mental
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.

* * * * *

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 sensi