THE VOLTAIC ARC. 17 



development of its brilliancy depends upon three concomitant 

 elements : first, the intensity of the current ; second, the 

 nature of the electrodes; third, the nature of the medium in 



which it is formed. These three elements affect also the 

 colour of the light : for example, with zinc electrodes it is 

 bluish : with silver, green ; with platinum, red ; and with 



2 



