are exposed; or, in other words,
upon the quantity of caloric with which they are penetrated[8]. _2dly_,
That it is extremely probable that air is a fluid naturally existing in
a state of vapour; or, as we may better express it, that our atmosphere
is a compound of all the fluids which are susceptible of the vaporous
or permanently elastic state, in the usual temperature, and under the
common pressure. _3dly_, That it is not impossible we may discover, in
our atmosphere, certain substances naturally very compact, even metals
themselves; as a metallic substance, for instance, only a little more
volatile than mercury, might exist in that situation.

Amongst the fluids with which we are acquainted, some, as water and
alkohol, are susceptible of mixing with each other in all proportions;
whereas others, on the contrary, as quicksilver, water, and oil, can
only form a momentary union; and, after being mixed together, separate
and arrange themselves according to their specific gravities. The same
thing ought to, or at least may, take place in the atmosphere. It is
possible, and even extremely probable, that, both at the first creation,
and every day, gasses are formed, which are difficultly miscible with
atmospheric air, and are continually separating from it. If these gasses
be specifically lighter than the general atmospheric mass, they must, of
course, gather in the higher regions, and form strata that float upon
the common air. The phenomena which accompany igneous meteors induce me
to believe, that there exists in the upper parts of our atmosphere a
stratum of inflammable fluid in contact with those strata of air which
produce the phenomena of the aurora borealis and other fiery meteors.--I
mean hereafter to pursue this subject in a separate treatise.

FOOTNOTES:

[8] The degree of pressure which they undergo must be taken into
account. E.

CHAP. III.

_Analysis of Atmospheric Air, and its Division into two Elastic Fluids;
the one fit for Respiration, the other incapable of being respired._

From what has been premised, it follows, that our atmosphere is composed
of a mixture of every substance capable of retaining the gasseous or
aeriform state in the common temperature, and under the usual pressure
which it experiences. These fluids constitute a mass, in some measure
homogeneous, extending from the surface of the earth to the greatest
height hitherto attained, of which the density continually decreases in