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BESIDES employing the argument arising from the reputation of Mr. Cavendish, which does not really affect the question of priority in the discovery, if established by other evidence, the advocates of Cavendish have made three principal assertions with the view of impugning M. Arago's accuracy. They have said, first, that Priestley “constantly maintained” that he had never found the weight of the water, produced in his experiment, equal to that of the gases exploded ; secondly, that an undue licence had been used, in substituting the term hydrogen for phlogiston, as used by Mr. Watt; and thirdly, that the conclusions of Cavendish, which were first stated to the Royal Society in his paper read on the 15th of January, 1784, must be supposed to have been included, or involved, in his experiments made in 1781.

The first of these assertions might well be termed by M. Arago “inconceivable,” when it is remembered that in Priestley's own paper he says,—“In order to judge more accurately “of the quantity of water so deposited, and to compare it " with the weight of the air decomposed, I carefully weighed

a piece of filtering paper, and then having wiped with it " all the inside of the glass vessel in which the air had been decomposed, weighed it again ; and I always found, as near

I could judge, the weight of the decomposed air in the moisture acquired by the paper."

In the very first pages


* Phil. Trans.,' 1783, p. 427.,

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of Mr. Watt's paper “on the Constituent Parts of Water," in describing Dr. Priestley’s experiment, it is said,—“ The “ moisture adhering to the glass after these deflagrations,

being wiped off, or sucked up by a small piece of sponge paper, first carefully weighed, was found to be exactly, or

very nearly, equal in weight to the airs employed.And,“ These two kinds of air unite with violence, they become “ red-hot, and, upon cooling, totally disappear. When the “ vessel is cooled, a quantity of water is found in it equal to the weight of the air employed."* So in Mr. Watt's Cor

. respondence," he finds on the side of the vessel a quantity of water equal in weight to the air employed.”+ And again, “No residuum, except a small quantity of water equal to their weight." I So also, “you will find the water, (equal in weight to the air), adhering to the sides of the vessel.” $ The circumstance of the equality of weight was indeed one of the facts on which Mr. Watt repeatedly states that he founded his deductions; and, as will presently be seen, it is of great importance in more points of view than one.

The substitution of the term hydrogen for phlogiston, had been so amply explained by M. Arago, in the note on that subject which accompanied Lord Brougham's Historical Note,ll that it might have been supposed no fair objection could have been raised to it by any one; even by the most injudicious and ill-informed partisan of Mr. Cavendish. M, Arago was also at the pains to produce a letter from Dr. Priestley to M. Lavoisier, dated 10th July, 1782, in which he says he has made “some experiments with inflammable air, that seem to “ prove that it is the same thing that has been called phlogiston.Dr. Priestley, in relating, in his paper of 1785, the theory which Mr. Watt had formed, says that he “concluded, &c., “ that water consists of dephlogisticated and inflammable air.But further, the professed difficulty might have been removed, if those who made it had chosen to profit by Mr. Watt's own

of Phil. Trans.,' 1784, pp. 332, 333.

† Mr. Watt to Mr. Gilbert Hamilton, 26 March, 1783.

# The same to the same, 22 April,


$ Mr. Watt to Mr. Fry, 28 April, 1783.

11 • Eloge of Watt,' p. 167.




note, (which, if they did not read, they at least ought to have read, and might have been supposed to have considered, because it is given both in the Philosophical Transactions' and in Lord Brougham's Historical Note), viz. : “ Previous to “ Dr. Priestley's making these experiments, Mr. Kirwan had proved, by very ingenious deductions from other facts, that inflammable air was, in all probability, the real phlogiston in

an aërial form. These arguments were perfectly convincing 6 to me."'*

So in Mr. Watt's paper we find these expressions :“ It “ was reasonable to conclude, that inflammable air must be the

pure phlogiston, or the matter which reduced the calces to “ metals ;" -the inflammable air being supposed to be wholly phlogiston;" ——" inflammable air or phlogiston ;—“it is

: worthy of inquiry whether the greater part of the heat let " loose was not contained in the phlogiston or inflammable air,” + &c. &c. So, also, in writing to Dr. Black on the 21st of April, 1783,—the very day on which his letter to Dr. Priestley was first written, although the second edition, read a year afterwards at the Royal Society, was written on the 26th of the same month,-he says, “therefore inflammable air is the

thing called phlogiston.So to Mr. Hamilton, on the 22rd of April, the first of the three deductions he states is, “pure inflammable air is phlogiston itself.Above all, in the same letter to Dr. Black, as if to exclude all possibility of any cavil being raised, on the ground of the language in which his theory is expressed, he further states his conclusion to be, " that water is composed of dephlogisticated and inflam"mable air."

Neither is the objection, thus groundlessly stated, an original one, nor has it now been for the first time effectually answered. For, nearly half a century ago, a very able pen thus wrote: “ We have said that the theory of Mr. Watt is “ now demonstrated to be true. To this assertion, an objec“tion may be raised from the language in which he states “ his theory; for he explains it by using the word 'phlo

* .Phil. Trans.,' 1784, p. 331.

Ibid., pp. 349, 350, 352.

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giston,' a word which is now exploded from philosophy as “ the name of an imaginary substance. But it is sufficient to reply, that Mr. Watt uses the word phlogiston as synonymous with inflammable air.”* And that hydrogen, and not any other sort of inflammable air, (such as the gas from charcoal, on which also Priestley experimented), was intended by Mr. Watt, is evident from the circumstance of the equality of the weight of the water, on which, as reported by Priestley, he 80 much dwells; but which could not have been attained except by that particular gas being employed : as well as by Mr. Watt's expression of the inflammable air being “ the “matter which reduced the calces to metals,”—a very exact description of hydrogen, and of no other gas.

It is evident that the term hydrogen, derived from the Greek word for water, and designating one of its constituents, could not have been invented till after the composition of that fluid had been ascertained. Lavoisier himself, the inventor of the term, did not use it till a later period; and he expressly says, in the beginning of his paper, “ The inflammable air “ which I understand when I mention it in this Memoir, is " that which is obtained, either from the decomposition of “ water by iron alone, or from iron and zinc dissolved in “ vitriolic and marine acids; and, as it appears proved that “ in all cases that air comes originally from water, I shall “ call it, when it presents itself in the aëriform state, aqueous

inflammable air; and when it is engaged in any combina“ tion, aqueous inflammable principle.That passage is one of those additions to the paper, which are said not to have been made till after November, 1783; for it contains an allusion to the experiments made with M. Meusnier, which had not been performed at that date, but were described in the Memoir read at Easter, 1784.

But in what respect was Cavendish superior to Mr. Watt on this point ? Even in 1784 he used neither the term hydrogen at all, nor uniformly the term inflammable air; for his conclusion is in that year thus stated :-“There seems

* Article • WATER,' Encyc. Brit., 1797.

“ the utmost reason to think that dephlogisticated air is only “ water deprived of its phlogiston, and that inflammable air is “ either phlogisticated water or else pure phlogiston; but in all probability the former,”—a conclusion infinitely more dim and distant from the truth than those which we have just cited from Mr. Watt's paper and letters. Such also is the language in which the rest of Mr. Cavendish's paper, on this subject, is couched ; and even with all the additional lights supplied by Watt and Lavoisier to guide him, it is undeniable that his conclusions are at least as much embarrassed and disguised as those of either of the others : while M. Arago, that equal justice might be done to all parties, used exactly the same substitution in speaking of Cavendish's labours; thus making them, as well as those of Mr. Watt, more intelligible to those accustomed only to the modern nomenclature.

Lastly, it has been asserted, that Cavendish's mere experiments, apart from the formation of any theory, “involved “ the notion, and established the fact,” of the composition of water. So in some sense did Priestley's;—so did Warltire's; nay, on the same principles, it might be hard to withhold the merit of priority from Macquer and Sigaud de Lafond, who, by the combustion of gases, produced water which appeared to them to be pure. It may be true that Macquer's data, so far as he has recorded them, were scarcely sufficient to have led him readily to form a just opinion on the subject. But Priestley and Warltire, in their experiments of 1781, came very much nearer the last experimental step afterwards arrived at by Cavendish : the loss of weight which Warltire detected after the combustion was almost imperceptible, and was at once to be accounted for by the least imperfection in his apparatus. Yet they both confidently attributed the formation of the dew to the mere deposition of suspended moisture.

So late as 1784, Meusnier and Lavoisier, in the commencement of their Memoir on the Decomposition of Water,* remark,

* "Mémoires de l'Académie ' for 1781, published in 1784.

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