WATT, 61 



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 com- 

 posed 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, 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. Priest- 

 ley, 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 M. De Luc, dated 26th November, 1783,t 



* 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 unpublished letters, of which the copies taken by his copying-machine, 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 perma- 

 nent 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.'] 



f The letter was addressed to M. J. A. De Luc, the well-known Gene- 

 vese philosopher, then a Fellow of the Royal Society, and Reader to Queen 

 Chariotte. He was the friend of Mr. Watt, who did not then belong to 

 the Society. M. 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 j but, hearing of it from Dr. Blagden, he obtained a loan of it 

 from Mr. Cavendish, and writes fco 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 same 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 



