PITH-BALLS AS ELECTROSCOPES. 139 



toward the resinous substance, A ; as represented 

 in Fig. 53. On 

 impinging against 

 the excited body A, 

 the elastic balls re- 

 bound, and, hav- 

 ing received excita- 

 tion by contact, are 

 repelled therefrom, 

 and also recipro- Fig - 



cally from each other; as represented in Fig. 54. 



If the balls, while thus ex- ~ 

 cited and diverging from a 

 piece of rubbed amber, seal- 

 ing-wax, or other resinous 

 substance, are brought near 

 a piece of glass, or other 

 vitreous substance similarly 

 excited by friction, they F<f ' 54> 



are moved directly toward it, and develop the 

 phenomenon called attraction. 



To explain this remarkable movement of the 

 excited balls from an excited piece of amber, or 

 other resinous substance, and their subsequent 

 movement toward a piece of rubbed glass, or 

 other vitreous substance, Dufay originated a the- 

 ory of " the existence of two different kinds of elec- 

 tricity, in the two different kinds of resinous and 

 of vitreous substances ; each having self-repellent 

 powers, and reciprocally attractive powers for the 



