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 imagination. This was the view of Kirchhoff, Helmholtz, Hertz and Boltzmann, 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 Ampère, it would be absurd to give for the same law two theoretical explanations and to maintain that the two explanations hold simultaneously; for a physicist 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 Thomson'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 guarded 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 equations 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
