 165. From the knowledge of 13 Quoted by John Davy in an editorial foot- the composition of the volatile alkali, ammonia, note in Works, V, p. 103, from a manuscript he was led to presume nitrogen as one of the lecture. .constituents of the fixed alkalies as well. As 14 W. Nicholson and A. Carlisle, Nicholson's evidence he cited the experiment in which he Journal of Natural Philosophy, Chemistry, and

exposed twenty-five pounds of washed lime the Arts, 4, 179-187 (1800). to the exhalations of putrifying beef blood for 15 Works, V, pp. 39-40. Phil. Trans., 97, eleven months. At the end of that time he 39 (1807).

249



ROBERT SIEGFRIED

limited while the "powers of our artificial instruments" are not, he ex-

pressed the hope " that the new mode of analysis may lead us to the discovery of the true elements of bodies..."6 In 1807, there was hardly a more

appropriate substance on which to test his ideas than potash. The successful results of this attempt were reported to the Royal Society on November 19, 1807, and printed in the Philosophical Transactions for 1808.17

The experiment which produced potassium for the first time was ex-

ceedingly simple. The opposite wires of a voltaic apparatus were connected to a piece of moist potash, and the new metallic substance appeared at the

negative wire. Davy quickly showed that the effect was independent of the presence of air or the composition of the wire. Gas, which proved to be oxygen, was emitted at the positive wire. No gas was noticed at the negative wire unless there was excess water. The same general results were obtained with soda.

Davy systematically determined the more obvious properties of the new metals and established not only their similarity to each other, but also their

unique differences from other substances. Of primary interest to Davy was the relation of these metals to the generally accepted system of chemical

knowledge. Davy asked, "Should the bases of potash and soda be called metals?" In spite of their low density, he decided that they should, for all their other properties were analogous to that class of bodies. With the concurrence of a number of " eminent scientific persons " he suggested that the names potassium and sodium be assigned "as implying simply the metals produced from potash and soda." 18

To determine the chemical relation of the new metals to the alkalies from which they had been obtained, Davy again depended upon analogies. He

pointed out that as in all previous electrical decomposition, the "combustible base" appeared at the negative surface, while oxygen appeared at the positive surface. This implied that the alkalies were the oxides of the new metals, an interpretation he confirmed by synthesis. When potassium was burned in oxygen, the gas was absorbed and nothing was emitted " which affected the purity of the residual air."

The alkalies produced were apparently dry . . . and their weights con- siderably exceeded those of the combustible matters consumed. ...

It appears then, that in these facts there is the same evidence for the decomposition of potash and soda into oxygen and two peculiar substances as there is for the decomposition of sulphuric and phosphoric acids and the metallic oxides into oxygen and their respective combustible bases.19

Davy's reference to sulfuric and phosphoric acids suggests his familiarity with and his emulation of Lavoisier's arguments on the nature of these acids.20

s6 Works, V, p. 54. Phil. Trans., 97, 54 (1808). (1807). 19 Works, V, pp. 63-65. Phil. Trans., 98, 7-9

17 Works, V, pp. 57-101. Phil. Trans., 98, 1-44 (1808). (1808). 20 Traite, vol. I, pp. 57 ff.

18 Works, V, p. 88. Phil. Trans., 98, 21

250




In spite of the clarity of this antiphlogistic interpretation, Davy pointed out that, " A phlogistic chemical theory might certainly be defended on the idea that the metals are compounds of certain unknown bases with the same matter as that existing in hydrogen; . . . but in this theory more unknown

principles would be assumed than in the generally received theory. It would be less elegant and less distinct." 21

Thus Davy's assignment of potassium and sodium to the list of simple substances was not based on a direct application of the definition of a simple substance. Rather he considered the new metals simple because of their

analogy with the other metals which in the "' generally received theory" were considered simple.

The knowledge of Davy's successful decomposition of the alkalies immediately created great interest and much published commentary. Although there appeared to be wide acceptance of Davy's interpretation of his results, there were many who disagreed. Among the latter group, the most important were Joseph Gay-Lussac and Louis Thenard. As early as January 1808, they concluded that, " there is no more reason to admit the composition of the alkalies than to regard them as simple bodies. It is possible to suppose that the metals obtained are only combinations of these alkalies with hydrogen." 22

The experiment upon which Gay-Lussac and Thenard based their conclusion that potassium was a hydride of potash, was not simple. They heated potassium in dry ammonia, some of which was absorbed while hydrogen was liberated. A fusible solid was also formed. When this solid was heated strongly, part of the original ammonia was regenerated, leaving a solid residue. Upon treating this residue with water, additional ammonia was obtained, the total now being equal to the quantity of ammonia originally consumed. Since there had also been some hydrogen produced, Gay-Lussac and Thenard concluded that since it could not have come from the ammonia, it must have come from the potassium.23

Davy learned of the work and views of Gay-Lussac and Thenard some time after he read his paper on the " Decomposition of the Earths" in

June 1808. Before that paper was published, he appended a long footnote in which he presented several arguments refuting the French chemists' view that potassium was a compound of potash and hydrogen.24 Ammonia has no affinity for potash by itself, said Davy; how then is it possible to conceive that the ammonia can drive out the hydrogen supposedly combined

21 Works, V, footnote pp. 89-90, Phil. Trans., 6 K + 6 NH, = 6 KNH, + 3 H, 98, 33 (1808). 6 KNH, + heat =2 K,N + 4 NH,

22 Ann. Chim. (Paris), 66, 217 (1808). Dis- 2 K,N + 2 H2O + 3 KOH = 2 NH3 satisfied with the small amounts of metal ob- Note that the six molecules of ammonia (NHa) tained by electrolysis, Gay-Lussac and Thenard used in the initial reaction are all regenerated devised the method of reducing the alkali by by the end of the sequence. There is also some iron turnings inside a strongly heated gun hydrogen produced. Gay-Lussac and Thenard barrel. This method quickly became preferred failed to take into account the hydrogen in for quantity production. the water used in the third step.

23 In modern symbols, this series of reactions 24 Works, V, pp. 134-137. Phil. Trans., 98, can be represented as follows. 365-366 (1808).

251



ROBERT SIEGFRIED

with the potash in potassium? He further pointed out that there was no evidence that the hydrogen produced came from the potassium, for all the ammonia originally consumed is not regenerated nor the potash formed

except after the addition of the water: ". .. and as the three bodies con- cerned in this experiment are potassium, ammonia, and water, the result ought to be potash, ammonia, and a quantity of hydrogen, equal to that evolved by the mere action of water on potassium, which is said to be the case." 25

Davy cited several additional examples of the chemical behavior of potassium and potash which seemed to argue against the relationship urged by Gay-Lussac and Thenard. In conclusion Davy said,

Could not the experiment of MM. Gay-Lussac and Thenard be explained, except on the supposition of the hydrogen being derived from the potassium, it would be a distinct fact in favor of the revival of the theory of phlogistion. It would not prove, however, that potassium is composed of hydrogen and potash, but that it is composed of hydrogen and an unknown basis; and that potash is this basis united to water.26

The inconsistency of the French chemists' interpretation was pointed out by Davy again after they had isolated boron and described its properties in an excellent paper in 1808.27 Gay-Lussac and Thenard reported the products of the reaction between boracic acid and potassium to be potassium borate, potash and a new combustible substance, now known as boron. The combustion of the new substances yielded boracic acid again and they correctly concluded that " boracic acid is really composed of oxygen and a combustible body ... of a particular nature, and that it can be placed along side of carbon, phosphorus, and sulfur." 28 The evidence and the arguments are quite like those of Lavoisier regarding the relationship between sulfur and phosphorus and their respective acids, as well as parallel to those used by Davy for the alkalies. Their interpretation is inconsistent, however, with the view that potassium is a hydride (hydruret) of potash. As Davy pointed out, "... the legitimate conclusion to be drawn from the process on their hypothesis, was, that they had made a hydruret of boracic acid." 29

The only other kind of evidence offered by Gay-Lussac and Thenard in support of their view of the composition of potassium is a very curious one. In a paper criticizing some of Davy's work on the nature of sulfur and phosphorus, they reacted potassium with hydrogen sulfide. The most striking result, they said, was that " it produced precisely the same quantity of hydrogen as if it had been treated by water or ammonia. This experiment may thus be cited as a new proof in favor of the existence of the hydrides." 30

25 Works, V, footnote p. 136. Phil. Trans., 98, 28 Ann. Chim. (Paris), 68, 173-174 (1808). 367 (1808). 29 Works, V, footnote p. 242. Phil. Trans.,

26 Works, V, p. 137. Phil. Trans., 98, 367 100, 33 (1810). (1808). 3 Ann. Chim. (Paris), 73, 237 (1810).

27 Ann. Chim. (Paris), 68, 169-174 (1808).

252




Shortly thereafter, Davy referred to this reasoning, but in the context of a discussion of " Dalton's law of proportions."

If such reasoning were to be adopted, as that metals are proved to be compounds of hydrogen, because in acting upon different combinations containing hydrogen, they produce the evolution of equal proportions of this gas, then it might be proved that almost any kind of matter is contained in any other. The same quantity of potash, in acting upon either muriate, sulphate, or nitrate of magnesia, will precipitate equal quantities of magnesia; but it would be absurd to infer from this, that potash contained magnesia, as one of its elements.... Any theory of metallization applicable to potash and soda, must likewise apply to the common metallic oxides.31

Here again Davy's appeal was to the analogy of the new metals with the old. Davy's arguments left Gay-Lussac and Thenard unconvinced, for it was

not until 1810 that they publicly admitted that the alkalies were oxides of the new metals. Meanwhile they made continuous corrections of various aspects of Davy's other work, which during this period was often highly speculative and based on hastily performed and unrepeated experiments.32

Gay-Lussac and Thenard were not the only chemists to disagree with Davy's conclusions regarding potassium and sodium, though he gave much more attention to their arguments than to those of the others. In the Bakerian Lecture for 1809, Davy summarized and unified the work he had been doing for the two previous years, confirmed his earlier work with new data and presented arguments against various interpretations of others. F. R. Curaudau had suggested that the new metals were composed of the alkalies united to charcoal, for upon burning them he had obtained not only the alkali but also carbonic acid.33 Davy pointed out that the charcoal was an accidental ingredient arising from Curaudau's method of preparation of the metals. The latter chemist had been the first to produce the new metals

by reduction of the alkalies with charcoal, and the presence of a little charcoal in the resulting metals could be expected. Davy could obtain no carbonic acid from the combustion of metals prepared by electrolysis or by

31 Works, V, pp. 280-281. Phil. Trans., 100, 72 (1810).

32 Though serving a useful function, most of the corrections had little to do directly with the nature of the alkalies and the new metals, and need not be described here. As a more positive contribution during this time, Gay- Lussac and Thenard could claim the preparation of boron and the first systematic report of its properties, the preparation and identifi- cation of the peroxides of potassium and sodium, and Gay-Lussac published independently his law of combining volumes. Davy, in addition to his work on the fixed alkalies, also prepared barium, strontium, and calcium, and before the end of 1810 he had read the first two papers dealing with the elementary nature of chlorine.

John Davy, editor of his brother's works, re-

ported that there was a very real personal aspect to this competition which produced so much gain for chemical knowledge. Shortly after Humphry had reported the production of potassium and sodium, he became seriously ill for several weeks. According to John Davy:

On his recovery, he found the subjects he had been investigating, seized upon by MM. Gay-Lussac, and Thenard, rather in the man- ner and feeling of contending generals in- tent on conquest, than of philosophical in- quirers, members of the common republic of science. This proceeding, it cannot be con- cealed, annoyed him at the time; especially as there was often a want of reference on their part to his previous labours: and it necessarily had the effect of hurrying on his researches . ..

Works, V, footnote p. 101. 33 Journal de physique, 66, 452-456 (1808).

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ROBERT SIEGFRIED

reduction with iron, even though he conducted very careful oxidations specifically to test Curaudau's assertion.34

Davy cited J. W. Ritter as believing that potassium and sodium were com-

pounds of hydrogen because of their extreme lightness. No argument, said

Davy, ... is more easily answered ... though soda is said to be lighter than potash, ... sodium is heavier than potassium.

On the theory which I have adopted, this circumstance is what ought to be expected. Potassium has a much stronger affinity for oxygen than sodium, and must condense it more; and the resulting higher specific gravity of the combination is a necessary consequence.35

Aside from the fact that Davy's comments are not really relevant to Ritter's suggestion, his argument is not supported by more recent measure- ments which show that the density of soda is greater than that of potash. Davy's argument is thus seen to be based on irrelevant factors. It is significant, however, that in this case and the preceding one with Curaudau, Davy did not appeal to any general principle or argue by analogy to other metals, but met ad hoc arguments with ad hoc refutations.

John Dalton also indicated that the low density of potassium "would seem to countenance the notion of its containing hydrogen." 36 Indeed Dalton's treatment of Davy's work on the alkalies was quite inconsistent. Early in the second part of volume one of his New System, Dalton appeared to accept the interpretation that the alkalies were oxides of the new metals, but in later pages he gave an extended discussion of potassium under the label of " hydruret of potash." 37 His change of views was based on the new knowledge that common potash contained hydrogen (potassium hy- droxide). Among other arguments he stated that when potash was elec- trolyzed, oxygen appeared at the positive wire, but no hydrogen appeared at the negative wire. Therefore the hydrogen must have combined with the potash to produce the potassium. Davy replied: "It is evident, that

adopting such a plan of reasoning, lead or copper might be proved to be hydrurets of their oxides; for when these metals are revived from their aqueous acid solutions, oxygen is produced at the positive surface, and no hydrogen at the negative surface." 38 Davy went on to say that when potash was subjected in quantity to large electrical currents, hydrogen was produced at the negative surface along with some hydride of potassium.

John Murray, who later resisted so strongly Davy's view on the elementary nature of chlorine, also disagreed with Davy's view on the alkali metals.

Murray considered potassium as made up of an unknown metallic base combined with hydrogen, a view earlier conjectured but not adopted by Davy. Like Dalton, Murray felt that since the hydrogen in the potash does

34 Works, V, p. 234. Phil. Trans., 100, 26 Philosophy, R. Bickerstaff, Manchester, vol. I, (1810). part 2 (1810), p. 262

35 Works, V, p. 235. Phil. Trans., 100, 26-27 37 Ibid., vol. I, part 2, pp. 484 if. (1810). 38 Works, V, footnote p. 322. Phil. Trans.,

36 John Dalton, New System of Chemical 101, 11 (1811).

254




not appear at the negative wire, it must go to make up the potassium produced. Murray also found that the oxidation product of potassium required the same amount of acid for neutralization as the usual potash and from which he concluded the two substances had the same composition. Since common potash contained hydrogen, so therefore did the potash obtained by the burning of potassium. The hydrogen could only have come from the potassium itself. Further evidence explainable by his view of the com-

position of potassium was given by Murray. When potash was reduced by iron, the resulting potassium was a little more dense than ordinary potassium, and Murray said this was due to its containing less hydrogen.39

The diverse suggestions of Curaudau, Ritter, Murray and many others not specifically mentioned here, seem unsystematic and ad hoc. Each chemist had seized upon a particular fact, alleged or real, and attempted to explain it without any attempt to relate the arguments to the generally accepted chemical system. If any one of these claims had been firmly established

experimentally, significant modification would have been forced on that

system. It should be pointed out also that Davy's refutations of each of these claims were also ad hoc, though probably because this was the easy way. Only in the case of Gay-Lussac and Thenard did he base his refutations

upon analogies to the generally accepted system of chemical knowledge. We are not justified in dismissing the views of these men as the petty

concerns of second-rate chemists, for Dalton, Davy, Gay-Lussac and Thenard were all willing to consider the possibility that potassium and sodium were

compound and not simple elements. Since the new substances were generally considered to be metals, any suggestions that they were compound forced a similar consideration for all metals, and the whole structure of the Lavoisier

system would have been placed in doubt. From the evidence, the conclusion that the system was in doubt seems inescapable. The difficulty lay in the

uncertainty of establishing a list of elements whose simple nature could be confidently accepted. There was no clearly recognized principle by which the simplicity of a substance could be determined.

In retrospect, the principle was already at hand in the conservation of

weight. This principle had long been used by chemists in quantitative analytical procedures and was well known both to Gay-Lussac and Thenard and to Davy. If Davy's view on potash and potassium were correct, potash is necessarily a compound and therefore should weigh more than the potassium from which it was made. The known weights were in accord with this as Davy well knew. On one occasion he even stated that, in decomposing the alkalies and the earths, " something has been separated from them which adds to their weight; and whether it can be considered as oxygen, or as

water, the inflammable body is less compounded, than the uninflammable substance resulting from its combustion." 40 In all the dozens of pages Davy wrote in discussion of the nature of the alkalies and the earths, this is the

only time he explicitly connected a weight relationship with his interpre-

39 Nicholson's Journal of Natural Philosophy, 4o Works, V, p. 135. Phil. Trans., 98, 366-

Chemistry, and the Arts, 28, 245 (1811). 367 (1808).

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ROBERT SIEGFRIED

tation. Though this knowledge undoubtedly contributed to his belief in the composition of alkalies and the earths, it is also clear that the weight relations did not seem to him the fundamental argument.

In 1810, Gay-Lussac and Thenard first announced a preference for the view that the new metals were simple bodies.41 This change of view had not come from any arguments of Davy, but was based on their own experiments with the peroxides of these metals whose discovery they reported. They treated the peroxides with carbon dioxide and with sulfur dioxide and obtained the dry salts along with some oxygen. They observed,

That there is not the slightest trace of moisture released in any case, and that the weights of products obtained correspond precisely to those of the oxide employed and the acid absorbed. Now as in the combustion of potassium and of sodium there is nothing released nor any volatile product formed, it is seen that if these metals are hydrides, it is necessary that the sulfates and carbonates of potash and soda, and without doubt all the salts which have these two alkalies for their bases, contain as much water as these hydrides are able to form in combining with oxygen, and that they retain it to a very high temperature; this is possible but it has not been proved up to now.42

Since they did not report the actual weights referred to, and the oxygen released in the reactions is not mentioned in the " precise correspondence " of weights of products and reactants, the weight argument is unconvincing. Here, as in the work of Davy, the appeal to weights is used to confirm rather than establish an interpretation.

Davy's comment on this paper further illustrates his continued emphasis on analogy. "MM. Gay-Lussac and Thenard have convinced themselves that potassium and sodium are not hydrurets of potash and soda, by a method similar to that which I adopted and published some months before, namely, by producing neutral salts from them." 43

But these two men were apparently not as convinced as Davy had thought. In 1811, they published their famous Recherches Physico-Chimiques in which they included a detailed " Discussion on the Nature of Potassium and Sodium." 44 With great care they recounted all the arguments on both sides of the question before expressing the opinion that they were " per- suaded with M. Davy, that potassium and sodium are particular metals." 45

As before, their opinion was based principally upon the behavior of the oxides with acids. They presented arguments for both hypotheses so " one

might choose that which appears the more probable." 46

The omission of the conservation of weight principle from the dispute over the alkali metals is surely one of the significant observations to be made from this study. It has been widely accepted that the enunciation of this principle for chemistry by Lavoisier provided an axiomatic basis

41 Ann. Chim. (Paris), 75, 95 (1810). 44 Recherches Physico-Chimiques, Chez Deter- 42 Ann. Chin. (Paris), 75, 93 (1810). ville, Paris, 1811, vol. II, pp. 215-264. 43 Works, V, footnote p. 323. Phil. Trans., 45 Ibid., vol. II, p. 257.

101, 11 (1811). 46 Ibid., vol. I, Introduction, xiv.

256




for quantitative chemistry. It is not intended here to discount the general validity of this view, but to point out that the significance of this principle is much clearer to us in the mid-twentieth century than it was to the chemists of the early nineteenth. Some of the specific difficulties of that earlier time should be described.

In order for the principle to be applied to potassium and its alkali, it was necessary to have shown that the weight of the potassium plus the weight of the oxygen absorbed in its combustion, was just equal to the weight of the alkali produced. Being unable to obtain a reliable weighing for the product, Davy based his analysis of it on the assumption that these weights were equal.

Since Gay-Lussac and Thenard had never combined hydrogen directly to potash, the principle could not be directly or reliably applied as a test for their hypothesis. When it was established in 1808 that ordinary potash contained water,47 Davy, Gay-Lussac and Thenard no longer always meant the same thing by the term potash, and the focus of their arguments became further confused.

Even had anyone attempted to perform a crucial weight-based experiment, it would not have answered finally the question of the simplicity of potassium. That metal might have contained hydrogen united to an unknown base, even as Davy had suggested, with potash being that base united to water. Such an idea cannot be tested by appeal to the conservation of weight, for this principle can be meaningfully applied only if all the reactants and all the products of a chemical change are separately identified and weighed. A direct test of the simplicity of potassium, or any other substance, could only be based on repeated failure to decompose it. Such negative evidence could be conclusive only with time, and there had not yet been time enough in 1810.

Meanwhile the only remaining argument for the simplicity of potassium was its analogy with the other metals which were already classified as simple in the generally accepted system. This, of course, is exactly the sort of argument Davy used from the very beginning of his work on the new metals.

The generally accepted system was not very firmly established, as evidence already presented in this paper suggests. Davy preferred the antiphlogistic system because of " a sense of its beauty and precision

" rather than from " a conviction of its permanency and truth." But " in the present state of our knowledge," he added, " it appears the best approximation that has been made to a perfect logic of chemistry." 48

Davy's extensive discussions of a possible revival of a modified phlogiston theory with hydrogen as the principle of combustion and metallization attest his belief that "the mature time for a complete generalization of chemical facts is yet far distant." 49 To him the essence of the antiphlogistic

47 J.P. J. d'Arcet, Ann. Chim. (Paris), 78, (1808). 175-190 (1808). 49 Works, V, p. 89. Phil. Trans., 98, 32

48 Works, V, p. 89. Phil. Trans., 98, 33 (1808).

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258 ROBERT SIEGFRIED

system was its use of the principle that " every body which was not yet decompounded, should be considered as simple." This " truly philosophical principle . . . was of the greatest use in promoting the progress of the science." 50 The new system and its nomenclature, which Lavoisier found

inseparable from the system itself, thus depended on the establishment of the list of elements; and the list of elements depended upon an appeal to the

system. For chemists of this time it was indeed " a period of perplexities." 51

50 Works, IV, p. 31. Elements of Chemical Press, London, 1930. Additional evidence sup- Philosophy, J. Johnson and Co., London, 1812, porting the view that the uncertainties con-

p. 44. cerning the chemical elements were a major 51 This phrase is used by J. C. Gregory as the problem of this time, will be presented in a

title of Chapter IV in The Scientific Achieve- subsequent paper. ments of Sir Humphry Davy, Oxford University



Documents

The Discovery of Potassium and Sodium, and the Problem of the Chemical Elements