IDi HISTORY OF ELECTRICITY. 



Dufay, whose experiments appear in the Memoirs of the I rench Academy, 

 in 1*733, 1*734, and 1737. * "I discovered," he says, " a very simple 

 principle, which accounts for a great part of the irregularities, and, if 

 I may use the term, the -caprices that seem to accompany most of the 

 experiments in electricity. This principle is, that electric bodies attract 

 all those that are not so, and repel them as soon as they are become 

 electric by the vicinity or contact of the electric body. . . . Upon 

 applying this principle to various experiments of electricity, any one 

 will be surprised at the number of obscure and puzzling facts which it 

 clears up." By the help of this principle, he endeavors to explain 

 several of Hawkesbee's experiments. 



A little anterior to Dufay's experiments were those of Grey, who, 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 packthread, 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 ascending to an upper window and 

 hanging down his ball, and, afterwards, by carrying the string horizon- 

 tally 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 destroyed 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 appeared from such experiments, and especially 2 from those made 

 by Dufny, that substances might be divided into electrics per se, and 

 non-electrics, or conductors. These terms were introduced by Desagu- 

 liers, 3 and gave a permanent currency to the results of the labors of 

 Grey and others. 



Another very important discovery belonging to this period is, that 



1 Priestley's History of Electricity, p. 45, ami the Memoirs quoted. 



2 J/fHt. Acad. Par. 1734. 3 Priestley, p. 66. 



