304 VOLTAIC ARC. SECT. XXIX. 



which appears between the poles of a very powerful battery, 

 depends upon the substance of the metal from whence it proceeds 

 and on the medium through which it passes. The spark from 

 zinc is blue, from silver it is green, from iron it is red and scin- 

 tillating precisely the colours afforded by these metals in their 

 ordinary combustion. But the colour varies also with the 

 medium through which the light passes, for when the medium is 

 changed a change takes place in the colour, showing an affection 

 of the intervening matter. A portion of the metal terminals or 

 poles is actually transmitted with every electrical or Voltaic dis- 

 charge, whence Mr. Grove concludes that the electrical discharge 

 arises, at least in part, from an actual repulsion and severance of 

 the electrified matter itself, which flies off at the points of least 

 resistance. He observes that " the phenomena attending the 

 electric spark or Voltaic arc tends to modify considerably our 

 previous idea of the nature of the electric force as a producer of 

 ignition and combustion. The Voltaic arc is perhaps, strictly 

 speaking, neither ignition nor combustion. It is not simply 

 ignition ; because the matter of the terminals is not merely 

 brought to a state of incandescence, but is physically sepa- 

 rated, and partially transferred from one terminal to another, 

 much of it being dissipated in a vaporous state. It is not com- 

 bustion; for the phenomena will take place independently of 

 atmospheric air, oxygen gas, or any of the bodies usually called 

 supporters of combustion ; combustion being in fact chemical 

 union attended with heat and light. In the Voltaic arc we may 

 have no chemical union, for if the experiment be performed in an 

 exhausted receiver, or in nitrogen, the substance forming the 

 terminals is condensed and precipitated upon the interior of the 

 vessel, in, chemically speaking, an unaltered state. Thus, to 

 take a very striking example, if the Voltaic discharge be taken 

 between zinc terminals in an exhausted receiver, a fine black 

 powder of zinc is deposited on the sides of the receiver; this can 

 be collected, and takes fire readily in air by being touched with 

 a match, or ignited wire, instantly burning into white oxide of 

 zinc. To an ordinary observer the zinc would appear to be burned 

 twice first in the receiver, where the phenomenon presents all 

 the appearance of combustion, and, secondly, in the real combus- 

 tion in air. With iron the experiment is equally instructive. Iron 

 is volatilized by the Voltaic arc in nitrogen, or in an exhausted 



