the regions of plain
fact, may, if they please, proceed no farther, that their delicacy be
not offended.

In extenuation of my offence, let it, however, be considered, that
_theory_ and _experiment_ necessarily go hand in hand, every process
being intended to ascertain some particular _hypothesis_, which, in
fact, is only a conjecture concerning the circumstances or the cause of
some natural operation; consequently that the boldest and most original
experimenters are those, who, giving free scope to their imaginations,
admit the combination of the most distant ideas; and that though many of
these associations of ideas, will be wild and chimerical, yet that
others will have the chance of giving rise to the greatest and most
capital discoveries; such as very cautious, timid, sober, and
slow-thinking people would never have come at.

Sir Isaac Newton himself, notwithstanding the great advantage which he
derived from a habit of _patient thinking_, indulged bold and excentric
thoughts, of which his Queries at the end of his book of Optics are a
sufficient evidence. And a quick conception of distant analogies, which
is the great key to unlock the secret of nature, is by no means
incompatible with the spirit of _perseverance_, in investigations
calculated to ascertain and pursue those analogies.

Sec. 1. _Speculations concerning the CONSTITUENT PRINCIPLES of the
different kinds of AIR, and the CONSTITUTION and ORIGIN of the
ATMOSPHERE, &c._

All the kinds of air that appear to me to be essentially distinct from
each other are _fixed air_, _acid_ and _alkaline_; for these, and
another principle, called _phlogiston_, which I have not been able to
exhibit in the form of _air_, and which has never yet been exhibited by
itself in _any form_, seem to constitute all the kinds of air that I am
acquainted with.

Acid air and phlogiston constitute an air which either extinguishes
flame, or is itself inflammable, according, probably, to the quantity of
phlogiston combined in it, or the mode of combination. When it
extinguishes flame, it is probably so much charged with the phlogistic
matter, as to take no more from a burning candle, which must, therefore,
necessarily go out in it. When it is inflammable, it is probably so much
charged with phlogiston, that the heat communicated by a burning candle
makes it immediately separate itself from the other principle with which
it was united, in which separation _heat_ is produced, as in