Whatever Davy’s manner might have been, it was not allowed to affect the admiration felt for his genius, and on December 13th, 1813, he was with practical unanimity elected a Corresponding Member of the First Class of the Institute.
During the last week of the preceding November Ampère had given Davy a small quantity of a substance which he had obtained from Clement, and which had been discovered by Courtois, a soap-boiler and manufacturer of saltpetre in Paris, in kelp or the ashes of sea-weeds. The substance had the extraordinary property of giving a violet-coloured vapour, but its true nature and relations were unknown, and it was commonly designated as X. Although actually known for some time previously, the first public notice of its existence was made by Clement at a meeting of the Institute on November 29th, 1813, and at the meeting on December 6th Gay Lussac presented a short note on the substance, to which he gave the name iode, and stated that it had analogies to chlorine. A week later—that is, on the day of Davy’s election to the Institute—a letter from him to Cuvier was read, in which he gave a general view of the chemical characters of the body; and on January 20th, 1814, a paper by him, dated Paris, December 10th, 1813, and entitled “Some Experiments and Observations on a new Substance which becomes a violet-coloured Gas by Heat,” was read to the Royal Society.
After reciting the above facts he explains why he has ventured to take up a subject on which Gay Lussac was still engaged. The explanation was no doubt necessary; he had evidently not forgotten Gay Lussac’s intrusion into his own field of work on the occasion of the discovery of the metals of the alkalis. He first draws attention to the peculiarities of the combination of the new substance with silver; this, he shows, is markedly different from silver chloride. He then forms this compound synthetically; forms also the combination with potassium by direct union, and describes its properties; studies the action of chlorine on the new substance, and notes the formation of the yellow solid chloride and the mode of its decomposition by water; prepares a number of metallic compounds; studies the action of the new substance on phosphorus, the nature of the product, and its mode of decomposition by water, with formation of the white crystalline phosphonium iodide and hydriodic acid gas. By acting on this gas with potassium he shows that it yields half its volume of hydrogen and forms the same product as by the direct union of the alkali metal with the new substance. He further finds that this gas is formed when the new substance and hydrogen are passed through a heated tube; it has a very strong attraction for water, which dissolves it to a large extent, and the concentrated solution rapidly becomes tawny. When the new substance is treated with potash solution it forms the same product as by its direct union with potassium, together with a salt precisely similar to potassium hyper-oxymuriate, and which, like that salt, is decomposed when heated, with evolution of oxygen. He shows that the new substance is expelled from its compounds when these are heated with chlorine. He studies the nature of the black fulminating compound discovered by Desormes and Clement by acting on the new substance with solution of ammonia, and concludes that it is analogous to the detonating oil of Dulong. He attempts to determine the combining proportion of the new substance, on the assumption that its compounds are analogous to those of chlorine, but he has to admit that his experiments have been made upon quantities too small to afford exact results. Nevertheless they prove that the value is much higher than those of the simple inflammable bodies, and higher even than those of most of the metals. He further shows that the combination with hydrogen must be one of the heaviest elastic fluids existing.
“From all the facts that have been stated, there is every reason to consider this new substance as an undecompounded body. In its specific gravity, lustre, colour, and the high number in which it enters into combination, it resembles the metals; but in all its chemical agencies it is more analogous to oxygen and chlorine; it is a non-conductor of electricity, and possesses, like these bodies, the negative electrical energy with respect to metals, inflammable and alkaline substances, and hence when combined with these substances in aqueous solution and electrized in the voltaic circuit, it separates at the positive surface; but it has a positive energy with respect to chlorine.... It agrees with chlorine and fluorine in forming acids with hydrogen.
“The name ione has been proposed in France for this new substance from its colour in the gaseous state, from ῐον, viola; and its combination with hydrogen has been named hydroionic acid. The name ione, in English, would lead to confusion, for its compounds would be called ionic and ionian. By terming it iodine, from ἱώδης violaceous, this confusion will be avoided, and the name will be more analogous to chlorine and fluorine.”
The rapidity with which Davy ascertained the properties and relations of the new substance was characteristic of him. A fortnight’s work—done partly at his hotel and partly in the laboratory of the young Chevreul, amidst a succession of interruptions caused by fêtes, levées, and visits of ceremony—sufficed to accumulate the material for his Royal Society paper, in which he gives with unerring precision, in spite of the small quantity of the matter at his disposal, the broad outlines of the chemistry of iodine. The paper shows him at his best: he seems to have seized, as if by instinct, upon the central fact of the analogy of iodine to chlorine, and he worked out the clue with a perspicacity and insight worthy of his genius.
As may be surmised, Davy’s action hardly contributed to his popularity with a certain section of the savants of Paris. Gay Lussac and Thenard were extremely angry with Ampère and Clement for having given him the material for his investigation, and the feeling broke out after the publication of Gay Lussac’s memoir in the Annales de Chimie in 1814. Davy in a note published in the Journal of the Royal Institution says:—
“Who had most share in developing the chemical history of that body [iodine], must be determined by a review of the papers that have been published upon it, and by an examination of their respective dates. When M. Clement showed Iodine to me, he believed that the hydriodic acid was muriatic acid; and M. Gay Lussac, after his early experiments, made originally with M. Clement, formed the same opinion, and maintained it, when I first stated to him my belief that it was a new and peculiar acid, and that Iodine was a substance analogous in its chemical relations to Chlorine.”
Davy left Paris towards the end of December, passing into Auvergne and thence to Montpellier, where he resumed his work on iodine. He then went to Genoa, where he made some experiments on the electricity of the torpedo, and about the middle of March arrived at Florence. In a letter to his brother John he says:—
“I have worked a good deal on iodine and a little on the torpedo. Iodine had been in embryo for two years. I came to Paris; Clement requested me to examine it, and he believed that it was a compound, affording muriatic acid. I worked upon it for some time, and determined that it was a new body, and that it afforded a peculiar acid by combining with hydrogen, and this I mentioned to Gay Lussac, Ampère, and other chemists. The first immediately ‘took the word of the Lord out of the mouth of His servant,’ and treated this subject as he had treated potassium and boron. The paper which I sent to the Royal Society on iodine I wrote with Clement’s approbation and a note published in the ‘Journal de Physique’ will vindicate my priority. I have just got ready for the Royal Society a second paper on this fourth supporter of combustion.
“The old theory is nearly abandoned in France. Berthollet, with much candour, has decided in favour of chlorine. I know no chemist but Thenard who upholds it at Paris, and he upholds it feebly, and by this time, probably, has renounced it.
“I doubt if the organ of the torpedo is analogous to the pile of Volta. I have not been able to gain any chemical effects by the shock sent through water; but I tried on small and not very active animals. I shall resume the inquiry at Naples, where I hope to be about the middle of May. In my journey I met with no difficulties of any kind, and received every attention from the scientific men of Paris, and the most liberal permission to go where I pleased from the government.
“I lived very much with Berthollet, Cuvier, Chaptal, Vauquelin, Humboldt, Morveau, Clement, Chevreul, and Gay Lussac. They were all kind and attentive to me; and, except for Gay Lussac’s last turn of publishing without acknowledgement what he had first learnt from me, I should have had nothing to complain of; but who can control self-love?
“It ought not to interfere with truth and justice; but I will not moralise nor complain. Iodine is as useful an ally to me as I could have found at home.”
At Florence he worked in the laboratory of the Accademia del Cimento on iodine and on the diamond. The results of his work on iodine he embodied in a paper read to the Royal Society on June 16th, 1814, which deals mainly with the iodates, or, as he preferred to call them, the oxyiodes. The object of his work on the diamond was to determine whether any peculiar matter separated from it during its combustion, and whether the gas formed in the process was precisely the same in its chemical nature as that produced by the combustion of plumbago and charcoal. At Florence he made use of the great burning-glass originally employed in the trials on the action of solar heat on the diamond instituted by Cosmo III., Grand Duke of Tuscany; he completed the research in the laboratory of the Accademia dei Lincei at Rome.