If two unequally
heated solid bodies be placed in contact, a continual approximation of
both to an intermediate temperature takes place.
In the case of the two gases, a separation may be effected by chemical
means; but in the other two cases the former state of things cannot be
restored by any natural process.
In the case of the conduction or diffusion of heat the process is not
only irreversible, but it involves the irreversible diminution of that
part of the whole stock of thermal energy which is capable of being
converted into mechanical work.
This is Thomson's theory of the irreversible dissipation of energy,
and it is equivalent to the doctrine of Clausius concerning the growth
of what he calls Entropy.
The irreversible character of this process is strikingly embodied in
Fourier's theory of the conduction of heat, where the formulae
themselves indicate, for all positive values of the time, a possible
solution which continually tends to the form of a uniform diffusion of
heat.
But if we attempt to ascend the stream of time by giving to its symbol
continually diminishing values, we are led up to a state of things in
which the formula has what is called a critical value; and if we
inquire into the state of things the instant before, we find that the
formula becomes absurd.
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