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 through a wire. _II.--Theories of Magnetism_ Certain bodies--as, for instance, the iron ore called loadstone, the earth itself, and pieces of steel which have been subjected to certain treatment--are found to possess the following properties, and are called magnets. If a magnet be suspended so as to turn freely about a vertical axis, it will in general tend to set itself in a certain azimuth, and, if disturbed from this position, it will oscillate about it. It is found that the force which acts on the body tends to cause a certain line in the body--called the axis of the magnet--to become parallel to a certain line in space, called the "direction of the magnetic force."