82 INFRA-RED EMISSION SPECTRA. 



the fact, that the filaments are metallic, electrical conductors with a high 

 reflecting power in the visible, and probably reflect uniformly high in the 

 infra-red, one would expect the distribution of energy in the spectrum to 

 follow a law similar to that of platinum, but with different constants. 

 The results thus far obtained from a study of such metals as tungsten 

 and osmium support this hypothesis. On the other hand, in the case of 

 oxides, which conduct electrolytically at high temperatures, there is no 

 data to form even a working hypothesis. All the oxides thus far examined 

 have no strong absorption bands near the visible spectrum; the only ex- 

 ception being the oxides of the rare earths, such as cerium, thorium, 

 lanthanum, didymium, erbium, etc., the compounds of which have strong, 

 sharply defined absorption bands in the visible, and at least some of these 

 have absorption bands in the infra-red. It seems to be a characteristic of 

 the oxides, that they have a low reflecting power (like transparent media, 

 electrical non-conductors) throughout the infra-red to about 8 n beyond 

 which point they have strong bands of metallic reflection. In this region 

 of metallic reflection, the emission will be suppressed l in proportion to the 

 reflecting power. In the rest of the spectrum, the emission will be pro- 

 portional to the absorbing power (general absorption), while at the point 

 where there is a band of selective absorption in the transmission spectrum, 

 there will be an emission band in the emission spectrum, provided the 

 radiation is a purely thermal one, following Kirchhoff's Law. 



In the case of the Auer mantle the emission spectrum 2 is a series of 

 emission bands at i to 2 /1, with practically no emission in the region from 

 4 to 7 n, while beyond 9 fi the spectrum is continuous, and is apparently 

 as intense as that of a complete radiator at the same temperature. 



In the case of the Nernst glower, which is a combination of the oxides 

 of cerium, thorium, and zirconium, belonging to the "rare earths," the 

 compounds of which are noted for their strong absorption bands, one 

 would expect the emission spectrum to show strong, sharp emission bands; 

 at least one would hardly expect the emission to follow the same general 

 law of spectrum energy distribution as is known for metals. This, how- 

 ever, has been done in the past, notably by Lummer and Pringsheim, 3 

 and by Mendenhall and Ingersoll. 4 



The first two investigators, from a rather cursory examination of the 

 Nernst filament, under normal power consumption, found a smooth con- 

 tinuous curve, with a maximum at wave-length ^ max =i.2 /. From this 

 and from the Wien displacement law, ^ max T= const, (const. = 2940 for 

 "black body," = 2630 for platinum), assuming that the Nernst glower 



1 Aschkinass, Verh. J. Phys. Ges., 17, p. 101, 1898. Rosenthal, Ann. der Phys. (3), 68, 

 p. 791, 1899. 



2 Rubens: Phys. Zeit., 6, p. 790, 1905. 



3 Lummer & Pringsheim: Verh. Deutsch. Phys. Gesell., 3, p. 36, 1901. 



4 Mendenhall & Ingersoll: Phys. Rev., 24, p. 230, 1907; 25, p. i, 1907. 



