the old, ought to have been but never were experimentally produced, he
was only able to do so because the principle of least action necessary
for his theory became evident in the case of those irreversible
phenomena which alone really exist in Nature. The energetists have
thus not succeeded in forming a thoroughly sound system, but their
efforts have at all events been partly successful. Most physicists are
of their opinion, that kinetic energy is only a particular variety of
energy to which we have no right to wish to connect all its other
forms.

If these forms showed themselves to be innumerable throughout the
Universe, the principle of the conservation of energy would, in fact,
lose a great part of its importance. Every time that a certain
quantity of energy seemed to appear or disappear, it would always be
permissible to suppose that an equivalent quantity had appeared or
disappeared somewhere else under a new form; and thus the principle
would in a way vanish. But the known forms of energy are fairly
restricted in number, and the necessity of recognising new ones seldom
makes itself felt. We shall see, however, that to explain, for
instance, the paradoxical properties of radium and to re-establish
concord between these properties and the principle of the conservation
of energy, certain physicists have recourse to the hypothesis that
radium borrows an unknown energy from the medium in which it is
plunged. This hypothesis, however, is in no way necessary; and in a
few other rare cases in which similar hypotheses have had to be set
up, experiment has always in the long run enabled us to discover some
phenomenon which had escaped the first observers and which corresponds
exactly to the variation of energy first made evident.

One difficulty, however, arises from the fact that the principle ought