of the Electric and Luminiferous Medium. 



277 



legitimate and feasible problem to seek out these aspects of the under- 

 lying unity which are the cause of the interactions and correlations 

 of these agencies, so far as they have yet been unravelled. The 

 mere recognition of the precise order that reigns in the larger work- 

 ings of the unlimited diversity that constitutes matter is itself the 

 strongest argument that the "common basis of the varieties of matter 

 involves something simple! and universal in its relations, with which 

 we are really in physics more intimately concerned than with the 

 infinity of arrangements and collocations that the molecules of matter 

 can individually and collectively assume. 



It follows from the analysis of the second of these memoirs that 

 the action formula can be completely expressed and developed in a 

 molecular theory sufficiently refined to take account of each electron 

 separately ; also that the main outline of the ordinary electrodynamic 

 theory, for finite systems of bodies treated as continuous aggregates, 

 can be developed from the action formula transformed so as to be 

 ■expressed in terms of matter in bulk, when the currents are specified 

 as circuital and the different regions are homogeneous as regards 

 electric and magnetic polarisation. But in questions of details of 

 mechanical action on matter, especially when in motion, and in 

 questions involving the distinction between the true current carried 

 by the matter and the total circuital current, and also involving 

 heterogeneity in the dielectric, the pure dynamics does not suffice, 

 and recourse must be had to direct processes of averaging, such as 

 are necessary in other domains of molecular theory, as for example 

 the theory of gases. 



3. The utility of an elastic solid model of the kind above described 

 is not to represent the eebher, but to enlarge our ideas : for the optical 

 phenomena show that the elasticity of the actual aether is of rota- 

 tional, not distortional or elastic solid, type. It is however also 

 explained in the memoir how, out of matter gyrostatically dominated, 

 it is theDretically possible to construct a model which will represent 

 the aether itself and it3 electrons for any assignable time, though 

 not for ever : the properties of the model there described would 

 gradually fade away, just as if matter were not eternal. A- main 

 difficulty in designing such a representation lies in the circumstance 

 that it must possess the property of perfect fluidity for irrotational 

 motions. 



The distinction between the features of elastic transmission in 

 the aether and in material elastic substances is brought out. The 

 former medium is a pure continuum of which elasticity, inertia, 

 and continuity of motion, are the sole ultimate and fundamental pro- 

 perties. Matter on the other hand is made up of discrete atoms or 

 singular points in the aether ; it has t he inertia which belongs to 

 these singularities individually; it has elasticity on account of their 



