all the space _o,
o_, (fig. 104.) between the inner and the outer ball, except the small
portion filled by the stem; and the results, therefore, are twice as
delicate as those with solid dielectrics.

1294. The insulation was good in all the experiments recorded, except Nos.
10, 15, 21, and 25, being those in which ammonia was compared with other
gases. When shell-lac is put into ammoniacal gas its surface gradually
acquires conducting power, and in this way the lac part of the stem within
was so altered, that the ammonia apparatus could not retain a charge with
sufficient steadiness to allow of division. In these experiments,
therefore, the other apparatus was charged; its charge measured and divided
with the ammonia apparatus by a quick contact, and what remained untaken
away by the division again measured (1281.). It was so nearly one-half of
the original charge, as to authorize, with this reservation, the insertion
of ammoniacal gas amongst the other gases, as having equal power with them.

P vi. _General results as to induction._

1295. Thus _induction_ appears to be essentially an action of contiguous
particles, through the intermediation of which the electric force,
originating or appearing at a certain place, is propagated to or sustained
at a distance, appearing there as a force of the same kind exactly equal in
amount, but opposite in its direction and tendencies (1164.). Induction
requires no sensible thickness in the conductors which may be used to limit
its extent; an uninsulated leaf of gold may be made very highly positive on
one surface, and as highly negative on the other, without the least
interference of the two states whilst the inductions continue. Nor is it
affected by the nature of the limiting conductors, provided time be
allowed, in the case of those which conduct slowly, for them to assume
their final state (1170.).

1296. But with regard to the _dielectrics_ or insulating media, matters are
very different (1167.). Their thickness has an immediate and important
influence on the degree of induction. As to their quality, though all gases
and vapours are alike, whatever their state; yet amongst solid bodies, and
between them and gases, there are differences which prove the existence of
_specific inductive capacities_, these differences being in some cases very
great.

1297. The direct inductive force, which may be conceived to be exerted in
lines between the two limiting and charged conducti