of natural knowledge to one of those points at which we must accept
the guidance of that faith by which we understand that "that which is
seen was not made of things which do appear."

One of the most remarkable results of the progress of molecular
science is the light it has thrown on the nature of irreversible
processes--processes, that is, which always tend towards and never
away from a certain limiting state. Thus, if two gases be put into
the same vessel, they become mixed, and the mixture tends continually
to become more uniform. If two unequally heated portions of the same
gas are put into the vessel, something of the kind takes place, and
the whole tends to become of the same temperature. 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