Fitzgerald — On a Model lUmtrafing Fropertie^ of Ether. 417 



prevent our supposing continuous rotation of its elements, wliich 

 seems almost necessary in order that the same quantities which 

 represent small motions in the light propagation may represent 

 known phenomena in electricity and magnetism. 



Although Professor Stokes seems to think that there is no con- 

 tradiction in supposing the ether to he a jelly, and at the same 

 time sufficiently little rigid to permit the free motion of matter 

 through it, nevertheless, there is no doubt that this is a serious 

 stumbling-block in the way of a general acceptance of the hypo- 

 thesis that the ether is, in all respects, like a thin jelly, and I 

 hardly think the difficulty diminished when its strains, as a rigid 

 body, are required to be capable of producing permanent electrical 

 forces. There are, of course, many ways in which matter may 

 move through the ether besides by displacing it ; as, for instance, in 

 the way in which a volume of liquid water might pass through ice, 

 namely, by dissolving in front, and by freezing as fast behind, 

 and such hypotheses do not require any limit to be assigned to the 

 rigidity of the ether. In all these cases it is, of course, evident, 

 that when it is once shown that the energy of the medium depends 

 on quantities which obey the laws of Maxwell's electric and mag- 

 netic induction and displacements, it follows that the forces on 

 the places that represent the electrified and magnetised bodies must 

 be the known electrical and magnetic attractions and repulsions ; 

 and one great difficulty in framing hypotheses as to the connection 

 of the ether and matter is in explaining how the matter moves 

 through the ether. 



A very beautiful theory of matter has been founded by Sir 

 William Thomson upon Helmholtz's theory of vortex motion in a 

 perfect liquid, and there seems no doubt that the simplest theory 

 as to the constitution of the ether is that it is a perfect liquid. It 

 seems unlikely, from Professor J. J. Thomson's investigations of 

 the properties of vortex atoms, that the simple hypothesis that an 

 atom is a mere vortex ring in a liquid otherwise at rest is a suffi- 

 cient hypothesis, and it seems almost impossible to explain electric 

 and magnectic phenomena without some further hypothesis. The 

 hypothesis that the ether is like a thin jelly in no way explains this 

 property, as it is the possession of properties analogous to rigidity 

 that requires explanation. Now, it seems certain that the only 

 way in which a perfect liquid can become everywhere endowed 



