366 Mr. Faraday's Researches in Electricity. {Series XL) 



fluence, they appeared in separate places, and resumed of 

 necessity their power of acting by induction on the electricity 

 of surrounding bodies. Had the effect depended upon a pe- 

 culiar relation of the contiguous particles of matter only, then 

 each half plate, d and^ should have shown positive force on 

 one surface and negative on the other. 



1247. Thus it would appear that the best solid insulators, 

 such as shell-lac, glass, and sulphur, have conductive pro- 

 perties to such an extent, that electricity can penetrate them 

 bodily, though always subject to the overruling condition of 

 induction (1 1 78.). As to the depth to which the forces pene- 

 trate in this form of charge of the particles, theoretically, it 

 should be throughout the mass, for what the charge of the 

 metal does for the portion of dielectric next to it, should be 

 done by the charged dielectric for the portion next beyond it 

 again ; but probably in the best insulators the sensible charge 

 is to a very small depth only in the dielectric, for otherwise 

 more would disappear in the first instance whilst the original 

 charge is sustained, less time would be required for the as- 

 sumption of the particular state, and more electricity would 

 re-appear as return charge. 



1248. The condition of time required for this penetration 

 of the charge is important, both as respects the general re- 

 lation of the cases to conduction, and also the removal of an 

 objection that might otherwise properly be raised to certain 

 results respecting specific inductive capacities, hereafter to be 

 given (1269. 1277.). 



1249. It is the assumption for a time of this charged state 

 of the glass between the coatings in the Leyden jar, which 

 gives origin to a well-known phaenomenon, usually referred to 

 the diffusion of electricity over the uncoated portion of the 

 glass, namely, the residual charge. The extent of charge 

 which can spontaneously be recovered by a large battery, after 

 perfect uninsulation of both surfaces, is very considerable, and 

 by far the largest portion of this is due to the return of elec- 

 tricity in the manner described. A plate of shell-lac six 

 inches square, and half an inch thick, or a similar plate of 

 spermaceti an inch thick, being coated on the sides with tin- 

 foil as a Leyden arrangement, will show this effect exceed- 

 ingly well. 



1250. The peculiar condition of dielectrics which has now 

 been described, is evidently capable of producing an effect 

 interfering with the results and conclusions drawn from the use 

 of the two inductive apparatus, when shell-lac, glass, &c. are 

 used in one or both of them (1192. 1207.); for upon dividing 

 the charge in such cases according to the method described 



