In the account that has been so far given, we have been dealing only with the equations of a hypothetical theory in which the only objects of consideration are the aether and free electricity. We must now pass on to the way in which this theory has been used to give an account of the electrical and magnetic properties of material bodies, and to the relation of these properties to the new point of view which has been outlined in the foregoing chapters.

The suggested conception of a material body is that it is pervaded, and partly or wholly constituted, by electric charges, which may be thought of, as has already been suggested, sometimes as point charges, sometimes as small nuclei of finite size. A molecule or atom is conceived to contain or to be composed of a group of such electrons, held together under their own mutual actions, and possibly in part by non-electromagnetic force. In addition to the electrons forming such groups there may (in the case of conducting bodies) be electrons moving between the molecules, or from one to another, so as to be capable of transporting a charge through any distance in the body.