216 Mr. Gr. A. Hemsalech : A Comparative Study 



as will be shown presently, a direct comparison between the 

 spectra emitted by iron compounds in various flames and in 

 the furnace at corresponding temperatures, has disclosed the 

 interesting fact that these spectra are identical in character, 

 from which we may conclude that the mode of excitation 

 must be the same in these two very widely different sources 

 of light. It seems therefore evident that the furnace spectra of 

 iron, up to a temperature of about 2500° C, are caused by the 

 action of heat on a chemical compound, and not simply by 

 vaporization of the metal which is placed in the furnace-tube. 

 The spectrum can therefore not be of purely thermal origin, 

 because its emission necessarily involves also some process 

 connected with the chemical change which the compound 

 molecule undergoes as it is acted upon by heat. In order to 

 better distinguish this mode of excitation from that which is 

 supposed to represent the direct thermal action on a simple 

 metal vapour, it will henceforth always be referred to as 

 ther mo -chemical excitation. In like manner the cone emission 

 of iron in the air-coal gas flame will be considered as caused 

 by chemical excitation, because in this case chemical actions 

 evidently play the more important role. 



§ 5. General Character of the Furnace Spectrum 

 of Iron. 



Most of the information regarding the character of the 

 iron emission as excited by thermo-chemical actions in the 

 furnace and in flames, was derived from an exhaustive exami- 

 nation of the many photographic records secured. All these 

 photographs were taken on ordinary plates, and therefore 

 they do not include the red end of the spectrum. This 

 deficiency is, however, justified in the present circumstances 

 because the low dispersion of my spectrograph did not allow 

 of accurate observations in this part of the spectrum. The 

 most objectionable factor in connexion with the low-dispersion 

 spectrograph is the relative intensification, especially in the 

 less refrangible region, of the continuous spectrum, which is 

 always present at furnace temperatures above 1500° C. This 

 continuous ground renders the observation of the line spectrum 

 most difficult, especially in its denser parts. 



As compared with the flame spectra of iron, the furnace 

 spectra of this element are less well developed in the violet 

 and ultra-violet parts of the spectrum; many of the lines in 

 this region reverse at the higher temperatures, and it seems, 

 as has already been remarked by Dr. King, that possibly the 

 shorter wave-lengths suffer an appreciable absorption in 

 passing out of the furnace through the cooler vapours near 

 the end. It is well to remember in this connexion that the 



