nature of things. Three years previously, Rankine also had put forth
some very remarkable ideas the full meaning of which was not at first
well understood. He it was who comprehended the utility of employing a
more inclusive term, and invented the phrase energetics. He also
endeavoured to create a new doctrine of which rational mechanics
should be only a particular case; and he showed that it was possible
to abandon the ideas of atoms and central forces, and to construct a
more general system by substituting for the ordinary consideration of
forces that of the energy which exists in all bodies, partly in an
actual, partly in a potential state.

By giving more precision to the conceptions of Rankine, the physicists
of the end of the nineteenth century were brought to consider that in
all physical phenomena there occur apparitions and disappearances
which are balanced by various energies. It is natural, however, to
suppose that these equivalent apparitions and disappearances
correspond to transformations and not to simultaneous creations and
destructions. We thus represent energy to ourselves as taking
different forms--mechanical, electrical, calorific, and chemical--
capable of changing one into the other, but in such a way that the
quantitative value always remains the same. In like manner a bank
draft may be represented by notes, gold, silver, or bullion. The
earliest known form of energy, _i.e._ work, will serve as the standard
as gold serves as the monetary standard, and energy in all its forms
will be estimated by the corresponding work. In each particular case
we can strictly define and measure, by the correct application of the
principle of the conservation of energy, the quantity of energy
evolved under a given form.

We can thus arrange a machine comprising a body capable of evolving