ble 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 permanent 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
tinfoil 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 progress 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 (1187, &c.), so as
nearly to fill the lower half of the space _o, o_ (fig. 104.) between it
and the inner ball; and then charges were divided in the manner already
described (1198. 1207.), each apparatus being used in turn to receive the
first charge before its division by the other. As the apparatus were known
to have equal inductive power when air was in both (1209. 1211.), any
differences resulting from the introduction of the shell-lac would show a
peculiar action in it, and if unequivocally referable to a specific
inductive influence, would establish the point sought to be sustained. I
have already referred to the precautions necessary in making the
experiments (1199, &c.); and with respect to the error which might be
introduced by the assumption of the peculiar state, it was guarded against,
as far as possible, in the first place, by operating quickly (1248); and,
afterwards, by using that dielectric as glass or sulphur, which assumed the
peculiar state most slowly, and in the least degree (1239. 1241.).

1257. The shell-lac hemisphere was put into app. i., and app. ii. left
filled with air. The results of an experiment in which the charge through
air was divided and reduced by the shel