minded men had during the eighteenth century. . . . The onlyether which has survived is that which was invented by Huygens to explain the propagation of light."[1]
Those ethers were working hypotheses which might be expected to give way wholly or in part to better ones constructed for working purposes under fuller knowledge. So, too, at first, was the luminiferous ether, which, as a hypothesis, had to be endowed arbitrarily with properties suited to the phenomena it was to account for, but the ether of modern science is accepted as beyond question. For example, Lord Kelvin says: ". . . This thing we call the luminiferous ether. That is the only substance we are confident of in dynamics. One thing we are sure of, and that is the reality and substantiality of the luminiferous ether."[2] It is not necessary here to go into the evidences of its reality, but our belief in it rests upon exactly the same kind of evidence and just as strong evidence as does our belief in the existence of any kind of matter. For we only infer the existence of any form of matter from its phenomena, and the phenomena of light, heat, magnetism, and electricity to the extent of a very large group are not only explainable but are best explainable by the assumption of the ether. The defect as yet in such an assumption lies in the fact that the ether is a substance of an unfamiliar kind. It is this want of familiarity that physicists to-day are doing their utmost to overcome, and the more it is examined the more are they impressed by the multiplicity of purposes which this one medium is competent to serve and which it seems to be serving. The time for doubting its existence is past—it is now only a question as to its nature and properties; and it is accepted as a fact, not merely a hypothesis, that the same medium is concerned, if not a principal factor, in the phenomena of light, heat, magnetism, electricity, and gravitation. Radiant heat and light are wave motion in the ether, and are similar forms of energy, the only difference being in the period of vibration. Their manifestation as energy only occurs when the vibrations affect matter, and this, the most difficult part of the subject, involves the relation between ether and ordinary forms of matter. We say "ordinary forms" of matter, because ether may or may not be considered a form of matter.
One of the great, the primary questions of science is. What is ether? The question. What is light? has found its answer, so too has the query as to heat and as to sound; as to electricity, not so assuredly or so definitely, but both it and magnetism are to find their explanation through this same medium in some way or
