posed
of minute isolated particles or molecules, which are always in
motion--vibrating, if you will. He must mentally magnify and
visualize these particles till he sees them quivering before him,
like tuning-forks held in the hand. Remember, then, that, like the
tuning-fork, each molecule would, if left to itself, quiver less and
less violently, until it ran down altogether, but that the motion thus
lessening is not really lost. It is sent out in the form of ether waves,
which can set up like motion in any other particles which they reach, be
they near or remote; or it is transmitted as a direct push--a kick,
if you will--to any other particle with which the molecule comes in
physical contact.

But note now, further, that our molecule, while incessantly giving out
its energy of motion in ether waves and in direct pushes, is at the same
time just as ceaslessly receiving motion from the ether waves made by
other atoms, and by the return push of the molecules against which it
pushes. In a word, then, every molecule of matter is at once a centre
for the distribution of motion (sending out impulses which affect,
sooner or later, every other atom of matter in the universe), and, from
the other point of view, also a centre for the reception of motion from
every direction and from every other particle of matter in the universe.
Whether any given molecule will on the whole gain motion or lose it
depends clearly on the simple mechanical principles of give and take.

From equally familiar mechanical principles, it is clear that our
vibrating molecule, in virtue of its vibrations, is elastic, tending to
be thrown back from every other molecule with which it comes in contact,
just as a vibrating tuning-fork kicks itself away from anything it
touches. And of course the vigor of the recoil will depend upon the
vigor of the vibration and the previous movements. But since these
movements constitute temperature, this is another way of saying that
the higher the temperature of a body the more its molecules will tend to
spring asunder, such separation in the aggregate constituting expansion
of the mass as a whole. Thus the familiar fact of expansion of a body
under increased temperature is explained.

But now, since all molecules are vibrating, and so tending to separate,
it is clear that no unconfined mass of molecules would long remain in
contiguity unless some counter influence tended to draw them together.
Such a counter influence i