76 CORRELATION OF PHYSICAL FORCES, 



matter of the terminals is not merely brought, to a state of 

 incandescence, but is physically separated and partially trans- 

 ferred from one electrode to another, much of it being dissipated 

 in a vaporous state. It is not combustion ; 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 electrodes is con- 

 densed, 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 the 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 pheno- 

 menon presents all the appearance of combustion, and secondly 

 in the real combustion in air. With iron the experiment is 

 equally instructive. Iron is volatilised by the voltaic arc in 

 nitrogen or in an exhausted receiver ; and when a scarcely 

 perceptible film has lined the receiver, this is washed with an 

 acid, which then gives, with ferrocyanide of potassium, the 

 Prussian-blue precipitate. In this case we readily distil iron, 

 a metal by ordinary means fusible only at a very high 

 temperature. 



Another strong evidence that the voltaic discharge consists 

 of the material itself of which the terminals are composed, is 

 the peculiar rotation which is observed in the light when iron 

 is employed, the magnetic character of this metal causing its 

 molecules to rotate by the influence of the voltaic current. 



If we increase the number of reduplications in a voltaic 

 series, we increase the length of the arc, and also increase its 

 intensity or power of overcoming resistance. With a battery 

 consisting of a limited number, say 100 reduplications, the 

 discharge will not pass from one terminal to the other without 

 first bringing them into contact, but if we increase the number 



