286 Mr. Gr. A. Hemsalech on the Origin of the 



From the results of these experiments we may safely 

 conclude that at temperatures above 2500° a column of iron 

 vapour in the furnace will carry a small portion of the heating- 

 current supplied to the tube. Also it is highly probable that 

 some connexion exists between the flow of electricity through 

 the iron vapour and the brilliant line spectrum observed 

 under these temperature conditions. The interior of the 

 furnace-tube may in effect be regarded as a low-tension arc, 

 in which the necessary degree of ionization and the gaseous 

 state of the metal are maintained by the heat from the carbon 

 tube. The spectrum of iron emitted under such conditions 

 should therefore approach that given by an ordinary arc 

 between iron poles, which indeed it does. 



§ 4. Persistence of the Iron Line Emission after the 

 Electric Current through the Furnace is broken. 



As has been observed by Dr. King, the iron lines remain 

 visible for some time after the current feeding the furnace is 

 broken, and he has therefore concluded that the furnace 

 radiation does not depend upon the existence of a potential 

 difference. I quite agree that this conclusion holds as 

 regards those lines which are caused by thermo-chemical 

 excitation, and there is little doubt that a spectrum, com- 

 posed of these lines, would be observed if the tube were 

 heated by other than electrical means, as in fact it is observed 

 in the mantles of the several flames examined. But with 

 regard to the so-called high-temperature lines, which become 

 prominent only after the metal has passed into the gaseous 

 stafe and fills the interior of the furnace-tube, does their 

 persistence, after the potential is taken off, really prove 

 that they were not, in the first place, excited by electric 

 actions ? 



I have shown in a series of experiments that in an electric 

 spark discharge between metal electrodes the emission of 

 luminous radiations by the metal vapour continues for an 

 appreciable time after the discharge has passed*. In these 

 experiments the spark employed was of the simplest type, 

 consisting of only one single oscillation, so that the metal 

 vapour, which was carried away from the spark-gap by 

 means of a current of air, was no longer under the action 



* Hemsalech, Comptes Renchis de V Academie des Sciences, vol cl. p. 1743 

 {1910); ibid, vol. cli. p. 220 & p. 668 (1910). 



