Five of Maxwells Papers | Page 7

we are
accustomed to argue as if such substances as air, water, or metal, which
appear to our senses uniform and continuous, were strictly and
mathematically uniform and continuous.
We know that we can divide a pint of water into many millions of
portions, each of which is as fully endowed with all the properties of
water as the whole pint was; and it seems only natural to conclude that

we might go on subdividing the water for ever, just as we can never
come to a limit in subdividing the space in which it is contained. We
have heard how Faraday divided a grain of gold into an inconceivable
number of separate particles, and we may see Dr Tyndall produce from
a mere suspicion of nitrite of butyle an immense cloud, the minute
visible portion of which is still cloud, and therefore must contain many
molecules of nitrite of butyle.
But evidence from different and independent sources is now crowding
in upon us which compels us to admit that if we could push the process
of subdivision still further we should come to a limit, because each
portion would then contain only one molecule, an individual body, one
and indivisible, unalterable by any power in nature.
Even in our ordinary experiments on very finely divided matter we find
that the substance is beginning to lose the properties which it exhibits
when in a large mass, and that effects depending on the individual
action of molecules are beginning to become prominent.
The study of these phenomena is at present the path which leads to the
development of molecular science.
That superficial tension of liquids which is called capillary attraction is
one of these phenomena. Another important class of phenomena are
those which are due to that motion of agitation by which the molecules
of a liquid or gas are continually working their way from one place to
another, and continually changing their course, like people hustled in a
crowd.
On this depends the rate of diffusion of gases and liquids through each
other, to the study of which, as one of the keys of molecular science,
that unwearied inquirer into nature's secrets, the late Prof. Graham,
devoted such arduous labour.
The rate of electrolytic conduction is, according to Wiedemann's theory,
influenced by the same cause; and the conduction of heat in fluids
depends probably on the same kind of action. In the case of gases, a
molecular theory has been developed by Clausius and others, capable

of mathematical treatment, and subjected to experimental investigation;
and by this theory nearly every known mechanical property of gases
has been explained on dynamical principles; so that the properties of
individual gaseous molecules are in a fair way to become objects of
scientific research.
Now Mr Stoney has pointed out[1] that the numerical results of
experiments on gases render it probable that the mean distance of their
particles at the ordinary temperature and pressure is a quantity of the
same order of magnitude as a millionth of a millimetre, and Sir William
Thomson has since[2] shewn, by several independent lines of argument,
drawn from phenomena so different in themselves as the electrification
of metals by contact, the tension of soap-bubbles, and the friction of air,
that in ordinary solids and liquids the average distance between
contiguous molecules is less than the hundred-millionth, and greater
than the two-thousand-millionth of a centimetre.
[1] Phil. Mag., Aug. 1868. [2] Nature, March 31, 1870.
These, of course, are exceedingly rough estimates, for they are derived
from measurements some of which are still confessedly very rough; but
if at the present time, we can form even a rough plan for arriving at
results of this kind, we may hope that, as our means of experimental
inquiry become more accurate and more varied, our conception of a
molecule will become more definite, so that we may be able at no
distant period to estimate its weight with a greater degree of precision.
A theory, which Sir W. Thomson has founded on Helmholtz's splendid
hydrodynamical theorems, seeks for the properties of molecules in the
ring vortices of a uniform, frictionless, incompressible fluid. Such
whirling rings may be seen when an experienced smoker sends out a
dexterous puff of smoke into the still air, but a more evanescent
phenomenon it is difficult to conceive. This evanescence is owing to
the viscosity of the air; but Helmholtz has shewn that in a perfect fluid
such a whirling ring, if once generated, would go on whirling for ever,
would always consist of the very same portion of the fluid which was
first set whirling, and could never be cut in two by any natural cause.
The generation of a ring-vortex is of course equally beyond the power

of natural causes, but once generated, it has the properties