394 WATT. 



It may further be "observed, that in another addition to the 

 paper, which is also in Sir C. Blagden's handwriting, and which 

 was certainly made after M. Lavoisier's memoir had arrived, 

 Mr. Cavendish for the first time distinctly states, as upon M . 

 Lavoisier's hypothesis, that water consisted 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 hy- 

 drogen ; but the former proposition is certainly the more dis- 

 tinct 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 dephlogisti- 

 cated air and phlogiston, deprived of part of their latent 

 heat."* The letter was received by Dr. Priestley and de- 

 livered 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 



* 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 and unpublished letters, of which the copies taken by his copying- 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) ex- 

 periments, had entertained the idea of the possibility of the conver- 

 sion 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 MB. JAMES WATT.] 



