It is also distinct from the change which
consists in losses or gains of heat. In chemical reactions, for
example, the entropy increases without the substances borrowing any
heat. When a perfect gas dilates in a vacuum its entropy increases,
and yet the temperature does not change, and the gas has neither been
able to give nor receive heat. We thus come to conceive that a
physical phenomenon cannot be considered known to us if the variation
of entropy is not given, as are the variations of temperature and of
pressure or the exchanges of heat. The change of entropy is, properly
speaking, the most characteristic fact of a thermal change.
It is important, however, to remark that if we can thus easily define
and measure the difference of entropy between two states of the same
body, the value found depends on the state arbitrarily chosen as the
zero point of entropy; but this is not a very serious difficulty, and
is analogous to that which occurs in the evaluation of other physical
magnitudes--temperature, potential, etc.
A graver difficulty proceeds from its not being possible to define a
difference, or an equality, of entropy between two bodies chemically
different.
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