of the Flame and Furnace Spectra of Iron. 215 



to the temperature of about 2500° C, i, e. in the neighbourhood 

 of the boiling-point of iron. Above this temperature, after 

 the furnace space is completely filled with iron vapour from 

 the boiling metal, the interior of the tube emits an extremely 

 brilliant white light of greenish hue. An intensely bright 

 line-spectrum, projected upon a most luminous continuous 

 spectrum, is now visible, and among the lines those of classes 

 II. and III. stand out more prominently. As will be shown 

 in the next communication, at this stage, the metal vapour in 

 the furnace carries part of the electric heating current, and 

 the line-spectrum of iron as emitted under these conditions is 

 of electric origin. 



§ 4. Origin of the Iron Spectrum emitted by the Furnace 

 at Temperatures beloiv 2500° C. 



The first traces of iron lines were obtained at a temperature 

 of only 1500° C, or at about the melting-point of the metal, 

 and the question naturally arises whether this emission is 

 really caused by the action of heat on iron metal or on some 

 compound of it. The number of lines and also the intensity 

 of the spectrum increase rapidly as the temperature is raised,, 

 but the general character of the spectrum changes but slowly 

 up to a temperature of about 2500° C, namely the boiling- 

 point of iron, after which a great change occurs. Now it 

 was found that iron spectra of precisely the same character 

 were obtained when iron metal was in the tube or when no 

 metal was present. Also the finely divided iron oxide blown 

 through the tube gave an identical spectrum, only, if airything, 

 a little more intense all round, as compared with the spectra 

 observed in the first two cases. Hence when the furnace is 

 run empty the iron spectrum emitted must be due to the 

 existence of iron in the substance of the furnace-tube. There 

 is little doubt that the iron, as well as most of the other im- 

 purities met with in the carbon, is chemically combined 

 with the latter in the form of carbide. Thus, as the furnace 

 gradually disintegrates in the interior, the iron carbide is set 

 free, and under the action of the prevailing heat the com- 

 pound molecule is dissociated or decomposed, which change 

 is accompanied by the emission of luminous radiations. Since 

 the spectrum emitted in this way is exactly the same in 

 character as that given by the action of the furnace heat on 

 iron oxide, the origin of the spectrum must be the same in 

 the two cases, namely dissociation of an iron compound. It 

 will be remembered that Dr. de Watteville and myself found 

 a similarity of the like kind to exist between the spectra given 

 by different compounds of iron when fed into flames. Now,. 



