e of
being imbibed by some substance or other, and consequently of being
_fixed_ in them, after they have been in an elastic state.

The term _mephitic_ is equally applicable to what is called _fixed air_,
to that which is _inflammable_, and to many other kinds; since they are
equally noxious, when breathed by animals. Rather, however, than either
introduce new terms, or change the signification of old ones, I shall
use the term _fixed air_, in the sense in which it is now commonly used,
and distinguish the other kinds by their properties, or some other
periphrasis. I shall be under a necessity, however, of giving names to
those kinds of air, to which no names had been given by others, as
_nitrous_, _acid_, and _alkaline_.

SECTION I.

_Of FIXED AIR._

It was in consequence of living for some time in the neighbourhood of a
public brewery, that I was induced to make experiments on fixed air, of
which there is always a large body, ready formed, upon the surface of
the fermenting liquor, generally about nine inches, or a foot in depth,
within which any kind of substance may be very conveniently placed; and
though, in these circumstances, the fixed air must be continually mixing
with the common air, and is therefore far from being perfectly pure, yet
there is a constant fresh supply from the fermenting liquor, and it is
pure enough for many purposes.

A person, who is quite a stranger to the properties of this kind of air,
would be agreeably amused with extinguishing lighted candles, or chips
of wood in it, as it lies upon the surface of the fermenting liquor; for
the smoke readily unites with this kind of air, probably by means of the
water which it contains; so that very little or none of the smoke will
escape into the open air, which is incumbent upon it. It is remarkable,
that the upper surface of this smoke, floating in the fixed air, is
smooth, and well defined; whereas the lower surface is exceedingly
ragged, several parts hanging down to a considerable distance within the
body of the fixed air, and sometimes in the form of balls, connected to
the upper stratum by slender threads, as if they were suspended. The
smoke is also apt to form itself into broad flakes, parallel to the
surface of the liquor, and at different distances from it, exactly like
clouds. These appearances will sometimes continue above an hour, with
very little variation. When this fixed air is very strong, the smoke of
a small quantit