engine, and that it is the same for all reversible machines working
between these two temperatures.

This is the very proposition of Carnot; but the proposition thus
stated, while very useful for the theory of engines, does not yet
present any very general interest. Clausius, however, drew from it
much more important consequences. First, he showed that the principle
conduces to the definition of an absolute scale of temperature; and
then he was brought face to face with a new notion which allows a
strong light to be thrown on the questions of physical equilibrium. I
refer to entropy.

It is still rather difficult to strip entirely this very important
notion of all analytical adornment. Many physicists hesitate to
utilize it, and even look upon it with some distrust, because they see
in it a purely mathematical function without any definite physical
meaning. Perhaps they are here unduly severe, since they often admit
too easily the objective existence of quantities which they cannot
define. Thus, for instance, it is usual almost every day to speak of
the heat possessed by a body. Yet no body in reality possesses a
definite quantity of heat even relatively to any initial state; since
starting from this point of departure, the quantities of heat it may
have gained or lost vary with the road taken and even with the means
employed to follow it. These expressions of heat gained or lost are,
moreover, themselves evidently incorrect, for heat can no longer be
considered as a sort of fluid passing from one body to another.

The real reason which makes entropy somewhat mysterious is that this
magnitude does not fall directly under the ken of any of our senses;
but it possesses the true characteristic of a concrete physical