currence of that
effect which closes all the phenomena. This discharge is not only in its
appearance and condition different to the former modes by which the
lowering of the powers was effected (1320. 1343.), but, whilst really the
same in principle, varies much from itself in certain characters, and thus
presents us with the forms of _spark_, _brush_, and _glow_ (1359.). I will
first consider _the spark_, limiting it for the present to the case of
discharge between two oppositely electrified conducting surfaces.

_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.

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