2S2 Mr. G. A. Hemsalech on the Origin of the 



and, as will be seen from the various results obtained, there 

 can be little doubt left, that the so-called high-temperature 

 emission of iron vapour in an electric tube resistance-furnace 

 is actually caused by the passage of an electric current 

 through the vapour. 



§ 2. General Observations on the Furnace Spectrum of 

 Iron Vapour at 2700° 0. 



As has already been recorded, the interior of the tube 

 furnace, working at atmospheric pressure, emits a purple 

 light up to about 2400°; above this temperature and up to 

 about 2500° the light emitted is of brilliant white, due no 

 doubt to carbon particles, since it gives a continuous spectrum. 

 Above the boiling-point of iron, when the gaseous metal has 

 pervaded the whole interior of the furnace, the colour of the 

 brilliant light emitted is of a decided greenish tint. Spectro- 

 scopic examination at this stage reveals, superposed on a 

 bright continuous ground, a most brilliant iron spectrum, in 

 which the group at 4957 is quite a prominent feature. Also 

 the Swan bands at 4737 and 5165 are now visible. Owing 

 to the low dispersion of my spectrograph, the finer details 

 of the iron spectrum are unfortunately more or less destroyed 

 by the continuous background which, even with short ex- 

 posures of one second or less, is an anno}<ing attribute of the 

 photographic records secured. Nevertheless the general 

 character of the spectrum is well brought out, as are also its 

 distinguishing features as compared with 'he corresponding- 

 flame spectrum. 



The spectrum of iron given by the furnace at 2700° 

 differs entirely from that observed at the same temperature 

 in the oxy-acetylene flame. Thus a large number of class III. 

 lines and, further, lines so far only obtained by means of 

 electric discharges, have been detected in this spectrum. 

 Of class III. lines the four doublets XX4872, 4891, 4920, 

 and 4957 stand out prominently. It will be remembered 

 that at the flame temperature of 2700° only traces have 

 been observed of three of these doublets ; they constitute, 

 however, an important group in the spectrum of the explosion 

 region of the air-coal gas-flame, and they are particularly 

 marked in the self-induction spark where electric actions 

 prevail. Now in electrical sources, such as arc and spark, 

 both components of each doublet are well developed, whereas 

 with chemical excitation in the explosion region only one 

 component is brought out. The low dispersion employed 



