It was first shown that if we consider only those electric and magnetic phenomena which can be exactly expressed by equations containing only electric, magnetic and gravitational quantities, with those of length, mass and time, that there are an infinite number of theories possible. If we suppose that the electric and magnetic quantities are such as we meet with in mechanics the number of possible theories will be much reduced. We have no right to make this assumption, however, and the reduction cannot be regarded as rigorous, but rather as having a certain degree of probability or as serving as an indication, and hence can only be justified by showing that the results obtained by its means are in accordance with observed facts. On this basis it was shown that one of two theories is the correct one, i. e., that either κ is a density and μ a compliancy or μ is a density and κ a compliancy. It was then shown that if either coefficient is a compliancy, then, when the corresponding intensity is varied, the corresponding coefficient should vary in the opposite direction. If neither theory were correct this change should not necessarily take place. It was found that there is such a relation existing between one of the coefficients and the corresponding intensity, i. e., between μ and H, whilst there is no such relation observable between the other coefficient and intensity. It was then shown that the rate of change of the coefficient μ with the intensity H was exactly that called for by the theory and a most rigorous experiment showed no deviation from the rate required not within the limits of experimental error. It was shown that this relation held not only for iron and other strongly magnetic substances, but also for a wide range of substances whose reluctivities varied from 0.0003 to 1000, and was hence probably a universal relation. Corroborative evidence was then given, showing that the reluctivity varied in different materials, in the same materials under temporary stress and in the same materials under permanent strain, in such a way as to accord with the theory. It was also shown that there was a relation between the coefficients of reluctance and of hysteresis which was also in accordance with the theory. It was then shown that there was evidence of such a connection between capacity and density as agreed with the theory. From this last relation the density and elasticity of the ether were determined, to a first approximation.