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 filled with quicksilver,
and kept inverted in a basin of the same .... With this apparatus, after
a variety of experiments.... on the 1st of August, 1774, I endeavored
to extract air from mercurius calcinatus per se; and I presently found
that, by means of this lens, air was expelled from it very readily.
Having got about three or four times as much as the bulk of my
materials, I admitted water to it, and found that it was not imbibed
by it. But what surprised me more than I can express was that a candle
burned in this air with a remarkably vigorous flame, very much like that
enlarged flame with which a candle burns in nitrous oxide, exposed to
iron or liver of sulphur; but as I had got nothing like this remarkable
appearance from any kind of air besides this particular modification of
vitrous air, and I knew no vitrous acid was used in the preparation of
mercurius calcinatus, I was utterly at a loss to account for it."(4)

The "new air" was, of course, ox