Line Spectrum emitted by Iron Vapour. 295 



b. Tliermo-cliemical excitation. Here the emission of light 

 radiations is caused by the action of heat on a chemical 

 compound of iron. The component atoms in the com- 

 pound remain chemically associated and therefore the 

 vibrations emitted are observed to be restricted in 

 development. This mode of excitation prevails in the 

 mantles of all the low and high temperature flames so 

 far examined, as also in the electric tube resistance- 

 furnace up to a temperature of nearly 2500° C. 



c. Chemical excitation. This involves the complete de- 

 composition, at a relatively low temperature, of an 

 iron compound and the formation of a new one, owing 

 to the existence of a strong chemical affinity between 

 iron and nitrogen. This mode of excitation has been 

 met with for the first time in the explosion region of the 

 air-coal gas flame. The spectrum to which it gives rise 

 presents a high degree of development. 



d. T hermo-electrical excitation. This accompanies the 

 discharge of electricity through iron vapour, which has 

 previously been strongly ionized through the action of 

 heat. It occurs in the electric tube resistance-furnace 

 at temperatures of over 2500° (J. and also in the two- 

 plate furnace. The ordinary electric arc between iron 

 poles may be regarded as a special case in which the 

 necessary degree of ionization is maintained auto- 

 matically by the application of a high voltage. 



e. Electrical excitation. Occurs in the capacity and self- 

 induction sparks passing between iron electrodes at 

 ordinary temperature. The radiating vapour is produced 

 by a direct action of the electric discharge on the 

 molecules in the surface-layer of the electrodes. The 

 vapour is hurled into the spark-gap with definite 

 velocity and its luminous vibrations, started in the 

 first instance by the disruption of the molecules at the 

 surface of the electrodes by the initial discharge 

 (capacity spark) or the first oscillation (self-induction 

 spark), are maintained or further developed by the 

 subsequent oscillations. 



§ 9. Concluding Remarks. 



In considering all the various facts observed in connexion 

 with the emission of the spectrum of iron we arrive at the 

 general conclusion that temperature, although often playino- 

 an important role in bringing about conditions favourable to 

 the effective actions of other agents, does not in itself suffice 



