um of entropy till the
moment when there should be produced a slow evolution in the contrary
direction bringing it back to the state from which it started. Thus,
in the infinity of time, the life of the Universe proceeds without
real stop.

This conception is, moreover, in accordance with the view certain
physicists take of the principle of Carnot. We shall see, for example,
that in the kinetic theory we are led to admit that, after waiting
sufficiently long, we can witness the return of the various states
through which a mass of gas, for example, has passed in its series of
transformations.

If we keep to the present era, evolution has a fixed direction--that
which leads to an increase of entropy; and it is possible to enquire,
in any given system to what physical manifestations this increase
corresponds. We note that kinetic, potential, electrical, and chemical
forms of energy have a great tendency to transform themselves into
calorific energy. A chemical reaction, for example, gives out energy;
but if the reaction is not produced under very special conditions,
this energy immediately passes into the calorific form. This is so
true, that chemists currently speak of the heat given out by reactions
instead of regarding the energy disengaged in general.

In all these transformations the calorific energy obtained has not,
from a practical point of view, the same value at which it started.
One cannot, in fact, according to the principle of Carnot, transform
it integrally into mechanical energy, since the heat possessed by a
body can only yield work on condition that a part of it falls on a
body with a lower temperature. Thus appears the idea that energies
which exchange with each other and correspond to equal quantities have
not the same qualitative value. Form has its importance, and there are
persons who prefer a golden louis to four pieces of five francs. The
principle of Carnot would thus lead us to consider a certain
classification of energies, and would show us that, in the
transformations possible, these energies always tend to a sort of
diminution of quality--that is, to a _degradation_.

It would thus reintroduce an element of differentiation of which it
seems very difficult to give a mechanical explanation. Certain
philosophers and physicists see in this fact a reason which condemns
_a priori_ all attempts made to give a mechanical explanation of the
principle of Carnot.

It is right, however, not to exagg