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
from which it resulted that in producing work an equivalent amount of
heat was destroyed. But the year 1842 is particularly memorable in the
history of science as the year in which Jules Robert Mayer succeeded,
by an entirely personal effort, in really enunciating the principle of
the conservation of energy. Chemists recall with just pride that the
_Remarques sur les forces de la nature animee_, contemptuously
rejected by all the journals of physics, were received and published
in the _Annalen_ of Liebig. We ought never to forget this example,
which shows with what difficulty a new idea contrary to the classic
theories of the period succeeds in coming to the front; but
extenuating circumstances may be urged on behalf of the physicists.

Robert Mayer had a rather insufficient mathematical education, and his
Memoirs, the _Remarques_, as well as the ulterior publications,
_Memoire sur le mouvement organique et la nutrition_ and the
_Materiaux pour la dynamique du ciel_, contain, side by side with very
profound ideas, evident errors in mechanics. Thus it often happens
that discoveries put forward in a somewhat vague manner by adventurous
minds not overburdened by the heavy baggage of scientific erudition,
who audaciously press forward in advance of their time, fall into
quite intelligible oblivion until rediscovered, clarified, and put
into shape by slower but surer seekers. This was the case with the
ideas of Mayer. They were not understood at first sight, not only on
account of their originality, but also because they were couched in
incorrect language.

Mayer was, however, endowed with a singular strength of thought; he
expressed in a rather confused mann