THE ELECTRIC AND LUMINIFEROUS MEDIUM. 
213 
acting on matter, for that would from this ultimate standpoint be a force acting on a 
strain-form spreading from its nucleus all through the medium, not a traction on a 
detinite surface bounding the matter. 
The fact is that transmission of force by a medium, or by contact action so-called, 
remains merely a vague phrase until the strain-properties of that medium are 
described ; the scientific method of describing them is to assign the mathematical 
function which represents its energy of strain, and thence derive its relations of 
stress by the principle of virtual work ; a real explanation of the transmission of a 
force by contact action must be taken to mean this process. Now in an elastic 
medium permeated by centres of permanent intrinsic strain, whether it be the 
rotational aether with its contained electrons, or an ordinary elastic solid jiermeated 
by polar strain-nuclei as described above, the specification of tiie strain-energy of the 
medium involves a mathematical function, not only of the displacement at each 
material point of the medium, but also of the positions of these intrinsic strain- 
centres which can move independently through it. To derive the play of internal 
force, this energy function must be varied with respect to all these independent 
quantities; the result is elastic fractional stress in the medium across every ideal 
interface, together loith forcive tending to displace each strain-centre, which we can 
consider either as resisted by extraneous constraint preventing displacement of the 
strain-centre, or as compensated by the reaction of the inertia of the strain-form 
against acceleration.''' Consider, for example, the analogy of the elastic solid medium, 
and suppose a portion of it to be slowly strained by extraneous force ; two strains 
are thereby set up in it, namely that strain which would be thus originated if the 
solid were initially devoid of intrinsic strain, and that strain which has to be super¬ 
posed in order to attain the new configuration of the intrinsic strain arising from the 
displacement of its nuclei. The latter part is conditioned by the displacement of 
these strain-centres, and in its production forces acting on them must be considered to 
assist, whose intensities may be determined as has been already done in the {ethereal 
problem. 
The attractions between material bodies are therefore not transmitted by the {ether 
in the way that mechanical tractions are transmitted by an ordinary solid, for it is 
electric force that is so transmitted : but neither are they direct actions at a distance. 
The point of view has been enlarged : the ordinary notion of the transmission of force, 
as framed mathematically by Lagrange and Green for a simple elastic medium 
without singularities, is not wide enough to cover the phenomena of a medium con¬ 
taining intrinsic strain-centres which can move about independently of the substance 
of the medium. But the same m{ithematical principles lead to the necessary exten¬ 
sion of the theory, when the energy function thus involves the positions of the 
* Thus when the medium is in equilibrium, there is in it only the static intrinsic strain diverging from 
these centres, which gives rise to the forces between them ; but when it is disturbed by radiation or 
otherwise, there is also the strain thenee arising. 
