It is well known that the conservation of energy was, at first,
regarded from the point of view of the reciprocal transformations
between heat and work, and that the principle received its first clear
enunciation in the particular case of the principle of equivalence. It
is, therefore, rightly considered that the scholars who were the first
to doubt the material nature of caloric were the precursors of R.
Mayer; their ideas, however, were the same as those of the celebrated
German doctor, for they sought especially to demonstrate that heat was
a mode of motion.

Without going back to early and isolated attempts like those of Daniel
Bernoulli, who, in his hydrodynamics, propounded the basis of the
kinetic theory of gases, or the researches of Boyle on friction, we
may recall, to show how it was propounded in former times, a rather
forgotten page of the _Mémoire sur la Chaleur_, published in 1780 by
Lavoisier and Laplace: "Other physicists," they wrote, after setting
out the theory of caloric, "think that heat is nothing but the result
of the insensible vibrations of matter.... 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 _Mémoire sur la Chaleur_, Sigaud de Lafond, a
professor of physics of great reputation, wrote: "Pure Fire, free from