LAWS OF ELECTRIC PHENOMENA. 11 



led to try to extend this communication to consi- 

 derable distances, first by ascending to an upper 

 window and hanging down his 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 become 

 a conductor of electricity. This he ascribed at first 

 to the smaller 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 de- 

 stroyed the effect. Thus Grey perceived that the 

 efficacy of the support depended on its being silk, 

 and he soon found other substances which answered 

 the same purpose. The difference, in fact, depended 

 on the supporting substance being electric, and 

 therefore not itself a conductor; for it soon ap- 

 peared from such experiments, and especially 2 from 

 those made by Dufay, that substances might be 

 divided into electrics per se, and non-electrics, or 

 conductors. These terms were introduced by De- 

 saguliers 3 , and gave a permanent currency to the 

 results of the labours of Grey and others. 



Another very important discovery belonging to 

 this period is, that of the two kinds of electricity. 

 This also was made by Dufay. " Chance," says he, 

 " has thrown in my way another principle more 



3 Mem. Acad. Par. 1734. 3 Priestley, p. 66. 



