l stress. The nature of the stress is, as Faraday pointed out,
a tension along the lines of force combined with an equal pressure in
all directions at right angles to these lines. This distribution of
stress is the only one consistent with the observed mechanical action on
the electrified bodies, and also with the observed equilibrium of the
fluid dielectric which surrounds them. I have, therefore, assumed the
actual existence of this state of stress.

Every case of electrification or discharge may be considered as a
motion in a closed circuit, such that at every section of the circuit
the same quantity of electricity crosses in the same time; and this is
the case, not only in the voltaic current, where it has always been
recognised, but in those cases in which electricity has been generally
supposed to be accumulated in certain places. We are thus led to a very
remarkable consequence of the theory which we are examining, namely,
that the motions of electricity are like those of an _incompressible_
fluid, so that the total quantity within an imaginary fixed closed
surface remains always the same.

The peculiar features of the theory as developed in this book are as
follows.

That the energy of electrification resides in the dielectric medium,
whether that medium be solid or gaseous, dense or rare, or even deprived
of ordinary gross matter, provided that it be still capable of
transmitting electrical action.

That the energy in any part of the medium is stored up in the form of a
constraint called polarisation, dependent on the resultant electromotive
force (the difference of potentials between two conductors) at the
place.

That electromotive force acting on a dielectric produces what we call
electric displacement.

That in fluid dielectrics the electric polarisation is accompanied by a
tension in the direction of the lines of force combined with an equal
pressure in all directions at right angles to the lines of force.

That the surfaces of any elementary portion into which we may conceive
the volume of the dielectric divided must be conceived to be
electrified, so that the surface density at any point of the surface is
equal in magnitude to the displacement through that point of the surface
_reckoned inwards_.

That, whatever electricity may be, the phenomena which we have called
electric displacement is a movement of electricity in the same sense as
the transference of a definite quantity of electricity thr