276 Mr. Gr. A. Hemsalech : Excitation of Spectra of Carbon y 



temperatures above 2500° C; even of the bright group at 

 4957, no trace was ever detected below 2500° C. I have 

 further pointed out, that this observation is quite in harmony 

 with the results of my investigations on the flame spectra of 

 iron. In this case also, there was no sign of class III. 

 group 4957 up to the temperature of the oxy-hydrogen 

 flame — namely, 2550° C, and only mere traces of it were 

 observed in the oxy-acetylene flame at 2700° C. But, when 

 the temperature of the tube-furnace was raised to 2700° (J.,. 

 the spectrum of iron emitted by it, besides being of great 

 brilliancy, showed a remarkably high degree of development, 

 with group 4957 as quite a conspicuous feature ; in fact the 

 character of the spectrum had completely changed and now 

 approached more that stage which it generally attains in 

 the arc. Thus, the spectrum of iron, as emitted by my tube- 

 furnace at 2700° C, was found to be entirely different from 

 that given by the oxy-acetylene flame at the same tem- 

 perature. Had the emissions of the iron spectra in these two 

 cases been solely controlled by temperature, then surely 

 the spectra given by these two sources would have been 

 identical, at least in so far as the relative intensities of the 

 lines were concerned. From the very fact that this was not 

 the case, and also from several other considerations, I had 

 arrived at the conclusion that the emission of class III. lines 

 by the tube-furnace was caused by part of the heating 

 current passing through highly ionized iron vapour, and 

 that the iron spectrum observed under these conditions 

 should be regarded as a low-tension arc spectrum. This 

 conclusion appeared to receive confirmation by the negative 

 results of an experiment with a special plate-furnace, in 

 which the acting electric field was greatly reduced. No 

 trace of class III. group 4957 was ever seen with this 

 furnace, although the observations were continued up to the 

 moment when the plate burnt through and the temperature, 

 as is well known, had reached a maximum value. As 

 Dr. King has rightly pointed out, I ought to have observed 

 at least those lines which are caused by the action of heat 

 upon the carbides of iron ; but unfortunately I had been 

 unable to obtain a photographic record of the spectrum on 

 account of a breakdown of the dynamos, and I have no 

 doubt that such a record would have shown class I. and II. 

 lines. As it was, however, I had concentrated my attention 

 upon the detection of group 4957 of iron and the Swan 

 bands of carbon, neither of which were observed. The 

 strong continuous spectrum, ahvays present, may possibly 

 have contributed to render the other lines less conspicuous, 



