IIo 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 Ampére, 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 lke 
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 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 
suecessor 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 
