430 JAMES WATT. 
in a memoir of the 21st of that month. The citation 
besides informs us only of one circumstance ; it is, that 
Cavendish had obtained water by the detonation of a 
mixture of oxygen and hydrogen, a result already proved 
by Warltire. 
In his April memoir, Priestley added a remarkable 
circumstance to those which resulted from the experi- 
ments of his predecessors, for he proved that the weight 
of the water deposited on the sides of the vase at the 
moment of the oxygen’s and hydrogen’s detonation, is 
the sum of the weight of both the gases. 
Watt, to whom Priestley communicated this important 
result, immediately saw in it, with the penetration of a 
superior mind, that water is not a simple body. 
He therefore wrote to his illustrious friend: “ What 
are the products of your experiments, water, light, and 
heat? Are we not then authorized from hence to con- 
clude that water is a union of oxyen and hydrogen gas, 
deprived of a portion of their latent or elementary heat ; 
that oxygen is water deprived of its hydrogen but united 
to latent heat and light ?” 
“If light be only a modification of heat, or only a cir- 
cumstance attendant on its manifestation, or a component 
part of hydrogen, oxygen gas must be water deprived of 
its hydrogen, but united to some latent heat.” 
This passage, so clear, so neat, so methodical, is ex- 
tracted from a letter by Watt, of the 26th of April, 
1783. The letter was communicated by Priestley to 
several learned men in London, and referred immediately 
after to Sir Joseph Banks, President of the Royal Society, 
to be read at a meeting of that learned society. Some 
circumstances, which I suppress because they are irrele- 
vant to our present purpose, retarded the reading by a 
