Then let a and b be discharged; the discharge destroys or neutralizes all external induction, and the coatings are therefore found by the carrier ball unelectrified; but it also removes almost the whole of the forces by which the electric charge was driven into the dielectric, and though probably a part of that charge goes forward in its passage and terminates in what we call discharge, the greater portion returns on its course to the surfaces of c, and consequently to the conductors a and b, and constitutes the recharge observed.

1246.  The following is the experiment on which I rest for the truth of this view.  Two plates of spermaceti, d and, f (fig. 114.), were put together to form the dielectric, a and b being the metallic coatings of this compound plate, as before.  The system was charged, then discharged, insulated, examined, and found to give no indications of electricity to the carrier ball.  The plates d and fwere then separated from each other, and instantly a with d was found in a positive state, and b with f in a negative state, nearly all the electricity being in the linings a and b.  Hence it is clear that, of the forces sought for, the positive was in one-half of the compound plate and the negative in the other half; for when removed bodily with the plates from each other’s inductive influence, they appeared in separate places, and resumed of necessity their power of acting by induction on the electricity of surrounding bodies.  Had the effect depended upon a peculiar relation of the contiguous particles of matter only, then each half-plate, d and f, should have shown positive force on one surface and negative on the other.

1247.  Thus it would appear that the best solid insulators, such as shell-lac, glass, and sulphur, have conductive properties to such an extent, that electricity can penetrate them bodily, though always subject to the overruling condition of induction (1178.).  As to the depth to which the forces penetrate in this form of charge of the particles, theoretically, it should be throughout the mass, for what the charge of the metal does for the portion of dielectric next to it, should be close by the charged dielectric for the portion next beyond it again; but probably in the best insulators the sensible charge is to a very small depth only in the dielectric, for otherwise more would disappear in the first instance whilst the original charge is sustained, less time would be required for the assumption of the particular state, and more electricity would re-appear as return charge.

1248.  The condition of time required for this penetration of the charge is important, both as respects the general relation of the cases to conduction, and also the removal of an objection that might otherwise properly be raised to certain results respecting specific inductive capacities, hereafter to be given (1269. 1277.)