at when its temperature is falling, or absorbing heat when
it is rising.

What he tried to make clear was, that both radiation and absorption were
going on at one and the same time; the radiation depending upon the body
itself, but the absorption depending upon the nature of the body. While
radiation and absorption are thus reciprocal, which implies that a good
radiator is a good absorber, and a bad radiator is a bad absorber, it
does not follow that all bodies radiate and absorb alike.

The capacity of bodies to radiate and to absorb differ considerably. Dr.
Franklin made several simple experiments to prove the relative powers of
radiation and absorption with several pieces of cloth. These were put
out on the snow, and exposed to the heat of the sun. He found that the
pieces which were dark in colour sank deepest into the snow, while those
which were lightest in colour sank the least. From this he inferred that
the darkest pieces were the best absorbers, and therefore the best
radiators, while the light-coloured cloths were the worst absorbers, and
therefore the worst radiators.

Radiation, therefore, may be said to be the propagation of a wave motion
through the Aether; and, as all motion is a source of power or energy,
we have in the radiation of heat from one body to another by the
aetherial waves, the transmission of a motive power capable of doing
work, either internal work as increasing the temperature of the molecule
or body, or external work as separating the atoms, or driving them
further apart. It can readily be seen that if the Aether were
frictionless, as has generally been supposed, the Aether could not have
any motive power at all, and therefore could not transmit heat from one
body to another. Professor Tyndall[10] on this point says, referring to
the cooling of a red-hot ball: "The atoms of the ball oscillate in a
resisting medium, which accepts their motion and transmits it on all
sides with inconceivable velocity." Now in the previous quotation given
in this article from the same authority, he states that the atoms are
immersed in the Aether. So that evidently in his opinion the Aether and
the resisting medium are one and the same. So that our assumption of the
gravitative property of the Aether is perfectly in accord with Professor
Tyndall's conception of the Aether, in so far as it concerns the
propagation of heat waves; and, as will be shown later on, heat and
light waves are due to the same phys