transformation--the melting of a certain mass of ice, for example--it
will be impossible, no matter how you set about it or whatever the
mechanism used, to associate this same transformation with the melting
of another weight of ice.

We can thus, in the transformation in question, obtain an appropriate
number which will sum up that which may be expected from the external
effect, and can give, so to speak, the price at which this
transformation is bought, measure its invariable value by a common
measure (for instance, the melting of the ice), and, without any
ambiguity, define the energy lost during the transformation as
proportional to the mass of ice which can be associated with it. This
measure is, moreover, independent of the particular phenomenon taken
as the common measure.


§ 3. THE PRINCIPLE OF CARNOT AND CLAUSIUS

The principle of Carnot, of a nature analogous to the principle of the
conservation of energy, has also a similar origin. It was first
enunciated, like the last named, although prior to it in time, in
consequence of considerations which deal only with heat and mechanical
work. Like it, too, it has evolved, grown, and invaded the entire
domain of physics. It may be interesting to examine rapidly the
various phases of this evolution. The origin of the principle of
Carnot is clearly determined, and it is very rare to be able to go
back thus certainly to the source of a discovery. Sadi Carnot had,
truth to say, no precursor. In his time heat engines were not yet very
common, and no one had reflected much on their theory. He was
doubtless the first to propound to himself certain questions, and