1233.  I think it expedient, next in the course of these experimental researches, to describe some effects due to conduction, obtained with such bodies as glass, lac, sulphur, &c., which had not been anticipated.  Being understood, they will make us acquainted with certain precautions necessary in investigating the great question of specific inductive capacity.

1234.  One of the inductive apparatus already described (1187, &c.) had a hemispherical cup of shell-lac introduced, which being in the interval between the inner bull and the lower hemisphere, nearly occupied the space there; consequently when the apparatus was charged, the lac was the dielectric or insulating medium through which the induction took place in that part.  When this apparatus was first charged with electricity (1198.) up to a certain intensity, as 400 deg., measured by the COULOMB’S electrometer (1180.), it sank much faster from that degree than if it had been previously charged to a higher point, and had gradually fallen to 400 deg.; or than it would do if the charge were, by a second application, raised up again to 400 deg.; all other things remaining the same.  Again, if after having been charged for some time, as fifteen or twenty minutes, it was suddenly and perfectly discharged, even the stem having all electricity removed from it (1203.), then the apparatus being left to itself, would gradually recover a charge, which in nine or ten minutes would rise up to 50 deg. or 60 deg., and in one instance to 80 deg..

1235.  The electricity, which in these cases returned from an apparently latent to a sensible state, was always of the same kind as that which had been given by the charge.  The return took place at both the inducing surfaces; for if after the perfect discharge of the apparatus the whole was insulated, as the inner ball resumed a positive state the outer sphere acquired a negative condition.

1236.  This effect was at once distinguished from that produced by the excited stem acting in curved lines of induction (1203. 1232.), by the circumstance that all the returned electricity could be perfectly and instantly discharged.  It appeared to depend upon the shell-lac within, and to be, in some way, due to electricity evolved from it in consequence of a previous condition into which it had been brought by the charge of the metallic coatings or balls.

1237.  To examine this state more accurately, the apparatus, with the hemispherical cup of shell-lac in it, was charged for about forty-five minutes to above 600 deg. with positive electricity at the balls h and B. (fig. 104.) above and within.  It was then discharged, opened, the shell-lac taken out, and its state examined; this was done by bringing the carrier ball near the shell-lac, uninsulating it, insulating it, and then observing what charge it had acquired.  As it would be a charge by induction, the state of the ball would indicate the opposite state of electricity in that surface of the shell-lac which had produced it.  At first the lac appeared quite free from any charge; but gradually its two surfaces assumed opposite states of electricity, the concave surface, which had been next the inner and positive ball; assuming a positive state, and the convex surface, which had been in contact with the negative coating, acquiring a negative state; these states gradually increased in intensity for some time.