38 Electric and Magnetic Science 



o 



that only a limited class of substances, among which the metals 

 were conspicuous, had the capacity of acting as channels for the 

 transport of the electric power ; to these Desaguliers, who. con- 

 tinued the experiments after Gray's death in 1736, gavfc^ the 

 name non-electrics or conductors. 



After Gray's discovery it was no longer possible to believe 

 that the electric effluvia are inseparably connected with the 

 bodies from which they are evoked by rubbing ; and it became 

 necessary to admit that these emanations have an independent 

 existence, and can be transferred from one body to another. 

 Accordingly we find them recognized, under the name of the 

 electric fluidft as one of the substances of which the world is 

 constituted. The imponderability of this fluid did not, for the 

 reasons already mentioned, prevent its admission by the side of 

 light and caloric into the list of chemical elements. 



The question was actively debated as to whether the electric 

 fluid was an element sui generis, or, as some suspected, was 

 another manifestation of that principle whose operation is seen 

 in the phenomena of heat. Those who held the latter view 

 urged that the electric fluid and heat can both be induced by 

 friction, can both induce combustion, and can both be transferred 

 from one body to another by mere contact ; and, moreover, that 

 the best conductors of heat are also in general the best con- 

 ductors of electricity. On the other hand it was contended that 

 the electrification of a body does not cause any appreciable rise 

 in its temperature; and an experiment of Stephen Gray's 

 brought to light a yet more striking difference. Gray,J in 1729,. 

 made two oaken cubes, one solid and the other hollow, and 

 showed that when electrified in the same way they produced 

 exactly similar effects ; whence he concluded that it was only 

 the surfaces which had taken part in the phenomena. Thus 

 while heat is disseminated throughout the substance of a body, 

 the electric fluid resides at or near its surface. In the middle of 



* Phil. Trans, xli. (1739), pp. 186, 193, 200, 209: Dissertation concerning 

 Electricity, 1742. 



t The Cartesians defined a fluid to be a body whose minute parts are in a 

 continual agitation. J Phil. Trans, xxxvii., p. 35. 



