". . . I had so little suspicion of the air from the mercurius
calcinatus, etc., being wholesome, that I had not even thought of
applying it to the test of nitrous air; but thinking (as my
reader must imagine I frequently must have done) on the candle
burning in it after long agitation in water, it occurred to me at
last to make the experiment; and, putting one measure of nitrous
air to two measures of this air, I found not only that it was
diminished, but that it was diminished quite as much as common
air, and that the redness of the mixture was likewise equal to a
similar mixture of nitrous and common air.... The next day I was
more surprised than ever I had been before with finding that,
after the above-mentioned mixture of nitrous air and the air from
mercurius calcinatus had stood all night, . . . a candle burned
in it, even better than in common air."

A little later Priestley discovered that "dephlogisticated air .
. . is a principal element in the composition of acids, and may
be extracted by means of heat from many substances which contain
them.... It is likewise produced by the action of light upon
green vegetables; and this seems to be the chief means employed
to preserve the purity of the atmosphere."

This recognition of the important part played by oxygen in the
atmosphere led Priestley to make some experiments upon mice and
insects, and finally upon himself, by inhalations of the pure
gas. "The feeling in my lungs," he said, "was not sensibly
different from that of common air, but I fancied that my
breathing felt peculiarly light and easy for some time
afterwards. Who can tell but that in time this pure air may
become a fashionable article in luxury? . . . Perhaps we may from