1238.  As the return action was evidently greatest instantly after the discharge, I again put the apparatus together, and charged it for fifteen minutes as before, the inner ball positively.  I then discharged it, instantly removing the upper hemisphere with the interior ball, and, leaving the shell-lac cup in the lower uninsulated hemisphere, examined its inner surface by the carrier ball as before (1237.).  In this way I found the surface of the shell-lac actually negative, or in the reverse state to the ball which had been in it; this state quickly disappeared, and was succeeded by a positive condition, gradually increasing in intensity for some time, in the same manner as before.  The first negative condition of the surface opposite the positive charging ball is a natural consequence of the state of things, the charging ball being in contact with the shell-lac only in a few points.  It does not interfere with the general result and peculiar state now under consideration, except that it assists in illustrating in a very marked manner the ultimate assumption by the surfaces of the shell-lac of an electrified condition, similar to that of the metallic surfaces opposed to or against them.

1239. Glass was then examined with respect to its power of assuming this peculiar state.  I had a thick flint-glass hemispherical cup formed, which would fit easily into the space o of the lower hemisphere (1188. 1189.); it had been heated and varnished with a solution of shell-lac in alcohol, for the purpose of destroying the conducting power of the vitreous surface (1254.).  Being then well-warmed and experimented with, I found it could also assume the same state, but not apparently to the same degree, the return action amounting in different cases to quantities from 6 deg. to 18 deg..

1240. Spermaceti experimented with in the same manner gave striking results.  When the original charge had been sustained for fifteen or twenty minutes at about 500 deg., the return charge was equal to 95 deg. or 100 deg., and was about fourteen minutes arriving at the maximum effect.  A charge continued for not more than two or three seconds was here succeeded by a return charge of 50 deg. or 60 deg..  The observations formerly made (1234.) held good with this substance.  Spermaceti, though it will insulate a low charge for some time, is a better conductor than shell-lac, glass, and sulphur; and this conducting power is connected with the readiness with which it exhibits the particular effect under consideration.

1241. Sulphur.—­I was anxious to obtain the amount of effect with this substance, first, because it is an excellent insulator, and in that respect would illustrate the relation of the effect to the degree of conducting power possessed by the dielectric (1247.); and in the next place, that I might obtain that body giving the smallest degree of the effect now under consideration for the investigation of the question of specific inductive capacity (1277.).