94 INFRA-RED EMISSION SPECTRA. 



wave-length and at the same temperature. Unfortunately the number of 

 radiating substances of which it is possible to determine even an approxi- 

 mate temperature is extremely small. Hence, the work presented on the 

 following pages can not be more than a qualitative proof of Kirchhoff's 

 law of proportionality between emission and absorption. It has numerous 

 applications, however, particularly in studying substances having sharp 

 emission (hence sharp absorption) bands. Numerous substances are given 

 here, of which it has not been practicable to study the absorption spectra. 

 Their emission spectra give us some idea of the nature of their absorption, 

 the only difference being that the emission bands are more intense, due to 

 the greater thickness (and the higher temperature) of the substance exam- 

 ined. Another feature of the results obtained, which is new, is the extreme 

 sharpness of the emission bands. Moreover, the maxima of the emission 

 and absorption bands coincide, although the temperatures at which the 

 two sets of observations were made differ by 800 to 1000 C. The posi- 

 tions of the sharp, well-defined maxima are not affected by change in 

 temperature. This is in marked contrast with the results of Konigsberger 1 

 for the limited region of the visible spectrum, in which he found that for 

 certain selectively absorbing substances the maximum of the absorption 

 band shifts toward the long wave-lengths with rise in temperature, and 

 with the results of Paschen on the emission bands of C0 2 and water-vapor, 

 which shifted with rise in temperature, some toward the long, others toward 

 the short wave-lengths. The data may also prove to be of use in deter- 

 mining whether pleochroism is an inter- or intra-molecular phenomenon. 

 For example, the absorption spectrum of adularia shows a band at 3.2 //, 

 which in the emission spectrum is shifted to 2.9 p.. The latter band is 

 characteristic of silicates, whether in the emission or absorption spectrum. 

 The present work may be considered an examination of the emission spectra 

 of electrical insulators, or transparent media. The only previous work 

 done on this subject is due to Rubens, 2 who examined the radiation from 

 the Auer mantle, as well as mantles composed of the pure oxides of cerium 

 and of thorium. The difficulty experienced by him was the elimination of 

 the emission spectrum of the hot gases which was superposed upon that 

 of the oxides composing the mantle. Hence, if he had examined a mantle 

 of zirconium oxide he would not have been able to detect an emission 

 band which occurs at 4.3 /. 



The substances used in the present investigation were in the form of 

 solid rods made in an oxy-hydrogen flame or in the form of thick layers 

 of the substance placed upon a heater. The rods were heated by an electric 

 current from the secondary of a 2000-volt (300-watt) transformer. These 

 rods had, of course, to be heated initially with an alcohol or blast lamp 

 until they became conducting, just as is necessary with the Nernst glower. 



1 Konigsberger: Ann. der Phys. (4), 4, p. 796, 1901. 



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



