excite the sense of vision; and, as a
sounding body has the air around it, through which it propagates its
vibrations, so also the luminous or heated body has a medium, called
aether, which accepts its motions and carries them forward with
inconceivable velocity. Radiation, then, as regards both light and
heat, is the transference of motion from the vibrating body to the
aether in which it swings: and, as in the case of sound, the motion
imparted to the air is soon transferred to surrounding objects,
against which the aerial undulations strike, the sound being, in
technical language, absorbed; so also with regard to light and heat,
absorption consists in the transference of motion from the agitated
aether to the molecules of the absorbing body.

The simple atoms are found to be bad radiators; the compound atoms
good ones: and the higher the degree of complexity in the atomic
grouping, the more potent, as a general rule, is the radiation and
absorption. Let us get definite ideas here, however gross, and purify
them afterwards by the process of abstraction. Imagine our simple
atoms swinging like single spheres in the aether; they cannot create
the swell which a group of them united to form a system can produce.
An oar runs freely edgeways through the water, and imparts far less of
its motion to the water than when its broad flat side is brought to
bear upon it. In our present language the oar, broad side vertical,
is a good radiator; broad side horizontal, it is a bad radiator.
Conversely the waves of water, impinging upon the flat face of the
oar-blade, will impart a greater amount of motion to it than when
impinging upon the edge. In the position in which the oar radiates
well, it also absorbs well. Simple atoms glide through the aether
without much resistance; compound ones encounter resistance, and hence
yield up more speedily their motion to the aether. Mix oxygen and
nitrogen mechanically, they absorb and radiate a certain amount of
heat. Cause these gases to combine chemically and form nitrous oxide,
both the absorption and radiation are thereby augmented hundreds of
times!

In this way we look with the telescope of the intellect into atomic
systems, and obtain a conception of processes which the eye of sense
can never reach. But gases and vapours possess a power of choice as
to the rays which they absorb. They single out certain groups of rays
for destruction, and allow other groups to pass unharmed.