lvanometer. The quantity absorbed
by the oxygen under those circumstances was too feeble to affect the
galvanometer; the gas, in fact, proved perfectly transparent to the
rays of heat. It had but a feeble power of radiation: it had an
equally feeble power of absorption.

The pile remaining in its position, a sheet of olefiant gas was caused
to issue from the same slit as that through which the oxygen had
passed. No one present could see the gas; it was quite invisible, the
light went through it as freely as through oxygen or air; but its
effect upon the thermal rays emanating from the cube was what might be
expected from a sheet of metal. A quantity so large was cut off, that
the needle of the galvanometer, promptly quitting the zero line, moved
with energy to its stops. Thus the olefiant gas, so light and clear
and pervious to luminous rays, was proved to be a most potent
destroyer of the rays emanating from an obscure source. The
reciprocity of action established in the case of oxygen comes out
here; the good radiator is found by this experiment to be the good
absorber.

This result, now exhibited before a public audience for the first
time, was typical of what had been obtained with gases generally.
Going through the entire list of gases and vapours in this way, we
find radiation and absorption to be as rigidly associated as positive
and negative in electricity, or as north and south polarity in
magnetism. So that if we make the number which expresses the
absorptive power the numerator of a fraction, and that which expresses
its radiative power the denominator, the result would be, that on
account of the numerator and denominator varying in the same,
proportion, the value of that fraction would always remain the same,
whatever might be the gas or vapour experimented with.

But why should this reciprocity exist? What is the meaning of
absorption? what is the meaning of radiation? When you cast a stone
into still water, rings of waves surround the place where it falls;
motion is radiated on all sides from the centre of disturbance. When
a hammer strikes a bell, the latter vibrates; and sound, which is
nothing more than an undulatory motion of the air, is radiated in all
directions. Modern philosophy reduces light and heat to the same
mechanical category. A luminous body is one with its atoms in a state
of vibration; a hot body is one with its atoms also vibrating, but at
a rate which is incompetent to