Five of Maxwells Papers | Page 9

or it might be enlarged by a layer of meteorites falling on it, or
its rate of revolution might slowly slacken, and yet it would continue to
be as much a planet as before.
But a molecule, say of hydrogen, if either its mass or its time of
vibration were to be altered in the least, would no longer be a molecule
of hydrogen.
If, then, we wish to obtain standards of length, time, and mass which
shall be absolutely permanent, we must seek them not in the
dimensions, or the motion, or the mass of our planet, but in the
wave-length, the period of vibration, and the absolute mass of these
imperishable and unalterable and perfectly similar molecules.
When we find that here, and in the starry heavens, there are
innumerable multitudes of little bodies of exactly the same mass, so
many, and no more, to the grain, and vibrating in exactly the same time,
so many times, and no more, in a second, and when we reflect that no
power in nature can now alter in the least either the mass or the period
of any one of them, we seem to have advanced along the path 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
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.
We thus arrive at the conception of a state of things which cannot be
conceived as the physical result of a previous state of things, and we

find that this critical condition actually existed at an epoch not in the
utmost depths of a past eternity, but separated from the present time by
a finite interval.
This idea of a beginning is one which the physical researches of recent
times have brought home to us, more than any observer of the course of
scientific thought in former times would have had reason to expect.
But the mind of man is not, like Fourier's heated body, continually
settling down into an ultimate state of quiet uniformity, the character of
which we can already predict; it is rather like a tree, shooting out
branches which adapt themselves to the new aspects of the sky towards
which they climb, and roots which contort themselves among the
strange strata of the earth into which they delve. To us who breathe
only the spirit of our own age, and know only the characteristics of
contemporary thought, it is as impossible to predict the general tone of
the science of the future as it is to anticipate the particular discoveries
which it will make.
Physical research is continually revealing to us new features of natural
processes, and we are thus compelled to search for new forms of
thought appropriate to these features. Hence the importance of a careful
study of those relations between mathematics and Physics which
determine the conditions under