On the other hand, Priestley found that air "phlogisticated" by
combustion or respiration could be "dephlogisticated," or have the
properties of pure common air restored to it, by the action of green
plants in sunshine. The question, therefore, would naturally arise--as
common air can be wholly phlogisticated by combustion, and converted
into a substance which will no longer support combustion, is it
possible to get air that shall be less phlogisticated than common air,
and consequently support combustion better than common air does?

Now, Priestley says that, in 1774, the possibility of obtaining air
less phlogisticated than common air had not occurred to him. [6] But in
pursuing his experiments on the evolution of air from various bodies by
means of heat, it happened that, on the 1st of August 1774, he threw
the heat of the sun, by means of a large burning glass which he had
recently obtained, upon a substance which was then called _mercurius
calcinatus per se_, and which is commonly known as red precipitate.

"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
well 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 air, exposed to iron or lime of
sulphur; but as I had got nothing like this remarkable appearance
from any kind of air besides this particular modification of
nitrous air, and I knew no nitrous acid was used in the preparation
of _mercurius calcinatus_, I was utterly at a loss how to
account for it.