round every atom and
molecule elastic envelopes or atmospheres, whose pressure is always
proportionate to their density.

[Footnote:8 _Phil Mag._, 1859.]

ART. 62. _Radiation and Absorption._--We have already seen (Art. 31)
that all matter is made up of atoms and molecules, each of which is
surrounded by its atmosphere of Aether. By means of the Aether, motion
in the form of light and heat may be transmitted from one atom and
molecule to another. The transmission of heat from one body to another
is termed Radiation, while the acceptance of heat is termed Absorption.
Tyndall defines Radiation as "the communication of molecular motion from
the heated body to the Aether in which it is immersed,"[9] and
Absorption, therefore, would be the acceptance of motion by the body
from the Aether. So that in Radiation, the atom, molecule, or body parts
with motion to the Aether, while in Absorption it gains motion from the
Aether.

Now in order for us to understand this theory of Radiation and
Absorption, it will be well for us if we look at a similar effect in the
sphere of music and sound. Let us suppose that we have two tuning-forks
of the same pitch, placed on a table at a distance of a foot from each
other. If we set one of the forks vibrating, the waves which it radiates
through the air will fall upon the other one, and will also set it in
vibration, because they are of the same period or size as those waves
which it would itself give off when sounded. Thus while one is losing
its motion, the other is gaining it, or while one is radiating motion,
the other is absorbing motion. This can readily be proved by stopping
the vibration of the first fork, when it will be found that the second
fork is now giving out a similar note to the first, although it was
silent at the commencement. Thus we have here an example of radiation
and absorption of sound, the success of the experiment depending upon
the fact that both forks shall have the same pitch. Again, it must be
noted, that if we have two tuning-forks both of which are of the same
pitch, and both vibrating at the same time, then, while one is radiating
sound and consequently losing motion to the other, yet at the same time
it is absorbing motion from the other. Because, if fork A can transfer
motion to fork B, the latter can equally transfer its motion to fork A,
and when both are vibrating together, each is the recipient of part of
the other's motion, while at the same time gi