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 The confirmation or refutation by experiment will therefore also decide whether or not this hypothesis is correct and in this respect whether or not it will be useful for further development.

The law that an empty cavity surrounded by walls of the same temperature contains blackbody radiation, also applies if the radiation emanates from gases which are sealed off from the cavity by means of transparent walls and from the outside by reflecting walls. However, the gases must have a finite absorptivity for all wavelengths. There is no doubt that there are gases such as carbonic acid and water vapor which emit heat rays simply by increasing their temperature. . Highly superheated vapors can be treated as gases, and by appropriately mixing different substances one can always imagine a gas mixture being produced which has a finite absorptivity for all wavelengths. But one must not think here of the radiation which the gases emit under the influence of electrical or chemical processes.

If one assumes a gas as a radiating body, Maxw ell's law of the distribution of velocities will apply if we take as our basis the kinetic theory of gases. The absolute temperature will be proportional to the mean kinetic energy of the gas molecules. This assumption has acquired a high degree of probability through the work of Clausius and Boltzmann and is supported by the investigations of Helmholtz on monocyclic systems, according to which both the kinetic energy and the absolute temperature have the property of being the integrating denominator of the differential of the energy supplied.

In order to avoid unnecessary details which would arise from considering different components of mixtures of gases, let us imagine a mixture