126 ELECTRICITY. 



see the electricity passing through the air as a 

 brilliant spark. It takes no sensible time to 

 pass through this, and if I were to take a long 

 metallic wire, no matter what the length, at 

 least as far as we are concerned ; and if I make 

 one end of it touch the outside, and the other 

 touch the knob at the top see how the elec- 

 tricity passes ! it has flashed instantaneously 

 through the whole length of this wire. Is not 

 this different from the transmission of heat 

 through this copper-bar (fig. 42), which has 

 taken a quarter of an hour or more to reach 

 the first ball ? 



Here is another experiment, for the purpose 

 of showing the conductibility of this power 

 through some bodies and not through others. 

 Why do I have this arrangement made of brass? 

 [pointing to the brass work of the electrical 

 machine, ^^7. 41]. Because it conducts electri- 

 city. And why do I have these columns made 

 of glass ? Because they obstruct the passage of 

 electricity. And why do I put that paper tassel 

 (fig* 43) at the top of the pole, upon a glass 

 rod, and connect it with this machine b^ means 

 of a wire ? You see at once that as soon as the 

 handle of the machine is turned, the electricity 



