clature, we thought it necessary to banish
all periphrastic expressions, which both lengthen physical language, and
render it more tedious and less distinct, and which even frequently does
not convey sufficiently just ideas of the subject intended. Wherefore,
we have distinguished the cause of heat, or that exquisitely elastic
fluid which produces it, by the term of _caloric_. Besides, that this
expression fulfils our object in the system which we have adopted, it
possesses this farther advantage, that it accords with every species of
opinion, since, strictly speaking, we are not obliged to suppose this to
be a real substance; it being sufficient, as will more clearly appear in
the sequel of this work, that it be considered as the repulsive cause,
whatever that may be, which separates the particles of matter from each
other; so that we are still at liberty to investigate its effects in an
abstract and mathematical manner.

In the present state of our knowledge, we are unable to determine
whether light be a modification of caloric, or if caloric be, on the
contrary, a modification of light. This, however, is indisputable, that,
in a system where only decided facts are admissible, and where we avoid,
as far as possible, to suppose any thing to be that is not really known
to exist, we ought provisionally to distinguish, by distinct terms, such
things as are known to produce different effects. We therefore
distinguish light from caloric; though we do not therefore deny that
these have certain qualities in common, and that, in certain
circumstances, they combine with other bodies almost in the same manner,
and produce, in part, the same effects.

What I have already said may suffice to determine the idea affixed to
the word _caloric_; but there remains a more difficult attempt, which
is, to give a just conception of the manner in which caloric acts upon
other bodies. Since this subtile matter penetrates through the pores of
all known substances; since there are no vessels through which it cannot
escape, and, consequently, as there are none which are capable of
retaining it, we can only come at the knowledge of its properties by
effects which are fleeting, and difficultly ascertainable. It is in
these things which we neither see nor feel, that it is especially
necessary to guard against the extravagancy of our imagination, which
forever inclines to step beyond the bounds of truth, and is very
difficultly restrained within th