It is not the purpose of this work to add another thread to the already tangled skein of scientific theory. It is written, rather, with the conviction, that it is impossible ever to get really behind or beyond phenomena; in the belief that the complete description of any natural process is simply the complete description of the associated phenomena, which may always be observed and co-ordinated but never ultimately explained. Phenomena must ever be accepted simply as phenomena--as the inscrutable manifestations of Nature. By induction from phenomena it is indeed possible to rise to working hypotheses, and thence, it may be, to general conceptions of Nature's order, and as already pointed out, it is to this method, of accepting phenomena, and of reasoning only from experimental facts, that all the advances of modern science are due. On the other hand, it is the neglect of this method--the departure, as it were, from Nature--which has led to the introduction into the scientific thought of the day of the various ethereal media with their extreme and contradictory properties. The use of such devices really amounts to an admission of direct ignorance of phenomena. They are, in reality, an attempt to explain natural operations by a highly artificial method, and, having no basis in fact, their whole tendency is to proceed, in ever-increasing degree, from one absurdity to another. It is quite possible to gain a perfectly true and an absolutely reliable knowledge of the properties of matter and energy, and the part which each plays, without resorting to speculative aids. All that is required is simply accurate and complete observation at first hand. The field of research is wide; all Nature forms the laboratory. By this method every result achieved may be tested and verified, not by its concurrence with any approved theory, however plausible, but by direct reference to phenomena. The verdict of Nature will be the final judgment on every scheme. It is on these principles, allied with the great generalisations with respect to the conservation of matter and energy, that this work is founded. As the result of a long, varied, and intimate acquaintance with Nature, and much experimental research in many spheres, the author has reached the conclusion, already foreshadowed in _Terrestrial Energy_, that the great principle of energy conservation is true, not only in the universal and generally accepted sense, but also in a particular sense with respect to all really separate bodies, such as planetary masses in space. Each of these bodies, therefore, forms within itself a completely conservative energy system. This conclusion obviously involves the complete denial of the transmission of energy in any form across interplanetary space, and the author, in this volume, now seeks to verify the conclusion by the direct experimental evidence of terrestrial phenomena.