... In the system we are now
examining, heat is the _vis viva_ resulting from the insensible
movements of the molecules of a body; it is the sum of the products of
the mass of each molecule by the square of its velocity.... We shall
not decide between the two preceding hypotheses; several phenomena
seem to support the last mentioned--for instance, that of the heat
produced by the friction of two solid bodies. But there are others
which are more simply explained by the first, and perhaps they both
operate at once." Most of the physicists of that period, however, did
not share the prudent doubts of Lavoisier and Laplace. They admitted,
without hesitation, the first hypothesis; and, four years after the
appearance of the _Memoire sur la Chaleur_, Sigaud de Lafond, a
professor of physics of great reputation, wrote: "Pure Fire, free from
all state of combination, seems to be an assembly of particles of a
simple, homogeneous, and absolutely unalterable matter, and all the
properties of this element indicate that these particles are
infinitely small and free, that they have no sensible cohesion, and
that they are moved in every possible direction by a continual and
rapid motion which is essential to them.
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