330 DISCOURSE ON THE STUDY 



only when disturbed by excitements of peculiar 

 kinds. The most effectual of these is friction, 

 which we have already observed to be a powerful 

 source of heat. Everybody is familiar with the 

 crackling sparks which fly from a cat's back when 

 stroked. These, by proper management, may be 

 accumulated in bodies suitably disposed to receive 

 them, and, although then no longer visible, give 

 evidence of their existence by the exhibition of a vast 

 variety of extraordinary phenomena, producing 

 attractions and repulsions in bodies at a distance, 

 admitting of being transferred by contact, or by 

 sudden and violent transilience of the interval of 

 separation, from one body to another, under the 

 form of sparks and flashes ; traversing with perfect 

 facility the substance of the densest metals, and a 

 variety of other bodies called conductors, but being 

 detained by others, such as glass, and especially 

 air, which are thence called non-conductors, pro- 

 ducing painful shocks and convulsive motions, and 

 even death itself if in sufficient quantity, in animals 

 through which they pass, and finally imitating, on a 

 small scale, all the effects of lightning. 



(369.) The study of these phenomena and their 

 laws until a comparatively recent period occupied 

 the entire attention of electricians, and constituted 

 the whole of the science of electricity. It appears, 

 as the result of their enquiries, that all the pheno- 

 mena in question are explicable on the supposition 

 that electricity consists in a rare, subtle, and highly 

 elastic fluid, which in its tendency to expand and 

 diffuse itself pervades with more or less facility the 

 substance of conductors, but is obstructed and de* 



