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
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.
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