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RADIATION. 2.01

the radiative power of. every vapor being found proportional
to its absorptive power.

The method of experiment here pursued, though not of
the simplest character, is still within your grasp. When
air is permitted to rush into an exhausted tube, the tem-
perature of the air is raised to a degree equivalent to the
vis viva extinguished.1 Such air is said to be dynamically
heated, and if pure, it shows itself incompetent to radiate,
even when a rock-salt window is provided for the passage
of its rays. But if instead of being empty the tube contain
a small quantity of vapor, then the warmed air will com-
municate heat by contact to the vapor, which will be thus
enabled to radiate. Thus the molecules of the vapor con-
vert into the radiant form the heat imparted dynamically
to the atoms of the air. By this process, which has been
called dynamic radiation, the radiative power of both vapors
and gases has been determined, and the reciprocity of their
radiation and absorption proved.”

In the excellent researches of Leslie, De la Provostaye
and Desains, and Balfour Stewart, the reciprocity of radia-
tion and absorption as regards solid bodies has been vari-
ously illustrated; while the labors, theoretical and experi-
mental, of Kirchhoff have given this subject a wonderful
expansion, and enriched it by applications of the highest
kind. To their results are now to be added the foregoing,
whereby gases and vapors which have been hitherto thought
inaccessible to experiments of this kind are proved to ex-
hibit the duality of radiation and absorption, the influence
on both of chemical combination being exhibited in the
most decisive and extraordinary way.

1 See page 20 for a definition of @229 WM.

9 When heated, air imparts its motion to another gas or vapor; the
transference of heat is accompanied by a change of vibrating period. The
dynamic radiation of vapors is rendered possible by the transmutation of
vibrations.

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