.

A month after he hears of Priestley's experiments, he writes Dr. Black
(April 21, 1783) that he "believes he has found out the cause of the
conversion of water into air." A few days later, he writes to Dr.
Priestley:

In the deflagration of the inflammable and dephlogisticated
airs, the airs unite with violence--become red-hot--and, on
cooling, totally disappear. The only fixed matter which remains
is _water_; and _water_, _light_, and _heat_, are all the
products. Are we not then authorised to conclude that water is
composed of dephlogisticated and inflammable air, or phlogiston,
deprived of part of their latent heat; and that
dephlogisticated, or pure air, is composed of water deprived of
its phlogiston, and united to heat and light; and if light be
only a modification of heat, or a component part of phlogiston,
then pure air consists of water deprived of its phlogiston and
of latent heat?

It appears from the letter to Dr. Black of April 21st, that Mr. Watt
had, on that day, written his letter to Dr. Priestley, to be read by him
to the Royal Society, but on the 26th he informs Mr. DeLuc, that having
observed some inaccuracies of style in that letter, he had removed them,
and would send the Doctor a corrected copy in a day or two, which he
accordingly did on the 28th; the corrected letter (the same that was
afterward embodied verbatim in the letter to Mr. DeLuc, printed in the
Philosophical Transactions), being dated April 26th. In enclosing it,
Mr. Watt adds, "As to myself, the more I consider what I have said, I am
the more satisfied with it, as I find none of the facts repugnant."

Thus was announced for the first time one of the most wonderful
discoveries recorded in the history of science, startling in its novelty
and yet so simple.

Watt had divined the import of Priestley's experiment, for he had
mastered all knowledge bearing upon the question, but even when this was
communicated to Priestley, he could not accept it, and, after making new
experiments, he writes Watt, April 29, 1783, "Behold with surprise and
indignation the figure of an apparatus that has utterly ruined your
beautiful hypothesis," giving a rough sketch with his pen of the
apparatus employed. Mark the promptitude of the master who had
deciphered the message which the experimenter himself could not
translate. He immediately writes in reply May 2, 1783:

I deny that your experiment ru