MODERN VIEWS AND PROBLEMS OF PHYSICS. 513 



minded men had during the eighteenth century. . . . The only- 

 ether which has survived is that which was invented by Huygens 

 to explain the propagation of light." * 



Those ethers were working hypotheses which might be ex- 

 pected 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 en- 

 dowed 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." f 

 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 im- 

 pressed 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 princi- 

 pal 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 



* Article on Ether in the Encyclopaedia Britannica. 



f Popular Lectures and Addresses, by Sir William Thomson, vol. i, p. 317. 

 VOL. XXV. 40 



