nciple thus put forward, we make the logical
deduction that one cannot hope to construct an engine which should
work for an indefinite time by heating a hot source and by cooling a
cold one. We thus come again into the route traced by Clausius, and
from this point we may follow it strictly.

Whatever the point of view adopted, whether we regard the proposition
of M. Perrin as the corollary of another experimental postulate, or
whether we consider it as a truth which we admit _a priori_ and verify
through its consequences, we are led to consider that in its entirety
the principle of Carnot resolves itself into the idea that we cannot
go back along the course of life, and that the evolution of a system
must follow its necessary progress.

Clausius and Lord Kelvin have drawn from these considerations certain
well-known consequences on the evolution of the Universe. Noticing
that entropy is a property added to matter, they admit that there is
in the world a total amount of entropy; and as all real changes which
are produced in any system correspond to an increase of entropy, it
may be said that the entropy of the world is continually increasing.
Thus the quantity of energy existing in the Universe remains constant,
but transforms itself little by little into heat uniformly distributed
at a temperature everywhere identical. In the end, therefore, there
will be neither chemical phenomena nor manifestation of life; the
world will still exist, but without motion, and, so to speak, dead.

These consequences must be admitted to be very doubtful; we cannot in
any certain way apply to the Universe, which is not a finite system, a
proposition demonstrated, and that not unreservedly, in the sharply
limited case of a finite system. Herbert Spencer, moreover, in his
book on _First Principles_, brings out with much force the idea that,
even if the Universe came to an end, nothing would allow us to
conclude that, once at rest, it would remain so indefinitely. We may
recognise that the state in which we are began at the end of a former
evolutionary period, and that the end of the existing era will mark
the beginning of a new one.

Like an elastic and mobile object which, thrown into the air, attains
by degrees the summit of its course, then possesses a zero velocity
and is for a moment in equilibrium, and then falls on touching the
ground to rebound, so the world should be subjected to huge
oscillations which first bring it to a maxim