Electrify the cork-balls as in the last experiment by glass, and
at the approach of an excited stick of wax their repulsion will be
increased. The effect will be the same, if the excited glass be
brought towards them, when they have been electrified by wax.
The bringing the excited glass to the end, or edge of the tin-tube,
in the third experiment, is supposed to electrify it positively, or
to add to the electrical fire it before contained; and therefore
some will be running off through the balls, and they will repel each
other. But at the approach of excited glass, which likewise _emits_
the electrical fluid, the discharge of it from the balls will be
diminished; or part will be driven back, by a force acting in a
contrary direction: and they will come nearer together. If the tube
be held at such a distance from the balls, that the excess of the
density of the fluid round about them, above the common quantity in
air, be equal to the excess of the density of that within them, above
the common quantity contained in cork; their repulsion will be quite
destroyed. But if the tube be brought nearer; the fluid without being
more dense than that within the balls, it will be attracted by them,
and they will recede from each other again.
When the apparatus has lost part of its natural share of this fluid,
by the approach of excited wax to one end of it, or is electrified
negatively; the electrical fire is attracted and imbibed by the balls
to supply the deficiency; and that more plentifully at the approach
of excited glass; or a body positively electrified, than before;
whence the distance between the balls will be increased, as the
fluid surrounding them is augmented. And in general, whether by the
approach or recess of any body; if the difference between the density
of the internal and external fluid be increased, or diminished; the
repulsion of the balls will be increased, or diminished, accordingly.