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 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 continued the experiments after Gray's death in 1736, gave 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 fluid, 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 conductors 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, 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