532 SPECIAL METHODS OF DISCOYERY. 





found additional bodies which became electric by friction. 

 In the year 1675, Sir Isaac Newton devised an experiment 

 of supporting a plate of glass upon thin slices of cork, and 

 rubbing its upper surface, and by this means discovered 

 that the lower side of the glass became electrically ex- 

 cited, and attracted light bodies beneath it. 



It was by means of new experiments that ' Grray, in 

 1729, discovered the properties of conductors. He found 

 that the attraction and repulsion which appear in electric 

 bodies are exhibited also by other bodies in contact with 

 the electric. In this manner he found that an ivory ball, 

 connected with a glass tube by a stick, a wire, or a pack- 

 thread, attracted and repelled a feather, as the glass itself 

 would have done. He was then led to try to extend this 

 communication to considerable distances, first by ascend- 

 ing to an upper window and hanging down the ball, and 

 afterwards, by carrying the string horizontally supported 

 on loops. As his success was complete in the former case, 

 he was perplexed by failure in the latter ; but when he 

 supported the string by loops of silk instead of hempen 

 cords, he found it again became a conductor of electricity. 

 This he ascribed at first to the small thickness of the silk 

 which did not carry off so much of the electric virtue ; 

 but from this explanation he was again driven, by finding 

 that wires of brass still thinner than the silk destroyed the 

 effect. Thus Gray perceived that the efficacy of the sup- 

 port depended on its being silk, and he soon found other 

 substances which answered the same purpose. The differ- 

 ence, in fact, depended on the supporting substance being 

 electric, and therefore not itself a conductor ; for it soon 

 appeared from such experiments, and especially from those 

 made by Du Fay, that substances might be divided into 

 electrics per SB, and non-electrics, or conductors. These 

 terms were introduced by Desagulier, and gave a perma- 



