THE following notes are intended as supplemental to a paper on the same subject by Lord Brougham.

       It appears to me that M. Arago has overrated the share that Watt really possessed in the discovery of the composition of water. In fact Blagden's account is confirmed by Mr. Watt himself, for he commences his letter to Dr. Priestley (26th April, 1783) thus:--
"On considering your very curious and important discoveries on the nature of phlogiston and dephlogisticated air, and on the conversion of water into air, and vice versâ, some thoughts have occurred (i.e. to me) on the probable causes of these phænomena*."

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* Guard Book of the Royal Society, vol. lxxiv.
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This agrees with what Dr. Priestley says in his first paper on this subject.
       "I was mentioning these ideas to Mr. Watt, in whose neighbourhood I have the happiness to be si- / p.4 / tuated, when he mentioned a similar idea of his, viz. that of the possibility of the conversion of water or steam into permanent air; saying, that some appearances in the working of his fire-engine had led him to expect this. He thought that if steam could be made red hot, so that all its latent heat should be converted into sensible heat, either this or some other change would probably take place in its constitution. The idea was new to me, and led me to attend more particularly to my former projects of a similar nature*."

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       * Philosophical Transactions, vol. lxxiii. p.416. See also the paper by Dr. Priestley, read Feb. 24th, 1785, in Phil. Trans. p.279-280. "In the experiements of which I shall now given an account, I was principally guided by a view to the opinions which have lately been advanced by Mr. Cavendish, Mr. Watt, and M. Lavoisier. Mr. Cavendish was of opinion that when air is decomposed water only is produced; and Mr. Watt concluded from some experiements, of which I gave an account to the Society, and also from some observations of his own, that water consisted of dephlogisticated and inflammable air, in which Mr. Cavendish and M. Lavoisier concur with him."
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       Mr. Watt's two letters, one addressed to Dr. Priestly and the other to Mr. Deluc, were read before the Royal Society; the first on the 22nd April, and the other on the 29th of April and the 6th of March 1784. The committee of papers which sat on the 20th of May in the same year, ordered both these letters to be printed. The first was dated 26th April, 1783, and, in a foot note at p.330, Watt explains the reason why it was not read sooner.
       "This letter Dr. Priestley received at London, and / p.5 / after showing it to several members of the Royal Society he delivered it to Sir Joseph Banks, the president, with a request that it might be read at some of the public meetings of the Society; but before that could be complied with, the author, having heard of Dr. Priestley's new experiments, begged that the reading might be delayed. The letter, therefore, was reserved until the 22nd of April last, when, at the author's request, it was read before the Society. It has been judged unnecessary to print that letter, as the essential parts of it are repeated, almost verbatim, in this letter to M. De Luc; but to authenticate the date of the author's ideas, the parts of it which are contained in the present letter are marked with double commas."
       Watt himself desired this note to be inserted in a letter to Dr. Blagden, dated Birmingham, May 27th, 1784, and preserved in the Guard Book of the Royal Society, No. 74*.

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       * The second letter addressed to Mr. Deluc was dated 26th November, 1783, and it was corrected by Watt in April, 1784, who then inserted the foot notes printed at pp. 330, 345, 351, and 353 of Phil. Trans., vol. lxxiv.; the concluding sentence was added then, and has evidently been inserted in the place of something else, not crossed out, but actually cut away from the original paper. I may here observe, that at the committee of papers, which sat on the 29th April 1784, it was "ordered that such cancels in Dr.Priestley's last paper relating to phlogiston, as the Doctor shall think fit, be printed at the expense of the Society;" this of course refers to the Doctor's first paper.
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       I think that there is sufficient evidence to show, / p.6 / that although perhaps Watt may have conceived an idea of the real composition of water, yet it was Dr. Priestley who first led him to consider the subject with any attention, and that both of them at the time thought it a matter of too little importance and too great uncertainty for any dispute or jealousy to arise.
       The ridicule with which their theories were received may be gathered from a review of Dr. Priestley's paper in the Gentleman's Magazine for 1789. I cannot help quoting part of this extraordinary paper, which was probably written by Dr. Robert Harrington, one of the most virulent opposers of Priestly and Watt:--
       "The decomposition of water is not the only chemical subject on which, in the pages of the Transactions of the Royal Society, contradictory conclusions have been drawn. We have been taught that water is compounded of inflammable and dephlogisticated air; and the truth of the doctrine has been supported by an appeal to experiments conducted in the bottles and glasses of the first experimenters of the age. In fact, however highly and however justly philosophers may value experimental inquiries, to us it appears evident that the rage for experiment has been carried much too far; for thousands of experiments have been made and obtruded on the public, within the last twenty years, from which no just or useful deductions have been drawn*."

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       * Mr. Richard Kirwan, in a paper read before the Royal Society in April 1782, had observed "the diminution of respirable air in common phlogistic processes," and attributed it to the gene- / p.7 / ration and absorption of fixed air. Mr. Cavendish, on the contrary, rightly maintained that it was owing to the production of water, which was formed by the union of the disengaged phlogiston with the dephlogisticated part of common air; and that fixed air is never produced in phlogistic processes, except some animal or vegetable substance is concerned in the operation, from whose decomposition it may arise. In the 74th volume of the Phil. Trans. may be found the dispute between Mr. Cavendish and Mr. Kirwan on their respective theories.
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M. Arago, after mentioning the discovery of hydrogen by Cavendish,and of oxygen by Priestley, with others, adds, that "in the midst of these remarkable discoveries water had always maintained its character of an element." In the year 1747, however, there was printed at London, in 4to, a small pamphlet, entitled, "A curious research into the element of water, containing many noble and useful experiments on that fluid body; being the conjunctive trials of Ambrose and John Godfrey, Chymists, from their late father's observations*."

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       * Letters from Ambrose Godfrey, their father, to Sir Hans Sloane, may be found in MSS. Sloane, 4045, in the British Museum, chiefly on water; and in MSS. Birch, 4432, fol. 149 r^o, is an "examination of the West-Ashton well water," by the same.
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I am not inclined to attach much value to this pamphlet, but its authors endeavour to prove that salt is the root and basis of all water, and always forms an essential part of its composition; and in one place they observe that "one of our said friends, some time since, told us that the experiment was contemplative and witty, and that a large quantity / p.8 / of water might be extracted from the air by attractive salts, which he knew by a tiresome chymical experiment."

      35, Alfred Place.
            Feb. 1st, 1840.