of the Electric and Luminiferous Medium. 



275 



2. The object of the present memoir is to further develope and 

 apply this conception of the relations of aether and matter, which has 

 thus been shown to form a working basis for optics and electro- 

 dynamics. According to this scheme the aether is the seat of the 

 elastic transmission of electric and magnetic force : but the pondero- 

 motive forces acting on its electrons or strain-centres, which repre- 

 sent matter, are derived directly from the energy without any 

 reference to such transmission ; and this has been taken as an objec- 

 tion to the theory. We are therefore led to a critique of the 

 sufficiency of the principle of step-by-step transmission or contact 

 action as a basis for a complete connected representation of the 

 play of physical agencies. 



With a view to thus attaining more precise ideas as to what con- 

 stitutes ultimate physical explanation, the properties of a material 

 elastic solid medium involving in its constitution centres from which 

 intrinsic strain spreads out into the surrounding parts, are noticed. 

 "The theory of fluid vortices, irrespective of any claim to form a 

 natural representation of material phenomena, has been of funda- 

 mental service as an illustration of what kind of interconnexion it is 

 conceivable to assume between matter and the universal aether : in 

 vortex-atom theories the fundamental reality as regards pressure 

 and inertia is transferred to the continuous fluid aether, and the 

 properties of the atomic matter, which had been the original source 

 of dynamical suggestion, reappear as secondary or derivative. Now 

 just as a vortex atom is a state of intrinsic motion which can travel 

 or flit through the fluid, independently of, and in addition to, motion 

 of the fluid itself, so also can a strain-centre in an elastic solid 

 medium move about independently of the medium.* Its motion 

 forms an element beyond and in addition to the changing strain 

 impressed on the parts of the solid medium by external influence. It 

 would be easy to construct a solid body involving such strain-centres, 

 and this circumstance gives an impression of actuality which may 

 be felt as wanting in an abstract theory : the material of the solid 

 would illustrate aether, the strain-form would represent an essential 

 of matter. Such a representation is a step beyond and outside the 

 ordinary idea of an elastic medium as the mere vehicle for transmis- 

 sion of forces : that notion, effective in its own sphere, would not 

 give any account of where or how these forces originate, or what is 

 the nature of the connexion by which the matter gets a grip upon 

 the impalpable aether. Nor was it to be expected that the mere idea 

 of contact action, which has come to be interpreted as an idea of 

 elastic transmission, could of itself give account of the actions of 



* The analogy is not quite complete ; for the vortices move so that their cores 

 are always made up of the same portions of the medium, while movement of a 

 strain-centre does not to any extent carry the medium with it. 



