4 PHYSIOLOGICAL PHYSICS. ichap. i. 



The nature of the electricity developed by friction 

 depends on the rubber as well as on the nature of the 

 thing rubbed. Glass receives a positive charge when 

 rubbed with silk, but a negative charge when rubbed 

 with cat-skin. Thus the same body may be either 

 positively or negatively electrified. 



Idioelectrics and anelectrics. It was at 

 first supposed that only certain bodies, like resin, 

 shellac, wax, sulphur, guttapercha, leather, glass, silk, 

 etc., could be electrified, and they were called idio- 

 electrics. Metals, carbon or coal, water, watery saline 

 solutions, etc., could not, it was believed, be electrified, 

 and they were, therefore, called anelectrics. 



Non-conductors and conductors are the 

 terms now used which correspond to idioelectrics and 

 anelectrics. They indicate the real facts. Idio- 

 electrics are really bodies which retain the electricity 

 with which they are charged at the place where it has 

 been received. Thus only the part of the glass or wax 

 that has been rubbed is electrified. In other words, 

 the electricity is not diffused over the surface of glass 

 or wax because they are non-conductors. Anelectrics 

 can be electrified ; but, as soon as the electricity has 

 been generated, it is conducted away over the whole 

 surface of the body, and thus becomes dissipated. 

 Even the term " non-conductor " is not strictly accu- 

 rate, because no body is an absolute non-conductor. 

 Some bodies, however, conduct very badly, and they 

 retain the electricity ; other bodies conduct very well, 

 and therefore electricity disappears as soon as it is 

 developed on the surface. Suppose, however, a bar of 

 metal is united to a bad conductor, say a rod of glass. 

 If then the metal, held by its glass support, be rubbed 

 vigorously, electricity will be produced, and will diffuse 

 itself over the whole metal surface; but the intervention 

 of the glass rod will prevent further escape, and the 

 usual signs of electrification will then be obtained. 



