the Occurrence of Enhanced Lines in the Arc. 289 



passing between metal poles using a feeble current with 

 small vapour production. If, on the contrary, the amount of 

 vapour was increased by heating one of the poles, the spark 

 lines would disappear. As a general result of all his observa- 

 tions Professor Hartmann arrives at the conclusion that the 

 spark lines do not correspond to a thermal radiation, but 

 rather to electro-luminescence. 



By carefully exploring the various regions of electric arcs 

 burning steadily between metal electrodes, Messrs. Fabry 

 and Buisson * and Professor Duffield f independently 

 discovered the emission of spark lines in the immediate 

 vicinity of the poles. Messrs. Fabry and Buisson attribute 

 the emission of spark lines under these conditions to the high 

 drop of potential which exists near the poles. According to 

 their view the ions would attain very high speeds near the 

 poles, and their collisions would give rise to the emission 

 of light radiations which could not be produced with low- 

 velocity ions. Temperature would in thiscase not intervene 

 at all. On the other hand, temperature by itself, if suffi- 

 ciently high, might give similar ionic velocities in the 

 absence of all electric actions. 



But spark lines have been observed also in flames under 

 certain conditions. Thus Messrs. Hemsalech and de Watte- 

 ville have shown that certain enhanced lines of iron are 

 emitted by the inner cone of the air-coal gas flame J. As 

 most of these lines were not seen in high temperature flames, 

 it seemed most plausible to connect their appearance in the 

 air-coal gas cone with the special chemical actions prevailing 

 therein. In discussing these results one of us directed 

 attention to the existence of several different types of 

 enhanced lines and, in a communication to the International 

 Union for Cooperation in Solar Research, proposed the 

 following provisional classification of enhanced lines § : — 



1st type. — Lines which are enhanced on passing from flames 

 of low to those of high temperature. Examples : X 3934 

 and A, 3968 of calcium. 



2nd type. — Lines emitted by the inner blue cone of the air- 

 coal gas flame but absent from the mantles of the 

 hotter flames. Only traces of a few of them have been 

 observed in the hottest region of the oxy-acetylene flame. 



* Fabry and Buisson, Journal de Physique, vol. ix. p. 929 (1910). 



t G. W. Duffield., Astrophysical Journal, vol. xxvii. p. 260 (1908). 



X Hemsalech and de Watteville, Comptes Rendus de V Academie des 

 Sciences, t. cxlvi. p. 1389 (1908). 



§ Hemsalech, Transactions Int. Union Solar Research, vol. iv. p. 145 

 (1914). 



Phil. Mag. S. 6. Vol. 43. No. 254. Feb. 1922. U 



