n fact exists, and is termed the "force" of
cohesion. This force is a veritable gravitation influence, drawing every
molecule towards every other molecule. Possibly it is identical with
gravitation. It seems subject to some law of decreasing in power with
the square of the distance; or, at any rate, it clearly becomes less
potent as the distance through which it operates increases.

Now, between this force of cohesion which tends to draw the molecules
together, and the heat vibrations which tend to throw the molecules
farther asunder, there seems to be an incessant battle. If cohesion
prevails, the molecules are held for the time into a relatively fixed
system, which we term the solid state. If the two forces about balance
each other, the molecules move among themselves more freely but maintain
an average distance, and we term the condition the liquid state. But if
the heat impulse preponderates, the molecules (unless restrained from
without) fly farther and farther asunder, moving so actively that when
they collide the recoil is too great to be checked by cohesion, and this
condition we term the gaseous state.

Now after this statement, it is clear that what the low-temperature
worker does when he would liquefy a gas is to become the champion of the
force of cohesion. He cannot directly aid it, for so far as is known it
is an unalterable quantity, like gravitation. But he can accomplish the
same thing indirectly by weakening the power of the rival force. Thus,
if he encloses a portion of gas in a cylinder and drives a piston down
against it, he is virtually aiding cohesion by forcing the molecules
closer together, so that the hold of cohesion, acting through a less
distance, is stronger. What he accomplishes here is not all gain,
however, for the bounding molecules, thus jammed together, come in
collision with one another more and more frequently, and thus their
average activity of vibration is increased and not diminished; in
other words, the temperature of the gas has risen in virtue of the
compression. Compression alone, then, will not avail to enable cohesion
to win the battle.

But the physicist has another resource. He may place the cylinder of gas
in a cold medium, so that the heat vibrations sent into it will be less
vigorous than those it sends out. That is a blow the molecule cannot
withstand. It is quite impotent to cease sending out the impulses
however little comes in return; hence the aggregate motion b