The electric spark or flash.

1406.  The spark is consequent upon a discharge or lowering of the polarized inductive state of many dielectric particles, by a particular action of a few of the particles occupying a very small and limited space; all the previously polarized particles returning to their first or normal condition in the inverse order in which they left it, and uniting their powers meanwhile to produce, or rather to continue, (1417.—­1436.) the discharge effect in the place where the subversion of force first occurred.  My impression is, that the few particles situated where discharge occurs are not merely pushed apart, but assume a peculiar state, a highly exulted condition for the time, i.e. have thrown upon them all the surrounding forces in succession, and rising up to a proportionate intensity of condition, perhaps equal to that of chemically combining atoms, discharge the powers, possibly in the same manner as they do theirs, by some operation at present unknown to us; and so the end of the whole.  The ultimate effect is exactly as if a metallic wire had been put into the place of the discharging particles; and it does not seem impossible that the principles of action in both cases, may, hereafter, prove to be the same.

1407.  The path of the spark, or of the discharge, depends on the degree of tension acquired by the particles in the line of discharge, circumstances, which in every common case are very evident and by the theory easy to understand, rendering it higher in them than in their neighbours, and, by exalting them first to the requisite condition, causing them to determine the course of the discharge.  Hence the selection of the path, and the solution of the wonder which Harris has so well described[A] as existing under the old theory.  All is prepared amongst the molecules beforehand, by the prior induction, for the path either of the electric spark or of lightning itself.

  [A] Nautical Magazine, 1834, p 229.

1408.  The same difficulty is expressed as a principle by Nobili for voltaic electricity, almost in Mr. Harris’s words, namely[A], “electricity directs itself towards the point where it can most easily discharge itself,” and the results of this as a principle he has well wrought out for the case of voltaic currents.  But the solution of the difficulty, or the proximate cause of the effects, is the same; induction brings the particles up to or towards a certain degree of tension (1370.); and by those which first attain it, is the discharge first and most efficiently performed.

  [A] Bibliotheque Universelle, 1835, lix. 275.