PHYSICAL FORCES 119 



by silk and which kind is called positive, or vitreous elec- 

 tricity ; the other (2), that produced on the wax by the 

 cloth, is termed negative, or resinous electricity. Further 

 each kind of electricity is communicable to a fresh object 

 by contact therewith. 



The experiment with the pith balls also shows that 

 (a) two bodies charged with similar kinds of electricity 

 repel each other, and (b) two bodies charged with different 

 kinds of electricity attract each other. 



A further examination of such bodies as we have 

 supposed to be experimented on, will show us 

 another most important law. If when the balls have 

 been repelled by the approach of the wax after previous 

 contact with it, the flannel, which has been used to rub 

 the wax, be brought near them, they will be attracted 

 by it, just as by a glass rod which has been rubbed with 

 silk ; while if the silk so used be brought near them, 

 they will be again repelled. This shows that at the 

 same time that the glass is acquiring positive electricity, 

 the silk which rubs it is acquiring negative electricity, 

 and that while the wax is acquiring negative electricity 

 the flannel is acquiring positive ; so that, bearing in mind 

 the extended use* given to two mathematical signs, the 

 electricity of both the glass and flannel may be dis- 

 tinguished by the sign + and that of the silk and wax 

 may be alike denoted by the sign - . Hence we see that 

 one kind of electricity cannot be evoked without at the 

 very same time evoking the opposite kind of electricity 

 somewhere else. So true is this, that if a piece of glass 

 have a small disc of metal attached on either side of it 

 and be suspended by a silk cord, then, if one of these 

 discs be made positively electrical, that alone will cause 



* See ante, p. 25. 



