theory of the facts which he discovered, or assisting in their rational
explanation, his influence to the end of his life was warmly exerted in
favour of error. From first to last, he was a stiff adherent of the
phlogiston doctrine which was prevalent when his studies commenced;
and, by a curious irony of fate, the man who by the discovery of what
he called "dephlogisticated air" furnished the essential datum for the
true theory of combustion, of respiration, and of the composition of
water, to the end of his days fought against the inevitable corollaries
from his own labours. His last scientific work, published in 1800,
bears the title, "The Doctrine of Phlogiston established, and that of
the Composition of Water refuted."

When Priestley commenced his studies, the current belief was, that
atmospheric air, freed from accidental impurities, is a simple
elementary substance, indestructible and unalterable, as water was
supposed to be. When a combustible burned, or when an animal breathed
in air, it was supposed that a substance, "phlogiston," the matter of
heat and light, passed from the burning or breathing body into it, and
destroyed its powers of supporting life and combustion. Thus, air
contained in a vessel in which a lighted candle had gone out, or a
living animal had breathed until it could breathe no longer, was called
"phlogisticated." The same result was supposed to be brought about by
the addition of what Priestley called "nitrous gas" to common air.

In the course of his researches, Priestley found that the quantity of
common air which can thus become "phlogisticated," amounts to about
one-fifth the volume of the whole quantity submitted to experiment.
Hence it appeared that common air consists, to the extent of
four-fifths of its volume, of air which is already "phlogisticated";
while the other fifth is free from phlogiston, or "dephlogisticated."