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200 FRAGMENTS OF SCIENCE.

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 ether undulations which speed through it
with the velocity of light. Our object now is to inquire
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 vapors to the same tempera-
ture, and then examine their power of discharging the
motion thus imparted to them upon the ether 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 thermo-electric pile, entirely
screened off from the hot ball, was exposed to the radiation
of the warm gas, and the deflection of a magnetic needle
connected with the pile declared the energy of the radia-
tion.

By this mode of experiment it was proved that the self-
same molecular arrangement which renders a gas a power-
ful absorber, renders it in the same degree a powerful
radiator—that the atom or molecule which is competent to
intercept the calorific waves is in the same degree compe-
tent to generate them. Thus, while the atoms of element-
ary gases proved themselves unable to emit any sensible
amount of radiant heat, the molecules of compound gases
were shown to be capable of powerfully disturbing the sur-
rounding ether. By special modes of experiment the same
was proved to hold good for the vapors of volatile liquids,

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