ut the mass, for what the charge of the
metal does for the portion of dielectric next to it, should be close by the
charged dielectric for the portion next beyond it again; but probably in
the best insulators the sensible charge is to a very small depth only in
the dielectric, for otherwise more would disappear in the first instance
whilst the original charge is sustained, less time would be required for
the assumption of the particular state, and more electricity would
re-appear as return charge.

1248. The condition of _time_ required for this penetration of the charge
is important, both as respects the general relation of the cases to
conduction, and also the removal of an objection that might otherwise
properly be raised to certain results respecting specific inductive
capacities, hereafter to be given (1269. 1277.)

1249. It is the assumption for a time of this charged state of the glass
between the coatings in the Leyden jar, which gives origin to a well-known
phenomenon, usually referred to the diffusion of electricity over the
uncoated portion of the glass, namely, the _residual charge_. The extent of
charge which can spontaneously be recovered by a large battery, after
perfect uninsulation of both surfaces, is very considerable, and by far the
largest portion of this is due to the return of electricity in the manner
described. A plate of shell-lac six inches square, and half an inch thick,
or a similar plate of spermaceti an inch thick, being coated on the sides
with tinfoil as a Leyden arrangement, will show this effect exceedingly
well.

* * * * *

1250. The peculiar condition of dielectrics which has now been described,
is evidently capable of producing an effect interfering with the results
and conclusions drawn from the use of the two inductive apparatus, when
shell-lac, glass, &c. is used in one or both of them (1192. 1207.), for
upon dividing the charge in such cases according to the method described
(1198. 1207.), it is evident that the apparatus just receiving its half
charge must fall faster in its tension than the other. For suppose app. i.
first charged, and app. ii. used to divide with it; though both may
actually lose alike, yet app. i., which has been diminished one-half, will
be sustained by a certain degree of return action or charge (1234.), whilst
app. ii. will sink the more rapidly from the coming on of the particular
state. I have endeavoured to avoid this