Consider an isothermal transformation. Instead of leaving the heat
abandoned by the body subjected to the transformation--water
condensing in a state of saturated vapour, for instance--to pass
directly into an ice calorimeter, we can transmit this heat to the
calorimeter by the intermediary of a reversible Carnot engine. The
engine having absorbed this quantity of heat, will only give back to
the ice a lesser quantity of heat; and the weight of the melted ice,
inferior to that which might have been directly given back, will serve
as a measure of the isothermal transformation thus effected. It can be
easily shown that this measure is independent of the apparatus used.
It consequently becomes a numerical element characteristic of the body
considered, and is called its entropy. Entropy, thus defined, is a
variable which, like pressure or volume, might serve concurrently with
another variable, such as pressure or volume, to define the state of a
body.
It must be perfectly understood that this variable can change in an
independent manner, and that it is, for instance, distinct from the
change of temperature.
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