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