THE ELECTRIC AND LUMINIFEROUS MEDIUM. 
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yEther contrasted ivitli Matter. 
7. The order of development here followed is thus avowedly based on the hypo¬ 
thesis that the aether is Sj very simple uniform medium, about which it may be 
possible to know all that concerns us ; and the present state of the theories of optics 
and electricity does much to encourage that idea. This procedure is of course at 
variance with the extreme application of the inductive canon, which would not allow 
the introduction of any hypothesis not based on direct observation and experiment. 
But though that philosophy has abundantly vindicated itself as regards the secondary 
properties of matter, which are amenable to direct examination, its rigid application 
would debar us from any theory of the aether at all, as we can only learn about it 
from circumstantial evidence. We could then merely go on heaping up properties on 
the aether, on the analogy of what is known of matter, as circumstances necessitated ; 
and this medium would be a sort of sink to dispose of relations that could not be 
otherwise explained. Whereas matter, with which we are familiar, is the really 
complicated thing on which all the maze of physical phenomena depends, so that it is 
doubtful whether much can ever be known definitely as to its ultimate dynamical 
constitution; our best chance is to try to approach it through tlie presumably simple 
and homogeneous aether in which it subsists. 
For example, it is found that the transmission of electrostatic force is affected by 
the constitution of the material dielectric through which it passes, and this is 
explained by a perfectly valid theory of polarization of the molecules of the matter: 
to press the analogy and ascribe the possibility of transmission through a vacuum to 
polarization of the aether may be convenient for some purposes of desci'iption, but in 
the majority of cases the impression is left that the so-called polarization of the 
aether is thereby exjdained. Whereas the processes being, almost certainly, of 
totally different character in the two cases, it will conduce to accurate thought to 
altogether avoid using the same term in the two senses, and to speak of the displace¬ 
ment of the aether which transmits electric force across a vacuum as producing 
polarization in the molecules of a material dielectric which exists in its path, which 
latter in turn affects the transmission of the electric force by reaction. In trying to 
pass beyond this stage, we may accumulate descriptive schemes of equations, which 
express, it may be with continually increasing accuracy, the empirical relations 
between these two phenomena ; but we can never reach very far below the surface 
without the aid of simple dynamical working hypotheses, more or less a priori, as to 
how this interaction between continuous aether and molecular matter takes j^lace. 
8. On the present viewq physical theory divides itself into two regions, but with a 
wide borderland common to both : the theory of radiation or the kinetic relations of 
this idtimate medium ; and the theory of the forces of matter which deals for the 
most part with molecular movements so slow that the surroundino’ aether is at each 
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instant practically in an ec|ui]ibrium condition, so that the material atoms practically 
