THE NATURE OF ELECTRICITY. 39 



THE SPAEK AND SNAP. As already stated, every 

 substance offers a certain degree of resistance to the 

 passage of electricity, and a result of this resistance 

 is the generation of heat, often accompanied with light, 

 and this effect varies as the resistance. 



Platinum is a metal of high resistance, while that of 

 copper is very low; and a. fine platinum wire will be 

 brought to a white heat by an electric current which 

 would scarcely change the temperature of a copper wire 

 of the same size. 



Air offers such high resistance that the passage of 

 electricity through it always produces both heat and 

 light, in the form of a bright spark. This occurs not 

 only when an electric charge passes through several 

 inches of it, but through the thinnest film ; the pres- 

 ence of heat, as well as light, being demonstrated by the 

 lighting of gas by a spark less than J inch in length. 



A sudden condensation of the air, forced forward 

 and laterally by the charge, has been suggested as 

 the probable cause. If such condensation takes place, 

 heat would certainly be the result, as when air is com- 

 pressed by mechanical means. And perhaps it would 

 be accompanied by light, though this is not probable, 

 as combustion and incandescence are the only known 

 means of producing artificial light besides that now 

 under consideration, either of which would imply the 

 presence of some other substance besides air. But the 

 hypothesis seems to assume the passage of some material 

 substance through the air to produce the condensation, 

 of which there is no evidence. 



But if, instead of condensation, we suppose undula- 

 tions to take place, giving great intensity of motion, 

 as the electric impulse, darting forward with incon- 



