ral
philosophy, to be informed of those facts which had been discovered by
others, before I turned my thoughts to the subject; which suggested, and
by the help of which I was enabled to pursue, my inquiries. Let it be
observed, however, that I do not profess to recite in this place _all_
that had been discovered concerning air, but only those discoveries the
knowledge of which is necessary, in order to understand what I have done
myself; so that any person who is only acquainted with the general
principles of natural philosophy, may be able to read this treatise,
and, with proper attention, to understand every part of it.

That the air which constitutes the atmosphere in which we live has
_weight_, and that it is _elastic_, or consists of a compressible and
dilatable fluid, were some of the earliest discoveries that were made
after the dawning of philosophy in this western part of the world.

That elastic fluids, differing essentially from the air of the
atmosphere, but agreeing with it in the properties of weight,
elasticity, and transparency, might be generated from solid substances,
was discovered by Mr. Boyle, though two remarkable kinds of factitious
air, at least the effects of them, had been known long before to all
miners. One of these is heavier than common air. It lies at the bottom
of pits, extinguishes candles, and kills animals that breathe it, on
which account it had obtained the name of the _choke damp_. The other is
lighter than common air, taking its place near the roofs of
subterraneous places, and because it is liable to take fire, and
explode, like gunpowder, it had been called the _fire damp_. The word
_damp_ signifies _vapour_ or _exhalation_ in the German and Saxon
language.

Though the former of these kinds of air had been known to be noxious,
the latter I believe had not been discovered to be so, having always
been found in its natural state, so much diluted with common air, as to
be breathed with safety. Air of the former kind, besides having been
discovered in various caverns, particularly the _grotta del Cane_ in
Italy, had also been observed on the surface of fermenting liquors, and
had been called _gas_ (which is the same with _geist_, or _spirit_) by
Van Helmont, and other German chymists; but afterwards it obtained the
name of _fixed air_, especially after it had been discovered by Dr.
Black of Edinburgh to exist, in a fixed state, in alkaline salts, chalk,
and other calcareous substan