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XXXIII. On the Character of the Light Radiations emitted by 

 the Vapours of Magnesium, Copper, and Manganese under 

 the Selective Actions of Thermo-cliemical and Thermo- 

 electrical Excitations. By G. A. Hemsalech*. 

 [Plates VIII. & IX.] 

 § 1. Introduction. 



THE flow of a heavy current of electricity through a 

 plate of graphite is accompanied at high temperatures 

 by the appearance of the red fringe. This phenomenon is 

 caused by part of the heating current passing outside the 

 plate as thermelectronic current through the strongly ionized 

 vapours in the protected space. The spectroscopic effect 

 produced by the thermelectronic current is quite distinct 

 from that caused by the action of the plate temperature 

 upon the chemical compounds which form the luminous 

 vapour t- In fact the difference between these two effects 

 is as striking as that which has been noted between the 

 spectra emitted by the explosion region and the outer mantle 

 of the air-coal gas flame. For the special case of iron 

 vapour the results of my experiments leave no room for 

 doubt that the character of the spectrum emitted by the 

 luminous vapour in the protected space is identical with 

 that observed in the outer mantles of flames. With regard 

 to the more complex spectrum observed in the explosion 

 region of the air-coal gas flame, I have attributed its origin 

 to the decomposition or breaking up of the iron compound, 

 owing to the existence of a strong chemical affinity between 

 the iron atom and nitrogen J . In like manner I have 

 ascribed the origin of the fringe emission to the electrolytic 

 decomposition of metal compounds by the thermelectronic 

 current. We should, therefore, in accordance with these 

 views, expect to observe some similarity between the spectra 

 caused by chemical excitation in the Bunsen cone and those 

 emitted under thermo-electrical excitation in the fringe, since 

 each of these processes is supposed to involve the breaking 

 up of a compound molecule. Thus we should anticipate 

 that the characteristic cone lines (class III.) of iron, cobalt, 

 nickel, &c. will be sensitive to the thermelectronic current 

 and form part of the fringe emission, whereas at the same 

 time the flame lines (classes I. and II.) of these elements 

 would be observed in the luminous vapour beneath the fringe. 



* Communicated by the Author. 



+ Hemsalech, Phil. Mag. ser. 6, vol. xxxix. p. 241 (1920). 



% Hemsalech, Phil. Mag. ser. 6, vol. xxxiv. p. 229 (1917). 



