No doubt what Priestley's friends repeatedly urged upon him--that he
would have escaped the heavier trials of his life and done more for the
advancement of knowledge, if he had confined himself to his scientific
pursuits and let his fellow-men go their way--was true. But it seems to
have been Priestley's feeling that he was a man and a citizen before he
was a philosopher, and that the duties of the two former positions are
at least as imperative as those of the latter. Moreover, there are men
(and I think Priestley was one of them) to whom the satisfaction of
throwing down a triumphant fallacy is as great as that which attends
the discovery of a new truth; who feel better satisfied with the
government of the world, when they have been helping Providence by
knocking an imposture on the head; and who care even more for freedom
of thought than for mere advance of knowledge. These men are the
Carnots who organise victory for truth, and they are, at least, as
important as the generals who visibly fight her battles in the field.

Priestley's reputation as a man of science rests upon his numerous and
important contributions to the chemistry of gaseous bodies; and to form
a just estimate of the value of his work--of the extent to which it
advanced the knowledge of fact and the development of sound theoretical
views--we must reflect what chemistry was in the first half of the
eighteenth century.

The vast science which now passes under that name had no existence.
Air, water, and fire were still counted among the elemental bodies; and
though Van Helmont, a century before, had distinguished different
kinds of air as _gas ventosum_ and _gas sylvestre_, and Boyle and Hales
had experimentally defined the physical properties of air, and
discriminated some of the various kinds of aeriform bodies, no one