Just as, in
elementary geometry, we can replace the postulate of Euclid by other
equivalent propositions, so the postulate of thermodynamics is not
necessarily fixed, and it is instructive to try to give it the most
general and suggestive character.
MM. Perrin and Langevin have made a successful attempt in this
direction. M. Perrin enunciates the following principle: _An isolated
system never passes twice through the same state_. In this form, the
principle affirms that there exists a necessary order in the
succession of two phenomena; that evolution takes place in a
determined direction. If you prefer it, it may be thus stated: _Of two
converse transformations unaccompanied by any external effect, one
only is possible_. For instance, two gases may diffuse themselves one
in the other in constant volume, but they could not conversely
separate themselves spontaneously.
Starting from the principle thus put forward, we make the logical
deduction that one cannot hope to construct an engine which should
work for an indefinite time by heating a hot source and by cooling a
cold one.
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