action does occur, and particles must, to a certain degree, be displaced.  A third mode, namely, that by sparks or brushes, may, because of its violent displacement of the particles of the dielectric in its course, be called the disruptive discharge; and a fourth may, perhaps, be conveniently distinguished for a time by the words convection, or carrying discharge, being that in which discharge is effected either by the carrying power of solid particles, or those of gases and liquids.  Hereafter, perhaps, all these modes may appear as the result of one common principle, but at present they require to be considered apart; and I will now speak of the first mode, for amongst all the forms of discharge, that which we express by the term conduction appears the most simple and the most directly in contrast with insulation.

P vii. Conduction, or conductive discharge.

1320.  Though assumed to be essentially different, yet neither Cavendish nor Poisson attempt to explain by, or even state in, their theories, what the essential difference between insulation and conduction is.  Nor have I anything, perhaps, to offer in this respect, except that, according to my view of induction, insulation and conduction depend upon the same molecular action of the dielectrics concerned; are only extreme degrees of one common condition or effect; and in any sufficient mathematical theory of electricity must be taken as cases of the same kind.  Hence the importance of the endeavour to show the connection between them under my theory of the electrical relations of contiguous particles.

1321.  Though the action of the insulating dielectric in the charged Leyden jar, and that of the wire in discharging it, may seem very different, they may be associated by numerous intermediate links, which carry us on from one to the other, leaving, I think, no necessary connection unsupplied.  We may observe some of these in succession for information respecting the whole case.

1322.  Spermnceti has been examined and found to be a dielectric, through which induction can take place (1240. 1246.), its specific inductive capacity being about or above 1.8 (1279.), and the inductive action has been considered in it, as in all other substances, an action of contiguous particles.

1323.  But spermaceti is also a conductor, though in so low a degree that we can trace the process of conduction, as it were, step by step through the mass (1247.); and even when the electric force has travelled through it to a certain distance, we can, by removing the coercitive (which is at the same time the inductive) force, cause it to return upon its path and reappear in its first place (1245. 1246.).  Here induction appears to be a necessary preliminary to conduction.  It of itself brings the contiguous particles of the dielectric into a certain condition, which, if retained by them, constitutes insulation, but if lowered by the communication of power from one particle to another, constitutes conduction.