474 JAMES WATT. 



time, as in Lavoisier's hypothesis, that water is a compound of 

 oxygen and hydrogen. Perhaps no essential difference will 

 be found between this conclusion and the one which Caven- 

 dish had asserted before, that oxygen gas is water deprived of 

 its phlogiston, for to render them identical it will suffice to 

 look upon phlogiston as hydrogen ; but to say of water that it 

 consists of oxygen and hydrogen, is, certainly embracing a 

 neater and less equivocal conclusion. I add, that in the orig- 

 inal part of his Memoir, in that which was read to the Royal 

 Society before the arrival of Lavoisier's Memoir in England, 

 Cavendish thinks it more correct to consider inflammable air 

 as phlogisticated water than as pure phlogiston." (P. 140, 

 Phil. Trans, for 1 784.) 



Now let us see what part Watt acted. The dates will here 

 be of importance. 



It appears that Watt wrote a letter to Dr. Priestley, on the 

 26th of April, 1783, in which he discusses the experiment of 

 igniting the two gases in closed vessels, and that by it he came 

 to the conclusion that water is composed of dephlogisticated 

 air and phlogiston, both of them deprived of a part of their 

 latent heat.* 



Priestley deposited the letter in the hands of Sir Joseph 

 Banks, with a request that it should be read at one of the ear- 

 liest meetings of the Royal Society. But Watt afterwards 

 wished the reading to be deferred, in order to have time to 

 see how his theory agreed with some of Priestley's recent ex- 



* We may feel quite safe in deducing from the inedited correspond- 

 ence of Watt, that he had already formed his theory on the composi- 

 tion of water in December, 1782, and probably earlier. At all events, 

 in his Memoir of the 21st of April, 1783, Priestley declares that, before 

 his own experiments, Watt had entertained the idea that steam might 

 be transformed into permanent gases. (P. 416, Phil. Trans. 1783.) 



Watt himself, in his Memoir (p. 335, Phil. Trans. 1784), declares 

 that, during several years, he had entertained an opinion that air is a 

 modification of water, and he makes known in detail the experiments 

 and the reasonings on which this opinion rested. (Note by Mr. Watt, 

 junior.) 



