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.

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.