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 surrounds
them exactly in the same situation as the particles of bodies are with
respect to the caloric; with this difference only, that the balls are
supposed to touch each other, whereas the particles of bodies are not in
contact, being retained at a small distance from each other, by the
caloric.
If, instead of spherical balls, we substitute solid bodies of a
hexahedral, octohedral, or any other regular figure, the capacity of the
intervals between them will be lessened, and consequently will no longer
contain the same quantity of sand. The same thing takes place, with
respect to natural bodies; the intervals left between their particles
are not of equal capacity, but vary in consequence of the different
figures and magnitude of their particles, and of the distance at which
these particles are maintained, according to the existing proportion
between their inherent attraction, and the repulsive force exerted upon
them by the caloric.
In this manner we must understand the following expression, introduced
by the English philosophers, who have given us the first precise ideas
upon this subject; _the capacity of bodies for containing the matter of
heat_. As comparisons with sensible objects are of great use in
assisting us to form distinct notions of abstract ideas, we shall
endeavour to illustrate this, by instancing the phenomena which take
place between water and bodies which are wetted and penetrated by it,
with a few reflections.
If we immerge equal pieces of different kinds of wood, suppose cubes of
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