556 POSITIVE AND NEGATIVE ELECTEIOITY. 



arranged in two great groups : each, body included in 

 either group repels every body in the same group, but 

 attracts every body in the other group. In order to 

 explain this fact and many other electrical phenomena, 

 it is supposed that there are two different electricities, 

 or rather electrical states, which bodies may assume, and 

 that bodies which are in the same state repel one another, 

 while those which are in an opposite state attract one 

 another; or, more briefly, like electricities repel, opposite 

 electricities attract each other. One of these electricities 

 is called positive, the other negative electricity. 



Of the substances already mentioned, sealing-wax, 

 sulphur, amber, ebonite, paper, become negatively elec- 

 trified when rubbed; glass becomes positive. If the 

 excited glass rod is held over the face, a sensation like 

 that produced by cobwebs spread over the face will 

 immediately be felt, and a peculiar smell may often be 

 noticed, not unlike that of phosphorus. The former 

 sensation arises from the attraction exerted by the glass 

 rod upon the small hairs which cover the surface of the 

 skin; the smell is due to the fact that the oxygen of the 

 air assumes a different condition in the vicinity of elec- 

 trical action ; in this condition oxygen is called ' ozone.' 



If the knuckle of a bent finger be drawn along the 

 excited rod, at a distance of a few millimetres from it, 

 a slight crackling sound will be heard, a faint prick will 

 be felt, and if the room is dark, a faint flash of light, 

 called the ' electric spark,' may be seen. 



Bodies may become electric, not only by friction, 

 but also by touching them with another body which is 

 already electrified. If an excited glass rod is brought 

 within a few centimetres of a few cork or pith balls 



