Non-existence of Density in the Elemental ^Eiher, 475 



sible medium of uniform density, endowed with those dyna- 

 mical properties which are embodied in his fundamental 

 equations. These properties are not very unlike the properties 

 attributed to an ordinary solid body ; and the question now 

 arises, whether these properties (or whatever are the real 

 dynamical properties of the medium in which are propagated 

 light, radiant heat, and other waves of electromagnetic stress) 

 are fundamental properties of the medium ; or whether, like 

 the properties of solids, liquids, or gases, they are the outcome 

 of events of a wholly different character happening at inter- 

 vals so short that the elements of volume (the dx dy dz's of 

 MacCullagh's formulas) contain vast numbers of them. Now 

 the dynamical properties of the luminiferous medium — 

 whether we use MacCullagh's or Cauchy's fundamental 

 equations — sufficiently resemble those of media which we 

 know to be " textured," to make the latter supposition the 

 more probable, after what we have found to be the real nature 

 of solid, liquid, and gaseous media. And this probability is 

 very much strengthened by the discovery made by Helmholtz 

 about a quarter of a century ago, of the persistence and dyna- 

 mical behaviour of vortex-rings and other vortex filaments in 

 a perfect incompressible fluid, and by the investigations to 

 which this discovery has led. 



One result of these investigations has been to suggest to Sir 

 William Thomson that the chemical atoms of which ponderable 

 matter consists may be simply vortex tangles in such a medium ; 

 and to suggest to Professor Fitz Gerald that the luminiferous 

 aether may be a medium of this kind permeated by straight 

 vortex filaments in all directions. Investigations are being 

 actively pushed forward with a view to ascertaining how far 

 these suppositions can be corroborated. Other hypotheses 

 which may be classed with these have been advanced by 

 Professor Hicks and others, but I select Professor Fitz Gerald's 

 and Sir William Thomson's, both because they seem, in our 

 present imperfect state of information, the best of their class. 

 and in order to give detiniteness to what further I have to say. 



Let us then imagine this room to be permeated by three 

 systems of wires. Let the first be a set of vertical wires from 

 the ceiling to the floor in rows parallel to the walls, and at 

 intervals from one another of one inch. Let a similar system 

 cross the room from side to side, passing midway between the 

 wires of the first set : and let a third system of wires run 

 from end to end of the room threading their way along the 

 middle of the clear passages that lie between the wires of the 

 other two systems. Let these wires represent straight vortex 

 filaments in a uniform incompressible medium devoid of 



