The question may be stated thus:  suppose A an electrified plate of metal suspended in the air, and B and C two exactly similar plates, placed parallel to and on each side of A at equal distances and uninsulated; A will then induce equally towards B and C. If in this position of the plates some other dielectric than air, as shell-lac, be introduced between A and C, will the induction between them remain the same?  Will the relation of C and B to A be unaltered, notwithstanding the difference of the dielectrics interposed between them?[A]

  [A] Refer for the practical illustration of this statement to the
  supplementary note commencing 1307, &c.—­Dec. 1838.

1253.  As far as I recollect, it is assumed that no change will occur under such variation of circumstances, and that the relations of B find C to A depend entirely upon their distance.  I only remember one experimental illustration of the question, and that is by Coulomb[A], 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 whole mass between that body and the surrounding conductors at which the induction terminates.  Charge depends upon induction (1171. 1178.); and if induction is related to the particles of the surrounding dielectric, then it is related 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 conclusion that might be drawn from Coulomb’s result; and therefore 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.

  [A] Memoires de l’Academie, 1787, pp. 452, 453.

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 consequence of the alkali it contains, however well-warmed and dried it may be, has a certain 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 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 that which I have found most difficult to obviate in the proving experiments.