ne can be common to all phenomena.

This extreme manner of regarding things is seductive by its
originality, but appears somewhat insufficient if, after enunciating
generalities, we look more closely into the question. From the
philosophical point of view it may, moreover, seem difficult not to
conclude, from the qualities which reveal, if you will, the varied
forms of energy, that there exists a substance possessing these
qualities. This energy, which resides in one region, and which
transports itself from one spot to another, forcibly brings to mind,
whatever view we may take of it, the idea of matter.

Helmholtz endeavoured to construct a mechanics based on the idea of
energy and its conservation, but he had to invoke a second law, the
principle of least action. If he thus succeeded in dispensing with the
hypothesis of atoms, and in showing that the new mechanics gave us to
understand the impossibility of certain movements which, according to
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 ot