246 INDUCTION OF 



glass, that is, to a body which resists it, it is changed 

 into motion, evidenced by the fracture of the glass ; 

 or at the point of junction of two metals, where its 

 passage is resisted, it becomes electricity. Elec- 

 tricity again passes almost without change along a 

 wire, until its quantity becomes too great for the 

 wire to convey, when that which is resisted assumes 

 the form of heat or light. The electric spark arises 

 only at those points at which the passage of the 

 current is resisted. 



The production of electricity from chemical action 

 may be traced to the same law. 



If the flame of an ordinary taper be observed, a 

 current of wax is seen rising towards the flame. 

 There exists also another invisible current, viz. that 

 of the air towards the same flame, but in the opposite 

 direction. The atmosphere, however, being a com- 

 pound body, this atmospheric current is necessarily 

 double, the nitrogen being repelled from the flame as 

 the oxygen approaches it. 



Conceive now a flame supported by hydrogen on 

 the one side and air upon the other. On the one 

 hand the hydrogen being attracted to the point of 



