when that gas was acid or alkaline, it could
not properly be removed by the air-pump, and yet required to be perfectly
cleared away. In such cases the apparatus was opened and emptied of gas;
and with respect to the inner ball _h_, it was washed out two or three
times with distilled water introduced at the screw-hole, and then being
heated above 212 deg., air was blown through to render the interior perfectly
dry.

1195. The inductive apparatus described is evidently a Leyden phial, with
the advantage, however, of having the _dielectric_ or insulating medium
changed at pleasure. The balls _h_ and B, with the connecting wire _i_,
constitute the charged conductor, upon the surface of which all the
electric force is resident by virtue of induction (1178.). Now though the
largest portion of this induction is between the ball _h_ and the
surrounding sphere _aa_, yet the wire _i_ and the ball B determine a part
of the induction from their surfaces towards the external surrounding
conductors. Still, as all things in that respect remain the same, whilst
the medium within at _oo_, may be varied, any changes exhibited by the
whole apparatus will in such cases depend upon the variations made in the
interior; and these were the changes I was in search of, the negation or
establishment of such differences being the great object of my inquiry. I
considered that these differences, if they existed, would be most
distinctly set forth by having two apparatus of the kind described,
precisely similar in every respect; and then, _different insulating media_
being within, to charge one and measure it, and after dividing the charge
with the other, to observe what the ultimate conditions of both were. If
insulating media really had any specific differences in favouring or
opposing inductive action through them, such differences, I conceived,
could not fail of being developed by such a process.

1196. I will wind up this description of the apparatus, and explain the
precautions necessary to their use, by describing the form and order of the
experiments made to prove their equality when both contained common air. In
order to facilitate reference I will distinguish the two by the terms App.
i. and App. ii.

1197. The electrometer is first to be adjusted and examined (1184.), and
the app. i. and ii. are to be perfectly discharged. A Leyden phial is to be
charged to such a degree that it would give a spark of about one-sixteenth
or one-twentieth