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.... The extreme tenacity and
the surprising mobility of its molecules are manifestly shown by the
ease with which it penetrates into the most compact bodies and by its
tendency to put itself in equilibrium throughout all bodies near to
it."

It must be acknowledged, however, that the idea of Lavoisier and
Laplace was rather vague and even inexact on one important point. They
admitted it to be evident that "all variations of heat, whether real
or apparent, undergone by a bodily system when changing its state, are
produced in inverse order when the system passes back to its original
state." This phrase is the very denial of equivalence where these
changes of state are accompanied by external work.

Laplace, moreover, himself became later a very convinced partisan of
the hypothesis of the material nature of caloric, and his immense
authority, so fortunate in other respects for the development of
science, was certainly in this case the cause of the retardation of
progress.

The names of Young, Rumford, Davy, are often quoted among those
physicists who, at the commencement of the nineteenth century, caught
sight of the new truths as to the nature of heat. To these names is
very properly added that of Sadi Carnot. A note found among his papers
unquestionably proves that, before 1830, ideas had occurred to him