Specific Induction, or Specific Inductive Capacity. 413 



will occur under such variation of circumstances, and that 

 the relations of B and C to A depend entirely upon their di- 

 stance. I only remember one experimental illustration of the 

 question, and that is by Coulomb*, in which he shows that 

 a wire surrounded by shell-lac took exactly the same quantity 

 of electricity from a charged body as the same wire in air. 

 The experiment offered to me no proof of the truth of the 

 supposition, for it is not the mere films of dielectric substances 

 surrounding the charged body which have to be examined 

 and compared, but the ixihole mass between that body and the 

 surrounding conductors at which the induction terminates. 

 Charge depends upon induction (1171. 1178.); and if induc- 

 tion relate to the particles of the surrounding dielectric, then 

 it relates to all the particles of that dielectric inclosed by the 

 surrounding conductors, 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 conclu- 

 sion that might be drawn from Coulomb's result ; and there- 

 fore had the apparatus made, which, with its use, has been 

 already described (1187, &c.), and which appears to me well 

 suited for the investigation of the question. 



1254f. Glass, and many bodies which might at first be con- 

 sidered as very fit to test the principle, proved exceedingly 

 unfit for that purpose. Glass, principally in consequence of 

 the alkali it contains, however well warmed and dried it may 

 be, has a certain degree of conducting power upon its sur- 

 face, 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 re- 

 jected, because of a slight degree of conducting power pos- 

 sessed by them ; and ultimately shell-lac and sulphur were 

 chosen, after many experiments, as the dielectrics best fitted 

 for the investigation. 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 

 indicate 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 ex- 

 periment, 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 induc- 

 tion sustained by that charged part, whether exerted through 



• Memoires de VAcadmie, 1787, pp. 452, 453. 



