extent not
accurately determined, but certainly far beyond what has hitherto been
imagined, for the temperature now existing at the surface of the
globe.

********************

14. Reciprocity of Radiation and Absorption.

Throughout the reflections which have hitherto occupied us, the image
before the mind has been that of a radiant source sending forth
calorific waves, which on passing among the molecules of a gas or
vapour were intercepted by those molecules in various degrees. In all
cases it was the transference of motion from the aether to the
comparatively quiescent molecules of the gas or vapour that occupied
our thoughts. We have now to change the form of our conception, and
to figure these molecules not as absorbers but as radiators, not as
the recipients but as the originators of wave-motion. That is to say,
we must figure them vibrating, and generating in the surrounding
aether undulations which speed through it with the velocity of light.
Our object now is to enquire whether the act of chemical combination,
which proves so potent as regards the phenomena of absorption, does
not also manifest its power in the phenomena of radiation. For the
examination of this question it is necessary, in the first place, to
heat our gases and vapours to the same temperature, and then examine
their power of discharging the motion thus imparted to them upon the
aether in which they swing.

A heated copper ball was placed above a ring gas-burner possessing a
great number of small apertures, the burner being connected by a tube
with vessels containing the various gases to be examined. By gentle
pressure the gases were forced through the orifices of the burner
against the copper ball, where each of them, being heated, rose in an
ascending column. A thermoelectric pile, entirely screened from the
hot ball, was exposed to the radiation of the warm gas, while the
deflection of a magnetic needle connected with the pile declared the
energy of the radiation.

By this mode of experiment it was proved that the selfsame molecular
arrangement which renders a gas a powerful absorber, renders it a
powerful radiator--that the atom or molecule which is competent to
intercept the calorific waves is, in the same degree, competent to
send them forth. Thus, while the atoms of elementary gases proved
themselves unable to emit any sensible amount of radiant heat, the
molecules of compound gases were shown to be capable of powerfully