7.

The coefficient of emission $ϵ$ depends, not only on the frequency $\nu$, but also on the condition of the emitting substance contained in the volume-element $d\tau$, and, generally speaking, in a very complicated way, according to the physical and chemical processes which take place in the elements of time and volume in question. But the empirical law that the emission of any volume-element depends entirely on what takes place inside of this element holds true in all cases (Prevost's principle). A body $A$ at $100°\mathrm{C}.$ emits toward a body $B$ at $0°\mathrm{C}.$ exactly the same amount of radiation as toward an equally large and similarly situated body $B^{\prime }$ at $1000°$ C. The fact that the body $A$ is cooled

by $B$ and heated by $B^{\prime }$ is due entirely to the fact that $B$ is a weaker, $B^{\prime }$ a stronger emitter than $A$.

We shall now introduce the further simplifying assumption that the physical and chemical condition of the emitting substance depends on but a single variable, namely, on its absolute temperature $T$. A necessary consequence of this is that the coefficient of emission $ϵ$ depends, apart from the frequency $\nu$ and the nature of the medium, only on the temperature $T$. The last statement excludes from our consideration a number of radiation phenomena, such as fluorescence, phosphorescence, electrical and chemical luminosity, to which E. Wiedemann has given the common name "phenomena of luminescence." We shall deal with pure "temperature radiation" exclusively.

A special case of temperature radiation is the case of the chemical nature of the emitting substance being invariable. In this case the emission takes place entirely at the expense of the heat of the body. Nevertheless, it is possible, according to what has been said, to have temperature radiation while chemical changes are taking place, provided the chemical condition is completely determined by the temperature.

[8.]8. Propagation.–-The propagation of the radiation in a medium assumed to be homogeneous, isotropic, and at rest takes place in straight lines and with the same velocity in all directions, diffraction phenomena being entirely excluded. Yet, in general, each ray suffers during its propagation a certain weakening, because a certain fraction of its energy is continuously deviated from its original direction and scattered in all directions. This phenomenon of "scattering," which means neither a creation nor a destruction of radiant energy but simply a change in distribution, takes place, generally speaking, in all media differing from an absolute vacuum, even in substances which are perfectly pure chemically.See, e.g., Lobry de Bruyn and L. K. Wolff, Rec. des Trav. Chim. des Pays-Bas 23, p. 155, 1904. The cause of this is that no substance is homogeneous in the absolute sense of the word. The smallest elements of space always exhibit some discontinuities on account of their atomic structure. Small impurities, as, for instance, particles of dust, increase the influence of scattering without, however, appreciably affecting its general character. Hence, so-called "turbid"

media, i.e., such as contain foreign particles, may be quite properly regarded as optically homogeneous,To restrict the word homogeneous to its absolute sense would mean that it could not be applied to any material substance. provided only that the linear dimensions of the foreign particles as well as the distances of neighboring particles are sufficiently small compared with the wave lengths of the rays considered. As regards optical phenomena, then, there is no fundamental distinction between chemically pure substances and the