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Reciprocity Of Radiation And Absorption

( Originally Published 1905 )

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 vapor, were intercepted by those molecules in various degrees. In all cases it was the transference of motion from the ether to the comparatively quiescent molecules of the gas or vapor 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 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, and vapors to the same temperature, their power of discharging the motion them upon the ether in which they was placed above a ring gas-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 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 self-same 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 them-selves unable to emit any sensible amount of radiant heat, the molecules of compound gases were shown to be capable of powerfully disturbing the surrounding ether. By special modes of experiment the same was proved to hold good for the vapors of volatile liquids, 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 easy to grasp. When air is permitted to rush into an exhausted tube, the temperature of the air is raised to a degree equivalent to the vis viva extinguished.` 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, the warmed air communicates its heat by contact to the vapor, the molecules of which convert into the radiant form the heat imparted to them by the atoms of the air. By this process also, which I have called Dynamic Radiation, the reciprocity of radiation and absorption has been conclusively proved.'

In the excellent researches of Leslie, De la Provostaye and Desains, and Balfour Stewart, the same reciprocity, as regards solid bodies, has been variously illustrated; while the labors, theoretical and experimental, 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 with the thermo-electric pile, are proved by it to exhibit the indissoluble duality of radiation and absorption, the influence of chemical combination on both being exhibited in the most decisive and extraordinary way.

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