no THE POPULAR SCIENCE MONTHLY 



electromagnetic terminology. It thus applies exactly to Fitzgerald's 

 and Larmor's resuscitation of McCullagh's rotational elastic theory, 

 which is found to be identical with the electromagnetic theory. 



I believe that I have thus given that definition of the ether which 

 best agrees with what Boltzmann calls the phenomenological view in 

 physics which attempts to exactly describe phenomena, without any 

 hypothesis, or any attempt at mechanical model to assist the imagina- 

 tion. This was the view of Kirchhoff, Helmholtz, Hertz and Boltz- 

 mann, and I believe it to be the most scientific. The English method, 

 of which Lord Kelvin was the leading example, demands concrete 

 models, which resemble the phenomena more or less, and which are 

 frequently changed. In the words of an acute French critic, M. 

 Duhem, for a geometer of the school of Laplace or Ampere, it would 

 be absurd to give for the same law two theoretical explanations and to 

 maintain that the two explanations hold simultaneously; for a phys- 

 icist of the school of Kelvin or Maxwell, there is no contradiction in 

 the same law being represented by two different models. I may also 

 quote Fitzgerald's words: 



I can not conclude without protesting strongly against Sir William Thom- 

 son's speaking of the ether as like a jelly. It is in some respects analogous to 

 one, but we certainly know a great deal too little about it to say that it is like 

 one. I also think that Sir William Thomson, notwithstanding his guarded 

 statements on the subject, is lending his overwhelming authority to a view of 

 the ether which is not justified by our present knowledge, and which may lead 

 to the same unfortunate results in delaying the progress of science as arose from 

 Sir Isaac Newton's equally gviarded advocacy of the corpuscular theory of optics. 



I feel that this protest is a very mild one, and that the attempt made 

 by Kelvin to determine the density and elasticity of the ether, from 

 very questionable assumptions, together with the recent attempts of 

 Lodge, based on equally naive conceptions of the nature of the ether as 

 a concrete substance, are greatly to be deplored. 



We come now to the most modern development of the ether theory. 

 Maxwell had, as has been said, accurately described the propagation 

 of the electromagnetic waves, and had given the differential equations 

 governing their propagation. It remained to add to these equa- 

 tions terms expressing the genesis of the waves, to show how these 

 resulted from the motion of charges of electricity. This was done in 

 an important series of papers begun in 1892 and continued until the 

 present by H. A. Lorentz, who may be characterized as the legitimate 

 successor of Maxwell. Not only did Lorentz add terms shown to be 

 necessary by the experiments of Rowland on the magnetic effect of 

 moving electric charges, and later by the deflection of the cathode rays 

 by a magnet, but he succeeded in showing for the first time how the 

 potentials determining the field were propagated in time through the 

 field, a result vainly sought by Gauss, Weber and Riemann, and almost 



