24t2 Mr. Gr. A. Hemsalech : Excitation of Spectra of Carbon^ 



§ 1. Introduction. 



BY directly comparing the spectra of iron as emitted by 

 flames of different temperatures with those given 

 by an electric carbon tube-resistance furnace heated to 

 corresponding temperatures, I arrived at the conclusion that 

 the electric furnace gives out two different kinds of emis- 

 sions caused by two separate and distinct modes of excitation. 

 The spectrum of one of these emissions was found to be 

 identical, at all temperatures up to 2500° C, with that 

 observed in the mantles of various flames, being indeed com- 

 posed of class I. and II. lines only. The spectrum of the 

 second emission, which was observed in the furnace at 

 temperatures of over 2500° C, showed in addition also 

 class III. lines, the group at A- 4957 being particularly pro- 

 minent. A special feature of the high temperature furnace 

 emission was the prominent appearance of the Swan spec- 

 trum of carbon *. As a result of further observations and 

 experiments I was led to attribute the cause of excitation of 

 the first emission, which as I had shown is controlled by the 

 temperature prevailing in flames and in the furnace, to 

 the action of heat upon a chemical compound of iron 

 (thermo-chemical excitation), and that of the second emis- 

 sion to the passage 'of an electric current through iron 

 vapour, ionized to a considerable degree by the high 

 temperature prevailing in the tube-furnace (thermo-electrical 

 excitation). To this latter mode of excitation I also ascribed 

 the emission of the Swan spectrum of carbon. 



In the high temperature tube-resistance furnace the spectra 

 of these two emissions are naturally always observed simul- 

 taneously and superposed upon each other ; hence no direct 

 evidence for the electrical origin of, for example, the carbon 

 bands could be obtained, although the Swan spectrum was 

 not observed by means of a special plate-furnace in which 

 the electric field had been considerably reduced in intensity. 

 As is well known, observations have to be made along the 

 axis of the furnace tube and, since these tubes are generally 

 of appreciable length, it is practically impossible to see what 

 is going on in the immediate vicinity of the inner wall, from 

 which the ionization currents are supposed to be passing out 

 into the enclosed space filled with conducting vapours. If, 

 however, we imagine a furnace tube with its jacket of car- 

 borundum powder, to be cut open longitudinally and rolled 

 out flat, so as to constitute a plate protected from loss of 



* Hemsalech, Phil. Mag-, vol. xxxvi. p. 209 ; ibid. p. 281 (1918). 



