." What
Newton had done for light Watt was held to have done for water.
Muirfield well says:

It is interesting in a high degree to remark that for him who
had so fully subdued to the use of man the gigantic power of
steam it was also reserved to unfold its compound natural and
elemental principles, as if on this subject there were to be
nothing which his researches did not touch, nothing which they
touched that they did not adorn.

Arago says:

In his memoir of the month of April, Priestley added an
important circumstance to those resulting from the experiments
of his predecessors: he proved that the weight of the water
which is deposited upon the sides of the vessel, at the instant
of the detonation of the oxygen and hydrogen, is precisely the
same as the weights of the two gases.

Watt, to whom Priestley communicated this important result, immediately
perceived that proof was here afforded that water was not a simple body.
Writing to his illustrious friend, he asks:

What are the products of your experiment? They are _water_,
_light_ and _heat_. Are we not, thence, authorised to conclude
that water is a compound of the two gases, oxygen and hydrogen,
deprived of a portion of their latent or elementary heat; that
oxygen is water deprived of its hydrogen, but still united to
its latent heat and light? If light be only a modification of
heat, or a simple circumstance of its manifestation, or a
component part of hydrogen, oxygen gas will be water deprived of
its hydrogen, but combined with latent heat.

This passage, so clear, so precise, and logical, is taken from a letter
of Watt's, dated April 26, 1783. The letter was communicated by
Priestley to several of the scientific men in London, and was
transmitted immediately afterward to Sir Joseph Banks, the President of
the Royal Society, to be read at one of the meetings of that learned
body.

Watt had for many years entertained the opinion that air was a
modification of water. He writes Boulton, December 10, 1782:

You may remember that I have often said, that if water could be
heated red-hot or something more, it would probably be converted
into some kind of air, because steam would in that case have
lost all its latent heat, and that it would have been turned
solely into sensible heat, and probably a total change of the
nature of the fluid would ensue