Early in his scientific career Priestley began investigations
upon the "fixed air" of Dr. Black, and, oddly enough, he was
stimulated to this by the same thing that had influenced
Black--that is, his residence in the immediate neighborhood of a
brewery. It was during the course of a series of experiments on
this and other gases that he made his greatest discovery, that of
oxygen, or "dephlogisticated air," as he called it. The story of
this important discovery is probably best told in Priestley's own
words:

"There are, I believe, very few maxims in philosophy that have
laid firmer hold upon the mind than that air, meaning atmospheric
air, is a simple elementary substance, indestructible and
unalterable, at least as much so as water is supposed to be. In
the course of my inquiries I was, however, soon satisfied that
atmospheric air is not an unalterable thing; for that, according
to my first hypothesis, the phlogiston with which it becomes
loaded from bodies burning in it, and the animals breathing it,
and various other chemical processes, so far alters and depraves
it as to render it altogether unfit for inflammation,
respiration, and other purposes to which it is subservient; and I
had discovered that agitation in the water, the process of
vegetation, and probably other natural processes, restore it to
its original purity....

"Having procured a lens of twelve inches diameter and twenty
inches local distance, I proceeded with the greatest alacrity, by
the help of it, to discover what kind of air a great variety of
substances would yield, putting them into the vessel, which I