erate the importance that should be
attributed to the phrase degraded energy. If the heat is not
equivalent to the work, if heat at 99 deg. is not equivalent to heat at
100 deg., that means that we cannot in practice construct an engine which
shall transform all this heat into work, or that, for the same cold
source, the output is greater when the temperature of the hot source
is higher; but if it were possible that this cold source had itself
the temperature of absolute zero, the whole heat would reappear in the
form of work. The case here considered is an ideal and extreme case,
and we naturally cannot realize it; but this consideration suffices to
make it plain that the classification of energies is a little
arbitrary and depends more, perhaps, on the conditions in which
mankind lives than on the inmost nature of things.

In fact, the attempts which have often been made to refer the
principle of Carnot to mechanics have not given convincing results. It
has nearly always been necessary to introduce into the attempt some
new hypothesis independent of the fundamental hypotheses of ordinary
mechanics, and equivalent, in reality, to one of the postulates on
which the ordinary exposition of the second law of thermodynamics is
founded. Helmholtz, in a justly celebrated theory, endeavoured to fit
the principle of Carnot into the principle of least action; but the
difficulties regarding the mechanical interpretation of the
irreversibility of physical phenomena remain entire. Looking at the
question, however, from the point of view at which the partisans of
the kinetic theories of matter place themselves, the principle is
viewed in a new aspect. Gibbs and afterwards Boltzmann and Professor
Planck have put forward some very interesting ideas on this subject.
By following the route they have traced, we come to consider the
principle as pointing out to us that a given system tends towards the
configuration presented by the maximum probability, and, numerically,
the entropy would even be the logarithm of this probability. Thus two
different gaseous masses, enclosed in two separate receptacles which
have just been placed in communication, diffuse themselves one through
the other, and it is highly improbable that, in their mutual shocks,
both kinds of molecules should take a distribution of velocities which
reduce them by a spontaneous phenomenon to the initial state.

We should have to wait a very long time for so extraordinary a