26 DR. FARADAY'S EXPERIMENTAL RESEARCHES IN ELECTRICITY. (SERIES XI.) 



the mere films of dielectric substances surrounding the charged body which have to 

 be examined and compared, but the whole mass between that body and the surround- 

 ing conductors at which the induction terminates. Charge depends upon induction 

 (II71. 1178.) ; and if induction relate to the particles of the surrounding dielectric, 

 then it relates to all the particles of that dielectric inclosed by the surrounding con- 

 ductors, and not merely to the few situated next to the charged body. Whether the 

 difference I sought for existed or not, I soon found reason to doubt the conclusion 

 that might be drawn from Coulomb's result ; and therefore had the apparatus made, 

 which, with its use, has been already described (1 187, &c.), and which appears to me 

 well suited for the investigation of the question. 



1254. Glass, and many bodies which might at first be considered as very fit to test 

 the principle, proved exceedingly unfit for that purpose. Glass, principally in conse- 

 quence of the alkali it contains, however well warmed and dried it may be, has a cer- 

 tain degree of conducting power upon its surface, dependent upon the moisture of 

 the atmosphere, which renders it unfit for a test experiment. Resin, wax, naphtha, 

 oil of turpentine, and many other substances were in turn rejected, because of a slight 

 degree of conducting power possessed by them ; and ultimately shell-lac and sulphur 

 were chosen, after many experiments, as the dielectrics best fitted for the investiga- 

 tion. No difficulty can arise in perceiving how the possession of a feeble degree of 

 conducting power tends to make a body produce effects, which would seem to indi- 

 cate that it had a greater capability of allowing induction through it than another 

 body perfect in its insulation. This source of error has been the one I have found 

 most difficult to obviate in the proving experiments. 



1255. Induction through shell-lac. — As a preparatory experiment, I first ascertained 

 generally that when a part of the surface of a thick plate of shell-lac was excited or 

 charged, there was no sensible difference in the character of the induction sustained 

 by that charged part, whether exerted through the air in the one direction, or through 

 the shell-lac of the plate in the other ; provided the second surface of the plate had 

 not, by contact with conductors, the action of dust, or any other means, become 

 charged (1203.). Its solid condition enabled it to retain the excited particles in a per- 

 manent position, but that appeared to be all ; for these particles acted just as freely 

 through the shell-lac on one side as through the air on the other. The same general 

 experiment was made by attaching a disc of tin foil to one side of the shell-lac plate, 

 and electrifying it, and the results were the same. Scarcely any other solid substance 

 than shell-lac and sulphur, and no liquid substance that I have tried, will bear this 

 examination. Glass in its ordinary state utterly fails ; yet it was essentially necessary 

 to obtain this prior degree of perfection in the dielectric used, before any further pro- 

 gress could be made in the principal investigation. 



1256. Shell-lac and air were compared in the first place. For this purpose a thick 

 hemispherical cup of shell-lac was introduced into the lower hemisphere of one of 

 the inductive apparatus (1 187, &c.), so as nearly to fill the lower half of the space o, o 



