274 Mr. Gr. A. Hem sale ch : Excitation of Spectra of Carbon, 



employed. But it is of interest to note that the lines of 

 these elements have been recorded as impurities by Dr. King 

 in a tube-furnace made of the same material as my plates — 

 namely, Acheson graphite *. This important observation 

 poiuts most forcibly to the presence of thermelectronic 

 currents in a tube resistance furnace. 



We now pass on to examine the relative behaviour of the 

 remaining constituents of the red fringe spectrum— namely, 

 the carbon bands. These, as was shown in § 7, consist of the 

 so-called cyanogen bands and those of the Swan spectrum. 

 The strong band at 3883 is the first to appear as the tem- 

 perature of the plate gradually rises, and its first traces 

 were detected already at 2500° C, whereas the complete 

 spectrum was observed only at about 3000° C, the Swan 

 bands being the last to come out. In conformity with the 

 more effective ionization of the vapours, and the absence 

 of an internal magnetic field in the tube-furnace, we shall 

 naturally expect to observe the carbon bands in the latter 

 already at much lower temperatures than with the single 

 plate. Now, according to my own observations f , the first 

 trace of the carbon spectrum — namely, the head of the 

 so-called cyanogen band at 3883, was noted already at 

 a temperature of only about 1900° O. As the furnace 

 temperature and consequently also the acting electric field 

 were increased, the carbon spectrum developed further by 

 the additional emission of the band at 4216. Finally, at 

 temperatures of above 2500° C, the bands of the Swan 

 spectrum came also into view and thus the spectrum attained 

 a similar degree of completeness to that observed in the red 

 fringe. From these observations we elicit the double fact 

 that the carbon spectrum emitted by the tube-furnace is not 

 only of precisely the same character as that given b}^ the 

 red fringe, but also that its gradual development with 

 increase of temperature and electric field proceeds in exactly 

 the same order as that noted therein. As I have already 

 pointed out, this order of development is just the reverse to 

 that observed when the emission is caused by chemical 

 excitation, such as prevails in the explosion regions of the 

 various hydrocarbon flames. Hence the emission of the 

 carbon bands in the tube-furnace cannot be ascribed to 

 chemical actions of this nature. Further, it cannot possibly 

 be due to thermal or thermo-chemical actions alone, because 

 none of these bands are emitted by the hottest regions in the 

 mantles of carbon flames. Finally, it has been conclusively 



* A. S. King-, Astrophysieal Journal, vol. xxxvii. p. 250 (1913). 

 t Hemsalech, Phil. Mag. vol. xxxvi. p. 225 (1918). 



