92 INFRA-RED EMISSION SPECTRA. 



of the ^ max (^ max T= 2620) it was found that the " constant " a decreased 

 from a 8.5 at 900 C. to a = 6.3 at iioo C. 



Using metal filament lamps (commercial no- volt) under "normal" 

 working conditions, for metallized carbon a 6.1, for tantalum a = 6.3, for 

 tungsten a = 6.6, and for osmium a = 6.9. In all cases the value of a 

 was found to decrease with rise in temperature. 



Theoretically, this variation in a must occur at some stage in the tem- 

 perature (either an abrupt decrease at some fixed temperature, or a uni- 

 form decrease throughout the range), otherwise a temperature would be 

 attainable at which the total radiation is greater than that of a complete 

 radiator at the same temperature. 



In the " black body " the reflection is zero. The Nernst glower, as 

 well as the oxides in general, have a very low reflecting power; hence, if 

 the radiating layer is of sufficient thickness, the emissivity of the oxides 

 must be almost as great as that of a" black body," as has just been found 

 for the Nernst glower, at high temperatures. Some oxides, having absorp- 

 tion bands in the visible spectrum, e. g., Ce0 2 , when in thin layers (Wels- 

 bach mantle) and at high temperatures will have a higher luminous effi- 

 ciency than the same material in thick rods, e.g., the Nernst glower. 

 In the next chapter it will be shown that the greater the electrical con- 

 ductivity the more continuous will be the emission spectrum of the oxides. 

 Furthermore, mixtures of oxides like mixtures of gases in a vacuum tube 

 have composite emission spectra. This indicates a possible method of 

 analysis of mineral solutions, and it is hoped to make a further examina- 

 tion into this question. 



