to which they are exposed.
It is difficult to comprehend these phenomena, without admitting them as
the effects of a real and material substance, or very subtile fluid,
which, insinuating itself between the particles of bodies, separates
them from each other; and, even allowing the existence of this fluid to
be hypothetical, we shall see in the sequel, that it explains the
phenomena of nature in a very satisfactory manner.
This substance, whatever it is, being the cause of heat, or, in other
words, the sensation which we call _warmth_ being caused by the
accumulation of this substance, we cannot, in strict language,
distinguish it by the term _heat_; because the same name would then very
improperly express both cause and effect. For this reason, in the memoir
which I published in 1777[3], I gave it the names of _igneous fluid_ and
_matter of heat_. And, since that time, in the work[4] published by Mr
de Morveau, Mr Berthollet, Mr de Fourcroy, and myself, upon the
reformation of chemical nomenclature, 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
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