But these, after all, are antique thoughts. Electric and Magnetic information has led us beyond them into a region of greater certainty and knowledge; so that now I am able to advocate a view of the Ether which makes it not only uniformly present and all-pervading, but also massive and substantial beyond conception. It is turning out to be by far the most substantial thing--perhaps the only substantial thing--in the material universe. Compared to ether the densest matter, such as lead or gold, is a filmy gossamer structure; like a comet's tail or a milky way, or like a salt in very dilute solution. To lead up to and justify the idea of the reality and substantiality, and vast though as yet largely unrecognised importance, of the Ether of Space, the following chapters have been written. Some of them represent the expanded notes of lectures which have been given in various places--chiefly the Royal Institution; while the first chapter represents a lecture before the Ashmolean Society of the University of Oxford in June, 1889. One chapter (viz. Chap. II) has already been printed as part of an appendix to the third edition of _Modern Views of Electricity_, as well as in the _Fortnightly_ and _North American Reviews_; but no other chapters have yet been published, though parts appear in more elaborate form in Proceedings or Transactions of learned societies. The problem of the constitution of the Ether, and of the way in which portions of it are modified to form the atoms or other constituent units of ordinary matter, has not yet been solved. Much work has been done in this direction by various mathematicians, but much more remains to be done. And until it is done, some scepticism is reasonable--perhaps laudable. Meanwhile there are few physicists who will dissent from Clerk Maxwell's penultimate sentence in the article "Ether" of which the beginning has already been quoted:-- "Whatever difficulties we may have in forming a consistent idea of the constitution of the æther, there can be no doubt that the interplanetary and interstellar spaces are not empty, but are occupied by a material substance or body, which is certainly the largest, and probably the most uniform body of which we have any knowledge." THE ETHER OF SPACE CHAPTER I THE LUMINIFEROUS ETHER AND THE MODERN THEORY OF LIGHT The oldest and best known function for an ether is the conveyance of light, and hence the name "luminiferous" was applied to it; though at the present day many more functions are known, and more will almost certainly be discovered. To begin with it is best to learn what we can, concerning the properties of the Interstellar Ether, from the phenomena of Light.