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We should have to wait a very long time for so extraordinary a
concourse of circumstances, but, in strictness, it would not be
impossible. The principle would only be a law of probability. Yet this
probability is all the greater the more considerable is the number of
molecules itself. In the phenomena habitually dealt with, this number
is such that, practically, the variation of entropy in a constant
sense takes, so to speak, the character of absolute certainty.
But there may be exceptional cases where the complexity of the system
becomes insufficient for the application of the principle of Carnot;--
as in the case of the curious movements of small particles suspended
in a liquid which are known by the name of Brownian movements and can
be observed under the microscope. The agitation here really seems, as
M. Gouy has remarked, to be produced and continued indefinitely,
regardless of any difference in temperature; and we seem to witness
the incessant motion, in an isothermal medium, of the particles which
constitute matter.


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