this energy; then we can force all the organs of this machine to
complete an entirely closed cycle, with the exception of the body
itself, which, however, has to return to such a state that all the
variables from which this state depends resume their initial values
except the particular variable to which the evolution of the energy
under consideration is linked. The difference between the work thus
accomplished and that which would have been obtained if this variable
also had returned to its original value, is the measure of the energy
evolved.

In the same way that, in the minds of mechanicians, all forces of
whatever origin, which are capable of compounding with each other and
of balancing each other, belong to the same category of beings, so for
many physicists energy is a sort of entity which we find under various
aspects. There thus exists for them a world, which comes in some way
to superpose itself upon the world of matter--that is to say, the
world of energy, dominated in its turn by a fundamental law similar to
that of Lavoisier.[5] This conception, as we have already seen, passes
the limit of experience; but others go further still. Absorbed in the
contemplation of this new world, they succeed in persuading themselves
that the old world of matter has no real existence and that energy is
sufficient by itself to give us a complete comprehension of the
Universe and of all the phenomena produced in it. They point out that
all our sensations correspond to changes of energy, and that
everything apparent to our senses is, in truth, energy. The famous
experiment of the blows with a stick by which it was demonstrated to a
sceptical philosopher that an outer world existed, only proves, in
reality, the existence of energy, and not that of matter. The stick in
itself is inoffensive, as Professor Ostwald remarks, and it is its
_vis viva_, its kinetic energy, which is painful to us; while if we