ishing this article, to say a few words relative
to the cause of the elasticity of gasses, and of fluids in the state of
vapour. It is by no means difficult to perceive that this elasticity
depends upon that of caloric, which seems to be the most eminently
elastic body in nature. Nothing is more readily conceived, than that one
body should become elastic by entering into combination with another
body possessed of that quality. We must allow that this is only an
explanation of elasticity, by an assumption of elasticity, and that we
thus only remove the difficulty one step farther, and that the nature of
elasticity, and the reason for caloric being elastic, remains still
unexplained. Elasticity in the abstract is nothing more than that
quality of the particles of bodies by which they recede from each other
when forced together. This tendency in the particles of caloric to
separate, takes place even at considerable distances. We shall be
satisfied of this, when we consider that air is susceptible of
undergoing great compression, which supposes that its particles were
previously very distant from each other; for the power of approaching
together certainly supposes a previous distance, at least equal to the
degree of approach. Consequently, those particles of the air, which are
already considerably distant from each other, tend to separate still
farther. In fact, if we produce Boyle's vacuum in a large receiver, the
very last portion of air which remains spreads itself uniformly through
the whole capacity of the vessel, however large, fills it completely
throughout, and presses every where against its sides: We cannot,
however, explain this effect, without supposing that the particles make
an effort to separate themselves on every side, and we are quite
ignorant at what distance, or what degree of rarefaction, this effort
ceases to act.

Here, therefore, exists a true repulsion between the particles of
elastic fluids; at least, circumstances take place exactly as if such a
repulsion actually existed; and we have very good right to conclude,
that the particles of caloric mutually repel each other. When we are
once permitted to suppose this repelling force, the _rationale_ of the
formation of gasses, or aeriform fluids, becomes perfectly simple; tho'
we must, at the same time, allow, that it is extremely difficult to form
an accurate conception of this repulsive force acting upon very minute
particles placed at great distances