Five of Maxwells Papers | Page 8

of
individuality, permanence in quantity, and indestructibility. It is also
the recipient of impulse and of energy, which is all we can affirm of
matter; and these ring-vortices are capable of such varied connexions
and knotted self-involutions, that the properties of differently knotted
vortices must be as different as those of different kinds of molecules
can be.
If a theory of this kind should be found, after conquering the enormous
mathematical difficulties of the subject, to represent in any degree the
actual properties of molecules, it will stand in a very different scientific
position from those theories of molecular action which are formed by
investing the molecule with an arbitrary system of central forces
invented expressly to account for the observed phenomena.
In the vortex theory we have nothing arbitrary, no central forces or
occult properties of any other kind. We have nothing but matter and
motion, and when the vortex is once started its properties are all
determined from the original impetus, and no further assumptions are
possible.
Even in the present undeveloped state of the theory, the contemplation
of the individuality and indestructibility of a ring-vortex in a perfect
fluid cannot fail to disturb the commonly received opinion that a
molecule, in order to be permanent, must be a very hard body.
In fact one of the first conditions which a molecule must fulfil is,
apparently, inconsistent with its being a single hard body. We know
from those spectroscopic researches which have thrown so much light
on different branches of science, that a molecule can be set into a state
of internal vibration, in which it gives off to the surrounding medium
light of definite refrangibility--light, that is, of definite wave-length and
definite period of vibration. The fact that all the molecules (say, of
hydrogen) which we can procure for our experiments, when agitated by
heat or by the passage of an electric spark, vibrate precisely in the same
periodic time, or, to speak more accurately, that their vibrations are
composed of a system of simple vibrations having always the same
periods, is a very remarkable fact.

I must leave it to others to describe the progress of that splendid series
of spectroscopic discoveries by which the chemistry of the heavenly
bodies has been brought within the range of human inquiry. I wish
rather to direct your attention to the fact that, not only has every
molecule of terrestrial hydrogen the same system of periods of free
vibration, but that the spectroscopic examination of the light of the sun
and stars shews that, in regions the distance of which we can only
feebly imagine, there are molecules vibrating in as exact unison with
the molecules of terrestrial hydrogen as two tuning-forks tuned to
concert pitch, or two watches regulated to solar time.
Now this absolute equality in the magnitude of quantities, occurring in
all parts of the universe, is worth our consideration.
The dimensions of individual natural bodies are either quite
indeterminate, as in the case of planets, stones, trees, &c., or they vary
within moderate limits, as in the case of seeds, eggs, &c.; but even in
these cases small quantitative differences are met with which do not
interfere with the essential properties of the body.
Even crystals, which are so definite in geometrical form, are variable
with respect to their absolute dimensions.
Among the works of man we sometimes find a certain degree of
uniformity.
There is a uniformity among the different bullets which are cast in the
same mould, and the different copies of a book printed from the same
type.
If we examine the coins, or the weights and measures, of a civilized
country, we find a uniformity, which is produced by careful adjustment
to standards made and provided by the state. The degree of uniformity
of these national standards is a measure of that spirit of justice in the
nation which has enacted laws to regulate them and appointed officers
to test them.
This subject is one in which we, as a scientific body, take a warm

interest; and you are all aware of the vast amount of scientific work
which has been expended, and profitably expended, in providing
weights and measures for commercial and scientific purposes.
The earth has been measured as a basis for a permanent standard of
length, and every property of metals has been investigated to guard
against any alteration of the material standards when made. To weigh
or measure any thing with modern accuracy, requires a course of
experiment and calculation in which almost every branch of physics
and mathematics is brought into requisition.
Yet, after all, the dimensions of our earth and its time of rotation,
though, relatively to our present means of comparison, very permanent,
are not so by any physical necessity. The earth might contract by
cooling,