PARTIAL RADIATORS. 



141 



of 8.5 and 9.03 /jl. This was predicted by Aschkinass 1 and experimentally 

 verified by Rosenthal, 2 with quartz, mica, and glass (see fig. 98). The 

 latter found the observed emission curves of these substances to coincide 

 precisely with the computed curves, using the observed reflecting power 

 of these minerals and the observed emission curve of a full radiator at the 

 same temperature. 



Since we have constantly before us examples of selective emission, such 

 as the Welsbach mantle, and high temperature radiation of such metals 

 as tungsten and tantalum, in the visible spectrum, it will be of interest to 

 consider the radiation of several substances having bands of selective 

 reflection in the infra-red. In fig. 99 is given an illustration of the marked 

 effect of selective reflection upon emission. The extraordinary reflection 

 of carborundum (SiC) has been found in only one other substance, viz, 



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40 



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20 



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90 



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ro 



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e-f, 

 O* 



60 D 

 o 

 -*i 



O 



50 0> 



-j 

 cr 

 o 



40^ 



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30 

 20 



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14 



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Fig. 99. Reflection from carborundum (a); Radiation of complete radiator at 300 abs. (6); 

 Radiation from carborundum at 300 abs. (c). 



quartz, and in the latter it is the result of two well-defined bands, at 8.5 

 and 9.03 ;x. The effect of such a band on the dispersion of this substance 

 must be very marked. In fact, the unusual dispersion in the visible spec- 

 trum found by others no doubt is greatly influenced by this band. In this 

 figure are given the computed energy curve for a perfect radiator (using 

 Planck's formula any temperature might have been selected instead of 

 300 abs.) and the computed energy curve of carborundum at the same 



1 Aschkinass: Verb. d. Deutsch. Phys. Ges., 17, p. 101, 1898, 

 3 Rosenthal: Ann. der Phys. (3), 68, p. 791, 1899. 



