The Anatomy of Atoms 23

the second and transfer to them a part of their kinetic
energy. Thus, the fast molecules will gradually slow down,
and the slow ones speed up, until a state of equilibrium
will be reached in which the molecules in both bodies
have equal average energies. We say then that both bodies
possess the same temperature, and that the "flow of heat"
from one into the other has ceased.

From such a view of the nature of heat and temperature,
it follows at once that there should exist a lowest possible
temperature, or an absolute zero, at which the molecules
of all material bodies are completely at rest. At this tem-
perature the constituent particles of any substance will
stick together, because of intermolecular cohesive forces,
and demonstrate the properties of a solid.

As the temperature rises, and the molecules begin to
move, there comes, sooner or later, a stage when the cohe-
sive forces are no longer able to keep the molecules rigidly
in their places, though still strong enough to prevent them
from flying apart. The body ceases to be rigid but still
keeps its finite volume, and we then have matter in the
liquid state. At still higher temperatures, the molecules
move so fast that they tear apart from each other and fly
off in all directions, thus forming a gas with a tendency
toward unlimited expansion. The fact that some substances
melt and evaporate at much lower temperatures than others
is simply explained by differences in the strength of the
cohesive forces of their respective molecules.

THE ENERGY OF MOLECULAR MOTION

Is there a direct empirical confirmation for these views;
may one actually observe this, thus far hypothetical, ther-
mal motion of molecules? The first step toward such a proof