oa X ; at) 
’ = 
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 1784.) 
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, 1788, in which he discusses the experiment of 
igniting the two gases in closed vessels, and that by it he carne 
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. 3835, 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.) 
