Lord Brougham on the Composition of Water. 319 



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 de- 

 phlogisticated air and phlogiston deprived of part of their latent heat."* 

 The letter was received by Dr Priestley, and delivered to Sir Joseph Banks, 

 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 l783,t with many further obser- 



• It may with certainty be concluded from Mr Watt's private and unpublished 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 in- 

 vented, 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 2181 April 1783, p. 416, states, that Mr Watt, prior to his (the Doctor's) ex- 

 periments, 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 reason- 

 ing upon which that deduction was founded. [Notk by Mr James Watt.] 



t 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 Char- 

 lotte. He was the friend of Mr Watt, who did not then belong to the So- 

 ciety. 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 1.5th January 1784 was read ; but, hear- 

 ing 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 



