Line Spectrum emitted by Iron Vapour. 293 



indeed what I have observed. Iron showed a most brilliant 

 spectrum in which the group 4957 was as marked as in the 

 tube-furnace at 2700°. Moreover, copper which gave no 

 line spectrum in the tube-furnace, emitted one under the 

 action of the stronger electric field which could be brought 

 to bear upon it in the two-plate furnace. 



A fuller discussion of these results, together with the de- 

 scription of a new and more practicable form of plate-furnace, 

 based on the principle explained above, will be reserved for a 

 subsequent communication. Suffice it to point out here that 

 the mode of excitation under these conditions is similar to that 

 underlying the emission of line spectra in the ordinary arc. 

 But whereas in the present case the necessary ionization 

 current is secured and maintained by special means at a 

 relatively low potential gradient, in the arc it is produced 

 and upheld automatically thanks to the existence of a high- 

 potential gradient. The high-temperature furnace spectrum 

 of iron as emitted either by a tube or a two-plate furnace 

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

 The line spectrum as obtained under these conditions is 

 brought about by the simultaneous actions of heat and of 

 electricity, and the process involved in its emission will be 

 referred to as ther mo-electrical excitation in distinction from 

 the more purely electrical mode of excitation which occurs 

 in the spark discharge as already mentioned in § 4. 



§ 8. Summary. 



I. All the results of the several observations and experiments 

 carried out in the course of this investigation harmonize 

 with the conclusion that the so-called high -temperature 

 furnace spectrum of iron, which is emitted above the 

 temperature of the boiling-point of this metal, is not 

 caused by purely thermal actions, but requires for its 

 emission the co-operation of electric forces. This con- 

 clusion is supported by the following observed facts : — 



a. The furnace spectrum of iron at 2700° C. is entirely 

 different from its flame spectrum at the same tempe- 

 rature. §§ 1 & 2. 



b. The relative behaviour of class III. lines, especially the 

 group of doublets at 4957, indicates that the high- 

 temperature furnace spectrum of iron approaches in 

 character that of the arc spectrum of this element. § 2. 



c. Direct experimental evidence has been furnished to the 

 effect that an ionization current will easily pass through 

 iron vapour in a tube-furnace. § 3. 



