ving off motion in the form
of sound waves itself. So that the power of a fork to radiate sound
waves equals its power to absorb sound waves. If now we apply this
simile to the atomic and molecular world, we shall be able to form a
mental picture as to what takes place in radiation and absorption.

All atoms and molecules are ever in a state of ceaseless motion, ever
moving, never still. All are creating Aether waves which move away with
the velocity of light. If, in the transmission of the waves by the
Aether, they fall upon another atom which can emit a wave of similar
length, in the same way that two tuning-forks emitted sound waves of the
same length, then the atom upon which the waves strike will be set in
vibration, as the second tuning-fork was set in vibration by the first.
We shall look again at the principle of wave motion in the next chapter.
Further, from the simile of the two forks, which absorb sound at the
same time that they radiate sound, we learn that an atom or body
radiates heat waves at the same time that it is absorbing heat waves.
Suppose that we have two bodies at equal temperatures, it must not be
thought that the radiation or absorption has ceased, for, according to
the simile used, they both still continue to vibrate and emit the
aetherial heat waves; but where we get equality of temperatures, there
we get equality of radiation and absorption. Before this equality of
temperatures, however, is reached, the hotter body will radiate more
heat waves than it absorbs, while the colder body will absorb more heat
waves than it emits. All bodies, whatever their temperature, are
incessantly radiating heat waves. This may be proved experimentally with
proper apparatus, as for example with an instrument known as the
thermopile. When, however, the total heat waves radiated out by a body
are less than it absorbs, the body gets gradually colder, and the
temperature decreases. So long as this is continued, so long will the
body continue to get colder and colder, until it arrives at the same
temperature as the surrounding bodies, at which point the total heat
waves radiated out will equal the total heat waves absorbed, and at that
point the temperature of the body will remain constant.

This aspect of temperature was first introduced by Prevost of Geneva in
1792, in an article in which he tried to explain the radiation from a
cold body. According to his reasoning, a body is not simply regarded as
radiating he