.
Immediately after withdrawing the apparatus from the boiler, the vapour
in the jar began to condense, and the mercury rose to its former
station; but it returned again to the aeriform state a few seconds after
replacing the apparatus in the boiler.

We have thus a certain number of substances, which are convertible into
elastic aeriform fluids by degrees of temperature, not much superior to
that of our atmosphere. We shall afterwards find that there are several
others which undergo the same change in similar circumstances, such as
muriatic or marine acid, ammoniac or volatile alkali, the carbonic acid
or fixed air, the sulphurous acid, &c. All of these are permanently
elastic in or about the mean temperature of the atmosphere, and under
its common pressure.

All these facts, which could be easily multiplied if necessary, give me
full right to assume, as a general principle, that almost every body in
nature is susceptible of three several states of existence, solid,
liquid, and aeriform, and that these three states of existence depend
upon the quantity of caloric combined with the body. Henceforwards I
shall express these elastic aeriform fluids by the generic term _gas_;
and in each species of gas I shall distinguish between the caloric,
which in some measure serves the purpose of a solvent, and the
substance, which in combination with the caloric, forms the base of the
gas.

To these bases of the different gases, which are hitherto but little
known, we have been obliged to assign names; these I shall point out in
Chap. IV. of this work, when I have previously given an account of the
phenomena attendant upon the heating and cooling of bodies, and when I
have established precise ideas concerning the composition of our
atmosphere.

We have already shown, that the particles of every substance in nature
exist in a certain state of equilibrium, between that attraction which
tends to unite and keep the particles together, and the effects of the
caloric which tends to separate them. Hence the caloric not only
surrounds the particles of all bodies on every side, but fills up every
interval which the particles of bodies leave between each other. We may
form an idea of this, by supposing a vessel filled with small spherical
leaden bullets, into which a quantity of fine sand is poured, which,
insinuating into the intervals between the bullets, will fill up every
void. The balls, in this comparison, are to the sand which sur