418 Scientific Proceedings, Royal Dublin Society. 



witli properties analogous to rigidity is by being everywhere in 

 motion. The most general supposition of this kind would be, that 

 it was what Sir William Thomson has called a vortex- sponge, i. e. 

 everywhere endowed with vortex motion, but with this motion so 

 mixed up as to have within any sensible volume an equal amount 

 of vortex motion in all directions. There are many ways in which 

 this supposition seems to be in accordance with what we know of 

 the properties of the ether. One would expect that electrostatic 

 forces would be due to the irrotational parts of the fluid, while the 

 electro-magnetic forces would be due to the rotational parts. 

 There are a great many ways in which such a vortex sponge could 

 be polarised. If it consisted of vortex filaments they might have 

 either a solenoidal or lamellar polarisation ; and if it consisted of 

 vortex rings they might also have their motions polarised so as to 

 move parallel to lines or planes which would correspond to solenoidal 

 'or lamellar polarisations. Besides this, the axes of the vortex fila- 

 ments or rings themselves might be bent, or their sections dis- 

 torted, introducing thus two new sources of forces. It is thus 

 visible that the proposed supposition introduces modes of pro- 

 ducing force between different parts of the medium in a sufficient 

 number of ways to explain not only electric and magnetic forces, 

 but cohesional and chemical forces. I have not made any suppo- 

 sition as to the nature of matter. The supposition that the ether 

 is a vortex sponge in a perfect liquid, does not diminish the number 

 of possible hypotheses as to the constitution of matter: on the 

 contrary, it very much increases the possible modes of action of 

 matter. I know very little as to the mechanics of a vortex sponge, 

 but some points mentioned by Sir William Thomson, in a Paper 

 read before the British Association at Swansea (Brit. Assoc. Eep., 

 Swansea, p. 474), seem in favour of the hypothesis. In the first 

 place, the energy of any polarised state of vortex motion is greater 

 than that of an unpolarised state, so that if the motion of matter 

 reduce the polarisation, there will be forces tending to produce that 

 motion. Now, in the case of two infinite planes separated by a region 

 of polarised vortex motion of any kind, it seems evident that, as 

 the forces due to a small vortex vary as a high power of the dis- 

 tance from it, the forces due to the polarisation between the planes 

 will depend on this polarisation, and not on the distance apart of 

 the planes. This is the characteristic of forces varying inversely 



