We are unable, in fact, to pass by any means, reversible or
not, from one to the other, so long as the transmutation of matter is
regarded as impossible; but it is well understood that it is
nevertheless possible to compare the variations of entropy to which
these two bodies are both of them individually subject.
Neither must we conceal from ourselves that the definition supposes,
for a given body, the possibility of passing from one state to another
by a reversible transformation. Reversibility is an ideal and extreme
case which cannot be realized, but which can be approximately attained
in many circumstances. So with gases and with perfectly elastic
bodies, we effect sensibly reversible transformations, and changes
of physical state are practically reversible. The discoveries of
Sainte-Claire Deville have brought many chemical phenomena into a
similar category, and reactions such as solution, which used to be
formerly the type of an irreversible phenomenon, may now often be
effected by sensibly reversible means. Be that as it may, when once the
definition is admitted, we arrive, by taking as a basis the principles
set forth at the inception, at the demonstration of the celebrated
theorem of Clausius: _The entropy of a thermally isolated system
continues to increase incessantly.
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