1784) had been read partly in November and December 1783, and additions were afterwards made to it. It was published in 1784. It contained Mr Lavoisier's account of his experiments in June 1783, at which, he says, Sir Charles Blagden was present; and it states that he told Mr Lavoisier of Mr Cavendish having “already burnt inflammable air in close vessels, and obtained a very sensible quantity of water." But he, Mr Lavoisier, says nothing of Sir Charles Blagden having also mentioned Mr Cavendish's conclusion from the experiment. He expressly states, that the weight of the water was equal to that of the two airs burnt, unless the heat and light which escape are ponderable, which he holds them not to be. His account, therefore, is not reconcilable with Sir Charles Blagden's, and the latter was most probably written as a contradiction of it, after Mr Cavendish's paper had been read, and when the Mémoires of the Académie were received in this country. These Mémoires were published in 1784, and could not certainly have arrived when Mr Cavendish's paper was written, nor when it was read to the Royal Society.

But it is further to be remarked, that the passage of Mr Cavendish's paper in Sir Charles Blagden's handwriting, only mentions the experiments having been communicated to Dr Priestley; they were made, says the passage, in 1781, and communicated to Dr Priestley, it is not said when, nor is it said that "the conclusions drawn from them,” and which Sir Charles Blagden says he communicated to Mr Lavoisier in summer 1783, were ever communicated to Dr Priestley; and Dr Priestley in his paper (referred to in Mr Cavendish's), which was read June 1783, and written before April of that year, says nothing of Mr Cavendish's theory, though he mentions his experiment.

Several propositions then are proved by this statement.

First, That Mr Cavendish in his paper, read 15th January 1784, relates the capital experiment of burning oxygen and hydrogen gases in a close vessel, and finding pure water to be the produce of the combustion.

Secondly, That in the same paper, he drew from this experiment the conclusion, that the two gases were converted or turned into water. Thirdly, That Sir Charles Blagden inserted in the same paper, with Mr Cavendish's consent, a statement that the experiment had first been made by Mr Cavendish in summer 1781, and mentioned to Dr Priestley, though it is not said when, nor is it said that any conclusion was mentioned to

Dr Priestley; nor is it said at what time Mr Cavendish first drew that conclusion. A most material omission.

Fourthly, That, in the addition made to the paper by Sir Charles Blagden, the conclusion of Mr Cavendish is stated to be, that oxygen gas is water deprived of phlogiston; this addition having been made after Mr Lavoisier's memoir arrived in England.

It may further be observed, that in another addition to the paper, which is in Mr Cavendish's handwriting, and which was certainly made after Mr Lavoisier's memoir had arrived, Mr Cavendish for the first time distinctly states, as upon Mr Lavoisier's hypothesis, that water consists of hydrogen united to oxygen gas. There is no substantial difference perhaps between this and the conclusion stated to have been drawn by Mr Cavendish himself, that oxygen gas is water deprived of phlogiston, supposing phlogiston to be synonymous with hydrogen; but the former proposition is certainly the more distinct and unequivocal of the two: and it is to be observed that Mr Cavendish, in the original part of the paper, i. e. the part read January 1784, before the arrival of Lavoisier's, considers it more just to hold inflammable air to be phlogisticated water than pure phlogiston, (p. 140).

We are now to see what Mr Watt did, and the dates here become very material. It appears that he wrote a letter to Dr Priestley on 26th April 1783, in which he reasons on the experiment of burning the two gases in a close vessel, and draws the conclusion, " that water is composed of dephlogisticated air and phlogiston deprived of part of their latent heat.”* The letter was received by Dr Priestley, and delivered to Sir Joseph Banks,

It may with certainty be concluded from Mr Watt's private and unpublished letters, of which the copies taken by his copying-machine then recently invented, are preserved, that his theory of the composition of water was already formed in December 1782, and probably much earlier. Dr Priestley, in his paper of 21st April 1783, p. 416, states, that Mr Watt, prior to his (the Doctor's) experiments, had entertained the idea of the possibility of the conversion of water or steam into permanent air. And Mr Watt himself, in his paper, Phil. Trans. p. 335, asserts, that for many years he had entertained the opinion that air was a modification of water, and he enters at some length into the facts and reasoning upon which that deduction was founded.-[NOTE BY MR JAMES WATT.]

with a request that it might be read to the Royal Society; but Mr Watt afterwards desired this to be delayed, in order that he might examine some new experiments of Dr Priestley, so that it was not read until the 22d April 1784. In the interval between the delivery of this letter to Dr Priestley and the reading of it, Mr Watt had addressed another letter to Mr De Luc, dated 26th November 1783,* with many further observations and reasonings, but almost the whole of the original letter is preserved in this, and is distinguished by inverted commas. One of the passages thus marked is that which has the important conclusion above mentioned; and that letter is stated in the subsequent one to have been communicated to several members of the Royal Society at the time of its reaching Dr Priestley, viz. April 1783.

In Mr Cavendish's paper as at first read, no allusion is to be found to Mr Watt's theory. But in an addition made in Mr Cavendish's own hand, after Mr Watt's paper had been read, there is a reference to that theory

The letter was addressed to Mr J. A. De Luc, the well known Genevese philosopher, then a Fellow of the Royal Society, and Reader to Queen Charlotte. He was the friend of Mr Watt, who did not then belong to the Society. Mr De Luc, following the motions of the Court, was not always in London, and seldom attended the meetings of the Royal Society. He was not present when Mr Cavendish's paper of 15th January 1784 was read; but, hearing of it from Dr Blagden, he obtained a loan of it from Mr Cavendish, and writes to Mr Watt on the 1st March following, to apprise him of it, adding that he has perused it, and promising an analysis. In the postscript he states, “In short, they expound and prove your system word for word, and say nothing of you." The promised analysis is given in another letter of the 4th of the some month. Mr Watt replies on the 6th, with all the feelings which a conviction he had been ill treated was calculated to inspire, and makes use of those vivid expressions which M. Arago has quoted; he states his intention of being in London in the ensuing week, and his opinion, that the reading of his letter to the Royal Society will be the proper step to be taken. He accordingly went there, waited upon the President of the Royal Society, Sir Joseph Banks, was received with all the courtesy and just feeling which distinguished that most honourable man, and it was settled that both the letter to Dr Priestley of 26th April 1783, and that to Mr De Luc of 26th November 1783, should be successively read. The former was done on the 22d, and the latter on the 29th April 1784.-[NOTE BY MR JAMES WATT.]

(Phil. trans. 1784, p. 140), and Mr Cavendish's reasons are given for not encumbering his theory with that part of Mr Watt's which regards the evolution of latent heat. It is thus left somewhat doubtful, whether Mr Cavendish had ever seen the letter of April 1783, or whether he had only seen the paper (of 26th November 1783) of which that letter formed a part, and which was read 29th April 1784. That the first letter was for some time (two months, as appears from the papers of Mr Watt) in the hands of Sir Joseph Banks and other members of the Society during the preceding spring, is certain, from the statements in the Note to p. 330; and that Sir Charles Blagden, the Secretary, should not have seen it seems impossible, for Sir Joseph Banks must have delivered it to him at the time when it was intended to be read at one of the Society's meetings (Phil. Trans. p. 330, Note), and as the letter itself remains among the Society's Records in the same volume with the paper into which the greater part of it was introduced, it must have been in the custody of Sir C. Blagden, It is equally difficult to suppose, that the person who wrote the remarkable passage already referred to, respecting Mr Cavendish's conclusions having been communicated to Mr Lavoisier in the summer of 1783 (that is, in June), should not have mentioned to Mr Cavendish that Mr Watt had drawn the same conclusion in the spring of 1783 (that is, in April at the latest). For the conclusions are identical, with the single difference, that Mr Cavendish calls dephlogisticated air, water deprived of its phlogiston, and Mr Watt says, that water is composed of dephlogisticated air and phlogiston.

We may remark, there is the same uncertainty or vagueness introduced into Mr Watt's theory which we before observed in Mr Cavendish's, by the use of the term Phlogiston, without exactly defining it.* Mr Cavendish leaves it uncertain, whether or not he meant by Phlogiston simply inflammable air, and he inclines rather to call inflammable air, water

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* Mr Watt, in a note to his paper of 26th November 1783, p. 331, observes, 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 aerial form. These arguments were perfectly convincing to me, but it seems proper to rest that part of the argument on direct experiment."-[NOTE BY MR JAMES WATT.]

1

united to phlogiston. Mr Watt says expressly, even in his later paper (of November 1783), and in a passage not to be found in the letter of April 1783, that he thinks that inflammable air contains a small quantity of water and much elementary heat. It must be admitted that such expressions as these on the part of both of those great men, betoken a certain hesitation respecting the theory of the composition of water. If they had ever formed to themselves the idea, that water is a compound of the two gases deprived of their latent heat,—that is, of the two gases, with the same distinctiveness which marks Mr Lavoisier's statement of the theory such obscurity and uncertainty would have been avoided.*

* Mr Watt, in his letter of 26th April 1783, thus expresses his theory and conclusions (Phil. Trans. p. 333): “Let us now consider what obviously happens in the case of the deflagration of the inflammable and dephlogisticated air. 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. This water is then the only remaining product of the process, and water, light, and heat, are all the products" (unless, he adds in the paper of November, there be some other matter set free, which escapes our senses). "Are we not then authorized to conclude, that water is composed of dephlogisticated air and phlogiston, deprived of their latent or elementary heat; that dephlogisticated or pure air is composed of water deprived of its phlogiston, and united to elementary heat and light; that the latter are contained in it in a latent state, so as not to be sensible to the thermometer or to the eye; and if light be only a modification of heat or a circumstance attending it, or a component part of the inflammable air, then pure or dephlogisticated air is composed of water deprived of its phlogiston and united to elementary heat?"

Is this not as clear, precise, and intelligible, as the conclusions of Mr Lavoisier?-[NOTE BY MR JAMES WATT.]

The obscurity with which Lord Brougham charges the theoretical conceptions of Watt and Cavendish does not appear to me well founded. In 1784, the preparation of two permanent and very dissimilar gases was known. Some called these gases, pure air and inflammable air; others, dephlogisticated air and phlogiston; and lastly, others, oxygen and hydrogen. By combining dephlogisticated air and phlogiston, water was produced equal in weight to that of the two gases. Water thenceforward was no longer a simple body, but a compound of dephlogisticated air and of phlogiston. The chemist who drew that conclusion, might have erroneous ideas as to the intimate nature of phlogiston, without throwing any uncertainty upon the merit of his first discovery. Even at this