electricity in an _instant_, and yet is charged far above the _mean_.

1270. Admitting for the present the general fact sought to be proved; then
1.5, though it expresses the capacity of the apparatus containing the
hemisphere of shell-lac, by no means expresses the relation of lac to air.
The lac only occupies one-half of the space _o, o_, of the apparatus
containing it, through which the induction is sustained; the rest is filled
with air, as in the other apparatus; and if the effect of the two upper
halves of the globes be abstracted, then the comparison of the shell-lac
powers in the lower half of the one, with the power of the air in the lower
half of the other, will be as 2:1; and even this must be less than the
truth, for the induction of the upper part of the apparatus, i.e. of the
wire and ball B. (fig. 104.) to external objects, must be the same in both,
and considerably diminish the difference dependent upon, and really
producible by, the influence of the shell-lac within.

* * * * *

1271. _Glass._--I next worked with glass as the dielectric. It involved the
possibility of conduction on its surface, but it excluded the idea of
conducting particles within its substance (1267.) other than those of its
own mass. Besides this it does not assume the charged state (1239.) so
readily, or to such an extent, as shell-lac.

1272. A thin hemispherical cup of glass being made hot was covered with a
coat of shell-lac dissolved in alcohol, and after being dried for many
hours in a hot place, was put into the apparatus and experimented with. It
exhibited effects so slight, that, though they were in the direction
indicating a superiority of glass over air, they were allowed to pass as
possible errors of experiment; and the glass was considered as producing no
sensible effect.

1273. I then procured a thick hemispherical flint glass cup resembling that
of shell-lac (1239.), but not filling up the space _o, o_, so well. Its
average thickness was 0.4 of an inch, there being an additional thickness
of air, averaging 0.22 of an inch, to make up the whole space of 0.62 of an
inch between the inductive metallic surfaces. It was covered with a film of
shell-lac as the former was, (1272.) and being made very warm, was
introduced into the apparatus, also warmed, and experiments made with it as
in the former instances (1257. &c.). The general results were the same as
with shell-lac, i.e. glass surpa