264 Mr. GL A. Hemsalech : Excitation of Spectra of Carbon, 



records secured in the two cases is that the furnace spectra 

 were exposed for two minutes, whereas the spectra obtained 

 with the graphite plate were given only six seconds, and 

 therefore the relative intensities are generally on a slightly 

 lower scale in the latter case. As regards iron, only the 

 ultra-violet end of the spectrum has been given here, but 

 the table contains all the lines observed in that region. 

 Between 3824 and 3860 a few lines have not been recorded 

 with the graphite plate, because the strong carbon band 

 at 3883 rendered their observation difficult. Similarly, the 

 line 4216 has not been observed, as it nearly coincides with 

 the head of a band. 



With regard to the temperature of the luminous vapour 

 nothing definite can be stated, as no data are available con- 

 cerning the temperature gradient on passing from the plate 

 downwards. The surface temperature of the plate was 

 nearly 3000° C. and the observations upon which the 

 tabular results are founded refer to the region situated 

 about one millimetre awaj r from the radiating surface. If 

 we compare these results with those given by the tube- 

 furnace at 2400° C. and bear in mind the fact that the 

 plate results were obtained with only 1/20 the exposure of 

 the furnace ones, we shall probably not be far w r rong by 

 assuming a temperature of at least 2700° C. for the luminous 

 vapour at the distance of about one millimetre from the hot 

 plate. It is interesting in this connexion to mention that 

 under these conditions class III. group 4957 of iron was not 

 observed. 



The great similarity between the fundamental characters 

 of the spectra, as given by the plate and the tube-furnace, 

 is so striking that there can be no room for doubt as to the 

 identity of the mode of excitation in the two cases. We are 

 therefore justified in concluding that the luminous emission 

 of the impurities which are expelled from the graphite and 

 carborundum and carried some distance downwards from 

 the plate, is caused by thermo-chemical excitation and is of 

 the same nature as that observed for these same elements 

 in the tube-furnace and in the mantles of various flames. 



§ 8. Cause of the sharp outline shown by the red fringe and 

 of the abrupt cessation of its spectrum emission. 



Sir Ernest Rutherford has very kindly directed my atten- 

 tion to the probable influence of the magnetic field due to 

 the heating current upon the motion of the electrons in the 

 red fringe. This magnetic field would tend to drive the elec- 

 trons back towards the plate and, consequently, to oppose 



