After postulating atomicity for the aether, we are then able to apply the Newtonian Law of Gravitation to it, which distinctly affirms that "every particle of matter attracts every other particle," and so we arrive at Thomas Young's fourth hypothesis given in the Philosophical Transactions of 1802, where he asserts that "All material bodies have an attraction for the aetherial medium, by means of which it is accumulated within their substance, and for a small distance around them in a state of greater density." He adds the significant remark that this hypothesis is opposed to that of Newton's. With an atomic and gravitative aether it is shown in Chapter IV. how the elasticity, density, and inertia of the medium are brought into harmony with all observation and experiments. In the succeeding chapters the new theory is applied to the phenomena of heat, light, electricity, and magnetism, and the principles enunciated therein are then applied to solar and stellar phenomena. One of the greatest stumbling-blocks to the discovery of the physical cause of gravitation, apart from the unphilosophical theory of the aether medium, lies in the fact that apparently the Law of Gravitation only recognizes a force of one kind. Dr. Lodge refers to this phase of the subject on page 39 of his _Modern Views of Matter_ just published. It is here where scientists have failed to solve the problem of universal gravitation, as there are _two_ forces at work in the solar system and not one; that is, if we are to accept the results of up-to-date experiments in relation to radiant light and heat as performed by Professor Lebedew, and Nichols and Hull of America. Their experiments conclusively prove that light waves exert a pressure upon all bodies on which they fall, and by no reasoning can this pressure be resolved into an attractive force. Herschel in his _Lectures on Scientific Subjects_ definitely refers to the existence of a repulsive force in the solar system, and asserts that it offers the most interesting prospect of any future discovery. The author has therefore attacked the problem of the cause of gravitation, by trying to solve the problem of the cause of the _repulsive_ force which has been experimentally demonstrated to exist by Professor Lebedew and others. In his efforts to ascertain the physical cause of the Centrifugal Force, he has been assisted by an unknown and original essay written by an unknown writer over twenty years ago. That unknown writer was the author's father, who wrote an essay on the _Complementary Law of Gravitation_, and if it had not been for that essay, the present work would never have been attempted.