The rivalry between the French and English chemists continued, but it took a new departure. Gay Lussac and Thenard had stolen a march on Davy by their discovery of a chemical method of making the metals of the alkalis, whereby they were able to use these metals as chemical reagents to greater advantage; but the tables were quickly turned. On July 12th, 1810, Davy read to the Royal Society his memorable paper “On the oxymuriatic Acid, its Nature and Combinations; and on the Elements of the muriatic Acid; with some Experiments on Sulphur and Phosphorus, made in the Laboratory of the Royal Institution.” This paper, in which he first demonstrates the nature of chlorine, is very short—only some twenty-six quarto pages—but it is unquestionably one of the most brilliant, as it is one of the most forcible of his productions.

Davy is here seen at his best. He is bold and yet wary, and as dexterous as trenchant; so confident is he in the strength of his position that he casts aside every argument that might tell in his favour, unless it is based on the most unimpeachable evidence. It is difficult to know what to admire most—the clearness of perception, the precision of the statement, the strictness of the logic, the aptness of the illustration, or the argumentative skill with which the whole is marshalled and presented. As a piece of induction, the memoir is a model of its kind, and as an exercise in “the scientific use of the imagination” it has few equals. Most scientific papers will stand a considerable amount of winnowing, and there is no assay-master more scrupulously strict than Time. “The more a science advances, the more it becomes concentrated in little books,” says Leibnitz; but the most fastidious of critics might read and re-read this work without wishing to omit or amend a sentence.

Every chemical student to-day is told that the elementary nature of chlorine was first demonstrated by Davy, and if the student is informed what Davy meant by the term “element,” the statement is not incorrect. What, however, Davy actually did was to demonstrate that the substance called oxymuriatic acid contained no oxygen; that it was a peculiar substance which “has not as yet been decompounded,” and therefore is “elementary as far as our knowledge extends.” The very character of the name which he suggested indicates this cautious and philosophical view. In making the suggestion, he says:—

“To call a body which is not known to contain oxygen and which cannot contain muriatic acid, oxymuriatic acid, is contrary to the principles of that nomenclature in which it is adopted; and an alteration of it seems necessary to assist the progress of discussion, and to diffuse just ideas on the subject. If the great discoverer of this substance [Scheele, who first observed it in 1774] had signified it by any simple name, it would have been proper to have recurred to it; but, dephlogisticated marine acid is a term which can hardly be adopted in the present advanced era of the science.

“After consulting some of the most eminent chemical philosophers in this country, it has been judged most proper to suggest a name founded upon one of its obvious and characteristic properties—its colour, and to call it chlorine, or chloric gas.H

“Should it hereafter be discovered to be compound, and even to contain oxygen, this name can imply no error, and cannot necessarily require a change.”

As the actual facts and arguments on which Davy based his views are seldom set forth in text-books, or presented to the student by teachers, it may be desirable to give a detailed account of his famous memoir. He begins by saying:—

“The illustrious discoverer of the oxymuriatic acid considered it as muriatic acid freed from hydrogen; and the common muriatic acid as a compound of hydrogen and oxymuriatic acid; and on this theory he denominated oxymuriatic acid dephlogisticated muriatic acid.

“M. Berthollet, a few years after the discovery of Scheele, made a number of important and curious experiments on this body; from which he concluded that it was composed of muriatic acid and oxygen; and this idea for nearly twenty years has been almost universally adopted.”

Having thus accurately stated the position, he proceeds to attack it. In the first place, he points out that Henry, ten years before, had shown that hydrogen could be produced from muriatic acid gas by the agency of electricity; this hydrogen was assumed by Henry to be due to water contained in the gas. Davy, in his Bakerian lecture of 1808, had shown that muriatic acid gas gave hydrogen when treated with potassium, and he had stated “that muriatic acid can in no instance be procured from oxymuriatic gas, or from dry muriates, unless water or its elements be present.”

Gay Lussac and Thenard had concluded “that muriatic acid gas contains about one-quarter of its weight of water; and that oxymuriatic acid is not decomposable by any substances but hydrogen, or such as can form triple combinations with it.”

He then points out, what he had already stated in a former paper, that charcoal freed from hydrogen and moisture by intense ignition in vacuo may be heated to whiteness by the voltaic battery in oxymuriatic or muriatic acid gases without affecting any change in them.

It now occurred to him that if the liquor of Libavius (stannic chloride) is a combination of muriatic acid and oxide of tin, as then surmised, oxide of tin ought to be separated from it by means of ammonia. On admitting ammonia gas to the tin chloride over mercury, the substances combined with great heat, a white solid was obtained; “some of it was heated to ascertain if it contained oxide of tin, but the whole volatilised, producing dense pungent fumes.” The experiment was repeated with every care, but no oxide of tin could be obtained.

He was next led to study the behaviour of ammonia with the substances he had formerly obtained, by the action of oxymuriatic gas on phosphorus (see p. 129). One of these is solid, and is now known as phosphorus pentachloride; the other is liquid, and is termed phosphorus trichloride.

“The first,” he says, “on the generally received theory of the nature of oxymuriatic acid, must be considered as a compound of muriatic acid and phosphoric acid. It occurred to me that if the acids of phosphorus really existed in these combinations, it would not be difficult to obtain them, and thus to gain proof of the existence of oxygen in oxymuriatic acid.”

He therefore brought ammonia gas into contact with the solid compound of oxymuriatic acid and phosphorus. Much heat was produced, and a white opaque powder was formed.

“Supposing that this substance was composed of the dry muriate and phosphate of ammonia; as muriate of ammonia is very volatile, and as ammonia is driven off from phosphoric acid, by a heat below redness I conceived that by igniting the product obtained I should procure phosphoric acid ... but found to my great surprise that it was not at all volatile nor decomposable at this degree of heat, and that it gave off no gaseous matter. The circumstance that a substance composed principally of oxymuriatic acid and ammonia should resist decomposition or change at so high a temperature induced me to pay particular attention to the properties of this new body.”

What he actually obtained was mainly a mixture of the so-called phospham and chlorophosphamide, remarkably stable substances, the characteristic properties of which he describes with accuracy. He then examined the action of ammonia gas on sulphur chloride, “the sulphuretted muriatic liquor of Dr. Thomson,” but as the compounds formed

“did not present the same uniform and interesting properties as that from the phosphoric sublimate, I did not examine them minutely: I contented myself by ascertaining that no substance known to contain oxygen could be procured from oxymuriatic acid in this mode of operation.”

He then shows that ammonia and oxymuriatic acid, in condensing to sal ammoniac with liberation of nitrogen, contrary to the general belief, form no water. According to Cruickshank, who appears to have been the first to make the observation, “hydrogenous gas” required rather more than its own volume of oxygenated muriatic acid to saturate it when a mixture of the two was exploded by means of the electric spark, “the products being water and muriatic acid.” Gay Lussac and Thenard had stated that no water was thus formed.

“I have attempted,” says Davy, “to make the experiment still more refined by drying the oxymuriatic acid and the hydrogen by introducing them into vessels containing muriate of lime [calcium chloride] and by suffering them to combine at common temperatures; but I have never been able to avoid a slight condensation; though in proportion as the gases were free from oxygen or water, this condensation diminished.I

“MM. Gay Lussac and Thenard have proved by a copious collection of instances, that in the usual cases where oxygen is procured from oxymuriatic acid, water is always present, and muriatic acid gas is formed; now as it is shewn that oxymuriatic acid gas is converted into muriatic acid gas by combining with hydrogen, it is scarcely possible to avoid the conclusion, that the oxygen is derived from the decomposition of the water, and consequently that the idea of the existence of water in muriatic acid gas, is hypothetical, depending upon an assumption which has not yet been proved—the existence of oxygen in oxymuriatic acid gas.

“MM. Gay Lussac and Thenard indeed have stated an experiment, which they consider as proving that muriatic acid gas contains one-quarter of its weight of combined water. They passed this gas over litharge, and obtained so much water; but it is obvious, that in this case, they formed the same compound as that produced by the action of oxymuriatic acid on lead; and in this process the muriatic acid must lose its hydrogen and the lead its oxygen; which of course would form water; these able chemists, indeed, from the conclusion of their memoir, seem aware, that such an explanation may be given, for they say, that the oxymuriatic acid may be considered as a simple body.”

He then repeats the experiments which first led him to suspect the existence of combined water in muriatic acid.

“When mercury is made to act upon 1 volume of muriatic acid gas, by voltaic electricity, all the acid disappears, calomel is formed, and about ·5 of hydrogen evolved.”

The same result is obtained by the use of potassium.

“And in some experiments made very carefully by my brother, Mr. John Davy, on the decomposition of muriatic acid gas, by heated tin and zinc, hydrogen, equal to about half its volume, was disengaged, and metallic muriates, the same as those produced by the combustion of tin and zinc in oxymuriatic gas, resulted.”

“It is evident from this series of observations, that Scheele’s view (though obscured by terms derived from a vague and unfounded general theory) of the nature of the oxymuriatic and muriatic acids, may be considered as an expression of facts; whilst the view adopted by the French school of chemistry, and which, till it is minutely examined, appears so beautiful and satisfactory rests in the present state of our knowledge upon hypothetical grounds.”

He then proceeds to explain the action of water upon the chlorides of tin, and phosphorus; and shows that it is by the decomposition of the water that the hydrogen is furnished to the oxymuriatic acid, and the oxygen to the tin and phosphorus.

“The vivid combustion of bodies in oxymuriatic acid gas, at first view, appears a reason why oxygen should be admitted in it; but heat and light are merely results of the intense agency of combination. Sulphur and metals, alkaline earths and acids become ignited during their mutual agency; and such an effect might be expected in an operation so rapid as that of oxymuriatic acid upon metals and inflammable bodies.”

“That the quantity of hydrogen evolved during the decomposition of muriatic acid gas by metals, is the same that would be produced during the decomposition of water by the same bodies, appears, at first view, an evidence in favour of the existence of water in muriatic acid gas; but as there is only one known combination of hydrogen with oxymuriatic acid, one quantity must always be separated. Hydrogen is disengaged from its oxymuriatic combination by a metal, in the same manner as one metal is disengaged by another from similar combinations.”

He once more shows that by the strongest analytical power he can command oxymuriatic acid fails to yield any substance differing from itself:

“I have caused strong explosions from an electrical jar, to pass through oxymuriatic gas, by means of points of platina, for several hours in succession; but it seemed not to undergo the slightest change.”

Such, then, are the reasons which induced Davy to consider that oxymuriatic acid contains no oxygen; that it had hitherto been “undecompounded,” and that, therefore, by the strict logic of chemistry, it was to be regarded as an elementary body. Had his paper concluded at this point, his position would have been unassailable, even in the light of nearly ninety years of subsequent work. But he could not stop here. Berthollet, the author of the prevailing theory, had discovered a salt then known as hyper-oxymuriate of potash, presumably capable of furnishing an acid termed by Chenevix hyper-oxygenised muriatic acid. This salt is now termed potassium chlorate, after the acid which Davy subsequently succeeded in isolating, and which, when the chlorine theory was generally accepted, was called chloric acid by Gay Lussac. The existence of the hyper-oxymuriate of potash was for a time a stumbling-block, and Davy sought to explain it on the assumption that it was nothing more than a triple compound of oxymuriatic acid, potassium, and oxygen.

“We have no right to assume the existence of any peculiar acid in it, or of a considerable portion of combined water; and it is perhaps more conformable to the analogy of chemistry to suppose the large quantity of oxygen combined with the potassium, which we know has an intense affinity for oxygen, and which from some experiments, I am inclined to believe, is capable of combining directly with more oxygen than exists in potash, than with the oxymuriatic acid which, as far as is known, has no affinity for that substance.”

It is perfectly true, as Davy surmised, that potassium can combine with more oxygen than is contained in potash, but it is no less true, as he himself proved by his discovery of the so-called euchlorine, that chlorine can combine with oxygen. Although he made several attempts to isolate Mr. Chenevix’s hyper-oxygenised muriatic acid, he was not successful at the time, and was evidently disposed to doubt its separate existence.

The remaining portion of the paper, although of interest as exemplifying Davy’s power of dealing with the broad issues which his views raise, need not detain us now. He seizes the opportunity, however, to correct his statements with regard to the presumed compound nature of sulphur and phosphorus, and gives details of observations, some of which, as in other of his papers, have been “discovered” by subsequent observers. Thus he states:—

“I have never been able to burn sulphur in oxygen without forming sulphuric acid in small quantities; but in several experiments I have obtained from 92 to 98 parts of sulphurous acid from 100 of oxygen in volume; from which I am inclined to believe that sulphurous acid consists of sulphur dissolved in an equal volume of oxygen.”

It was hardly to be expected that views so entirely opposed to the convictions of chemists at the time should pass unchallenged. Berzelius, the countryman of Scheele, warmly defended the doctrine of the French School, and yet another Scotch professor sought to show that Davy was still “vera troublesome.” The controversy, in which Davy himself took little part, occasioned considerable stir at the period, and was even of interest outside philosophical circles. The discussion was not without its uses, inasmuch as it led to fresh discoveries. The noise of it all, however, is now forgotten. Berzelius eventually enjoined his cook to speak no longer of oxymuriatic acid: “Thou must call it chlorine, Anna; that is better.” Dr. Murray, with the pertinacity of his race, still clung to the old doctrine, and defended it with no little dialectical subtlety, but he alone was faithful among the faithless. It is true there has been an occasional flutter in the dovecots since these times, and the faith of chemists in the validity of Davy’s teaching has been once or twice assailed, but as yet it has survived all assaults.

The Royal Institution possesses a book which no lover of science can regard with other than reverential interest. It is a small, well-bound quarto of some 386 manuscript pages, of notes taken by Michael Faraday, when a bookbinder’s apprentice, of the last of Davy’s lectures at the Institution. A Mr. Dance—his name deserves to be held in remembrance—had given the youth a ticket for the lectures, and Faraday, perched in the gallery over the clock, had zealously followed the expositions of the brilliant lecturer, and had subsequently, when asking for an engagement at the Institution, sent in these notes, neatly written out and embellished with drawings of the apparatus, to the Professor as evidence of the applicant’s “knowledge, diligence and order.” Among the lectures is one on chlorine, given on March 14th, 1812, the notes of which are as characteristic of the auditor as of the lecturer. We read:—

“Accustomed for years to consider the chemical principles of the French School of Physical Sciences as correct, I had adopted them and put faith in them until they became prejudices, and I even felt unwilling to give them up when my judgment was fully convinced by experiment that they were erroneous. I know that this is the case in some degree with almost every person; he is unwilling to believe that he is wrong and therefore feels averse to adopt what is right when it opposes his principles.”

Then follows an account of various experiments showing the properties of chlorine, and the proofs that it contains no oxygen:—

“Oxygen does combine with chlorine. I have ventured to name the compound euchlorine; it is of a very bright yellow-green colour. Names should represent things not opinions for in the last case they often tend to misrepresent and mislead.

“Had Mr. Berthollet obtained oxygen from chlorine there would have been no error in his theory, but by not attending to the minute circumstances of his experiment, by not ascertaining that the water present acted no part and was not decomposed he fell into an error, and of course all the conclusions he drew were false and erroneous. Nothing should be allowed but what can be proved by experiment, and nothing should be taken for granted upon analogy or supposition.”

Faraday concludes as follows:—

“Mr. Davy now proceeded to comment and make observations on the former theory of chlorine gas. Here I was unable to follow him. The plan which I pursue in taking of notes is convenient and self-sufficient with respect to the theoretical and also the practical part of the lecture, but for the embellishments and ornaments of it it will not answer. Mr. Davy’s language at those times is so superior (and indeed throughout the whole course of the lecture) that then I am infinitely below him, and am incapable of following him even in an humble style. Therefore I shall not attempt it; it will be sufficient to give a kind of contents of it. He said that hypotheses should not be considered as facts and built upon accordingly. Nevertheless, if cautiously pursued, they might lead to mature fruit. That nothing should be taken for granted unless proved. By considering oxygen as contained in chlorine the whole chemical world had been wrapped in error respecting that body for more than one-third of a century.

“He noticed that all the truly great scientific men were possessed of great humility and diffidence of their own opinions and powers. He spoke of Scheele, the discoverer of chlorine; observed that he possessed a truly philosophical spirit, gave up his opinions when he supposed them to be erroneous, and without hesitation or reluctance adopted those of others which he considered more correct; admired his spirit and recommended it to all philosophers; compared it to corn, which looked but simple and insignificant in blossom, and asked for little praise, yet was the support of man.”

In his fifth Bakerian lecture, “On some of the Combinations of Oxymuriatic Gas and Oxygene, and on the chemical Relations of these Principles to inflammable Bodies,” read before the Royal Society on November 15th, 1810, he still further developed his ideas respecting the nature of chlorine. Gay Lussac and Thenard, who had convinced themselves that potassium and sodium are not hydrates of potash and soda, had made known the fact that potassium can combine with oxygen in more than one proportion; and Davy had confirmed their conclusion, seeing in it a further proof of his views concerning the constitution of the hyper-oxymuriate of potash. He then studied the behaviour of a large number of the metals and their oxides with chlorine, making in many cases quantitative determinations, from which very fair approximations to the combining proportions or atomic weights of the substances may be deduced. Thus, he says “the number representing the proportion in which mercury combines must be about 200,” and that “the quantity of chlorine in corrosive sublimate is exactly double that in calomel, and that the orange oxide contains twice as much oxygen as the black, the mercury being considered the same in all.” The atomic weight of silver deducible from the amount of chlorine taken up by that metal during its conversion into horn-silver is almost exactly the value obtained by the most rigorous analyses of modern times. It is, however, noteworthy that in this paper Davy is brought into sharp conflict with Dalton, and there is a characteristic exhibition of temper in the way in which he protests against the manner in which Dalton had sought to use certain of his numerical estimations in deducing the weights of atoms. The comparative merits of Mr. Higgins and John Dalton as the real authors of the explanation of the laws of chemical combination have now been fully and finally assessed, but it was wholly unnecessary for the purpose of Davy’s contention to underrate the originality of the Manchester chemist. Dalton was no doubt wrong in the assumption that 47 represented the weight of the atom of nitrogen, and Davy was right in pointing out the invalidity of the basis on which this assumption rested, and in his statement that 13·4 more nearly represented the smallest proportion in which nitrogen is known to combine. Davy says:—

“I shall enter no further at present into an examination of the opinions, results, and conclusions of my learned friend; I am however obliged to dissent from most of them, and to protest against the interpretations that he has been pleased to make of my experiments; and I trust to his judgment and candour for a correction of his views.

“It is impossible not to admire the ingenuity and talent with which Mr. Dalton has arranged, combined, weighed, measured, and figured his atoms; but it is not, I conceive, on any speculations upon the ultimate particles of matter, that the true theory of definite proportions must ultimately rest. It has a surer basis in the mutual decomposition of the neutral salts, observed by Richter and Guyton de Morveau, in the mutual decompositions of the compounds of hydrogen and nitrogen, of nitrogen and oxygen, of water and the oxymuriatic compounds; in the multiples of oxygen in the nitrous compounds; and those of acids in salts, observed by Drs. Wollaston and Thomson; and above all, in the decompositions by the Voltaic apparatus, where oxygen and hydrogen, oxygen and inflammable bodies, acids and alkalies, &c., must separate in uniform ratios.”

It has been alleged that Davy in thus expressing himself offered a kind of factious opposition to the views of Dalton. In so far as they were atomic, this is possibly true, for Davy never brought himself to regard the fact of chemical combination occurring in definite proportions as admitting of the simple mechanical explanation of Dalton, which he considered too speculative. That, however, he did ample justice to Dalton’s merits ultimately will be seen from the terms in which he speaks of them on the occasion of the award to Dalton in 1826 of the first of the Royal medals. In one of his unfinished Dialogues, written shortly before his death, “On the Powers which act upon Matter and produce Chemical Changes,” he thus expresses himself:—

“The atomic doctrine, or theory, has been embraced by several modern chemists; but the development of it is owing to Mr. Dalton who seems to have been the first person to generalize the facts, of chemistry relating to definite proportions.... Mr. W. Higgins appears to have had only some loose idea of particles combining with particles, without any profound views of the quantity being unalterable; and there is good reason for thinking that these ideas, as he expresses them, were gained from another source, Dr. Bryan Higgins, who many years before supported the notion, that chemical substances were formed of molecules, either simple or compound, surrounded by an atmosphere of heat; and his views, though not developed with precision, approached nearer to those of Mr. Dalton, than those of his cousin. But neither of these gentlemen attempted any statical expressions; and to Richter and Dalton belongs the exclusive merit of having made the doctrine practicable. As a theoretical view, other authors have a claim to it, and the early followers of Newton, such as Kiel, Hartley, and Marzucchi, all attempted a corpuscular chemistry, founded upon figure, weight, and attractive power of the ultimate particles of matter; but this chemistry was of no real use, and had no other foundation than in the imagination. Indeed, in my opinion, Mr. Dalton is too much of an Atomic Philosopher; and in making atoms arrange themselves according to his own hypothesis, he has often indulged in vain speculation; and the essential and truly useful part of his doctrine, the expression of the quantities in which bodies combine, is perfectly independent of any views respecting the ultimate nature either of matter or its elements.”

He concludes the paper in which he so minutely studied the action of chlorine upon oxides by asking, if it be said that the oxygen arises from the decomposition of the oxymuriatic gas and not from the oxides, why is it always the quantity contained in the oxide that is evolved? And why in some cases, as those of the peroxides of potassium and sodium, it bears no relation to the quantity of oxymuriatic gas?

“When potassium is burnt in oxymuriatic gas, a dry compound is obtained. If potassium combined with oxygen is employed, the whole of the oxygen is expelled, and the same compound formed. It is contrary to sound logic to say, that this exact quantity of oxygen is given off from a body not known to be compound, when we are certain of its existence in another; and all the cases are parallel.”

An argument in favour of the existence of oxygen in chlorine might be derived from the circumstance of the formation of the latter gas by the action of muriatic acid on peroxides. Davy found that, by heating muriatic acid gas in contact with dry peroxide of manganese, water was rapidly formed and oxymuriatic gas produced.

“Now as muriatic acid gas is known to consist of oxymuriatic gas and hydrogen, there is no simple explanation of the result, except by saying that the hydrogen of the muriatic acid combined with oxygen from the peroxide to produce water.”

The bleaching power of chlorine had been explained by Scheele on the supposition that it destroyed colours by combining with phlogiston. Berthollet considered it to act by supplying oxygen. Davy then made the well-known experiment proving that the dry gas “is incapable of altering vegetable colours, and that its operation in bleaching depends entirely upon its property of decomposing water and liberating its oxygen.” It had been supposed that oxymuriatic acid gas was capable of being condensed and crystallised at a low temperature. He shows that it was only damp chlorine or its solution in water that yielded any solid product. He exposed the pure gas, dried by muriate of lime, to a temperature of -40° F., without observing any change. It is curious, however, that liquid chlorine had actually been obtained by Northmore five years before by heating the so-called hydrate of chlorine under pressure. The phenomenon was misunderstood, and it was reserved for Faraday, in 1823, to show that the product was actually the liquefied gas.

Davy, who was not always happy in his suggestions as to chemical nomenclature, proposed to denote the compounds of oxymuriatic gas by the names of their bases with the termination ane.

“Thus, argentane may signify horn-silver; stannane Libavius’s liquor; antimonane, butter of antimony; sulphurane, Dr. Thomson’s sulphuretted liquor, and so on for the rest.... In cases when two or more proportions of inflammable matter combine with one of gas; or two or more of gas with one of inflammable matter, it may be convenient to signify the proportions by affixing vowels before the name, when the inflammable matter predominates, and after the name when the gas is in excess; and in the order of the alphabet, a signifying two, e, three, i, four and so on.”

Thus he called phosphorus pentachloride phosphorana, and the trichloride phosphorane, because there was a larger percentage proportion of phosphorus in the latter compound than in the former. That Davy was not unaware of the difficulties and inconveniences of such a system of nomenclature may be inferred from what he says in his “Elements” concerning the names for the two chlorides of mercury, the true composition of which he was the first to discover:—

“The names mercurane and mercurana which may be adopted to signify the relations of their composition, are too similar to each other to be safely used as familiar appellations for the two substances, as corrosive sublimate is a powerful poison, calomel an excellent medicine.”

In matters of chemical nomenclature Davy was a great latitudinarian. All that he contended for was that names should be independent of all speculative views, and should rather be derived from some simple and invariable property. It is remarkable, however, that he who invented the happy term “chlorine” should have objected to the word “cyanogen.” At the close of the short paper “On the Prussic Basis and Acid,” in which he first made known the existence of the cyanides of phosphorus and of iodine, he said:—

“I wish M. Gay Lussac could be prevailed upon to give up the inexpressive and difficult names of cyanogen and hydrocyanic acid, and to adopt the simple ones of prussic gas and prussic acid.”

By treating the potassium hyper-oxymuriate of Berthollet (potassium chlorate) with hydrochloric acid, a greenish-yellow explosive gas is obtained which Chenevix had referred to as “hyper-oxygenised muriatic acid,” and as indicating the existence of a compound of oxymuriatic gas and oxygen in a separate state. Davy, as we have seen, was at first inclined to doubt the existence of this substance, and to consider the gas as simply chlorine. But on comparing it with chlorine prepared in other ways he perceived a difference; its solution in water was of lemon yellow or orange colour; when treated with mercury it becomes of a brilliant yellow green. It is, moreover, highly explosive, especially when heated, even at the warmth of the hand, when it loses its vivid colour, and is resolved into a mixture of oxygen and chlorine. Metals, arsenic, phosphorus, charcoal, nitric oxide, act upon it in a manner different from that of chlorine. Davy makes use of these differences as a proof of the correctness of his views of the nature of chlorine.

“If the power of bodies to burn in oxymuriatic gas depended upon the presence of oxygen, they all ought to burn with much more energy in the new compound; but copper and antimony, and mercury and arsenic and iron and sulphur have no action upon it, till it is decomposed; and they act then according to their relative attractions on the oxygen, or on the oxymuriatic gas. There is a simple experiment which illustrates this idea. Let a glass vessel containing brass foil be exhausted, and the new gas admitted, no action will take place; throw in a little nitrous gas [nitric oxide], a rapid decomposition occurs, and the metal burns with great brilliancy.

“As the new compound in its purest form is possessed of a bright yellow-green colour, it may be expedient to designate it by a name expressive of this circumstance and its relation to oxymuriatic gas. As I have named that elastic fluid Chlorine; so I venture to propose for this substance the name Euchlorine, or Euchloric gas from e? and ??????. The point of nomenclature I am not inclined to dwell upon. I shall be content to adopt any name that may be considered as most appropriate by the able chemical philosophers attached to this Society” [the Royal Society].

Euchlorine was subsequently discovered by Soubeiran to be a mixture of chlorine and chlorine peroxide, a gas which Davy himself afterwards isolated in a pure state. It is however obvious from the accounts he gives that even in his first paper he must have been experimenting with a fairly pure product, due probably to the circumstance that he had collected the mixed gases over mercury, which retains the greater part of the chlorine. Former experimenters had collected the gas over water, which dissolves the chlorine peroxide more readily than the chlorine. Madame de StaËl once observed that an interesting book might be written on the important consequences which have sprung from little differences. It ought to be noted, however, that Davy had himself doubts whether his euchlorine was not a mixture of chlorine and the gas which he subsequently discovered, and to which he says: “I shall not propose to give any name till it is determined whether euchlorine is a mixture or a definite compound.”

It has been stated that Davy discovered the two chlorides of phosphorus. In a paper read to the Royal Society on June 18th, 1812, “On some Combinations of Phosphorus and Sulphur and on some other Subjects of Chemical Inquiry,” he reverts to these substances, as they “offer decided evidences in favour of an idea that has been for some time prevalent among many enlightened chemists and which I have defended in former papers published in the Philosophical Transactions; namely that bodies unite in definite proportions, and that there is a relation between the quantities in which the same element unites with different elements.”

He first makes a determination, singularly accurate for the time, of the amount of chlorine contained in the lower chloride, and finds that 13·6 grains on decomposition with water afforded 43 grains of horn-silver; theory requires 42·6 grains. By synthetical experiments he came to the conclusion that the amount of chlorine absorbed by phosphorus to form the higher chloride was exactly double that contained in the lower chloride: he found that 3 grains of phosphorus combined with 20 grains of chlorine: in reality it should require only 17¾ grains.

He shows that by treatment with water the lower chloride yields phosphorous acid, the properties and mode of decomposition of which by heat he accurately describes. He further concludes, as the logical consequence of his view of the composition of the two chlorides, and the mode of their decomposition by water, that phosphorous acid contains half the amount of oxygen present in phosphoric acid, the quantity of phosphorus being the same. It is noteworthy that in his argument, as indeed on all subsequent occasions when he speaks of the decomposition of water in definite proportions, he regards water as composed of 2 combining proportions of hydrogen and 1 of oxygen, and the number representing it as 17, oxygen being regarded as 15. Certain of his statements considered in the light of subsequent work are interesting. Thus he says:—

“A solid acid volatile at a moderate degree of heat, may be produced by burning phosphorus in very rare air, and this seems to be phosphorous acid free from water; but some phosphoric acid, and some yellow oxide of phosphorus are always formed at the same time.”

He also observes that unless the product of the combustion of phosphorus is strongly heated in oxygen it contains phosphorous acid as well as phosphoric acid. He further states that sulphurous acid (sulphur dioxide) consists of equal weights of oxygen and sulphur, which is almost strictly true, and that sulphuretted hydrogen is composed of 1 combining proportion of sulphur and 2 of hydrogen, although his values for the combining proportions of sulphur and oxygen are incorrect. He repeats Dalton’s experiment of the formation of “solid sulphuric acid” by the mutual action of sulphur dioxide and nitric oxide, and shows that the substance is only produced in presence of vapour of water; the two substances, he says, then “form a solid crystalline hydrate; which when thrown into water gives off nitrous gas and forms a solution of sulphuric acid.” This substance is the so-called “leaden-chamber crystal,” or nitrosulphonic acid, the existence of which was first made known by Scheele.

Davy’s conclusions concerning the composition of the oxides and chlorides of phosphorus were subsequently contested by Berzelius and Dulong, who showed that although the amount of chlorine in the lower chloride was identical with that which he had found, the ratio of this amount to that in the higher chloride was as 3 to 5, and not as 1 to 2, and that the same ratio held good as regards the oxygen in phosphorous oxide and phosphoric oxide. Davy, six years afterwards, repeated his experiments, but without discovering the fallacy in his first observations.

The other incidents in Davy’s scientific career may be most conveniently dealt with in connection with his personal history.

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