the total current was a continuous streaming flow; there proved to be no necessity, in the case of systems at rest, for keeping distinct the current of conduction, the current arising from changing electric polarization in a dielectric substance, and the displacement current belonging to free aether apart from matter altogether: the only hypothesis he required was that there is an aethereal current of such amount as to complete into a single circuital stream all the types of true electric flux which are associated with matter. These distinctions however become essential as soon as the theory is to take cognizance of the motion of the matter, especially in the domain of radiation where a mere equilibrium theory, contemplating the convection unaltered of its electric field along with the matter, is not a valid approximation. Then convection, relative to the aether, of electric charge and of dielectric polarization, contributes to the total current, as well as the change of aethereal elastic displacement and of material polarization. The problem thus presents itself in the form of two media, the aether and the matter, each with its own motion, but both occupying the same space; and some idea has to be formed of the interconnexions by which they influence each other. If we treat them both by the methods of continuous analysis, the only way open is to assume the most general linear relations between the two sets of variables representing the properties and states of the two media, and subsequently try to reduce the generality by aid of experimental indications. This is a well-tried course of procedure in abstract physics, and has been very effective under simpler and more easily grasped conditions: but even if successful it could hardly help us to mentally realize the connexion between aether and matter, while on the other hand the philosophical objections to filling the same space with several different media have been widely felt and emphasized.
17. Possibly the only sound procedure is the one which recommends itself on purely philosophical grounds. From remote ages the great question with which, since Newton's time, we have been familiar under the somewhat misleading antithesis of contact versus distance actions, has engaged speculation, — how it is that portions of matter can interact on each
