THE ELECTRIC AND LUMINIFEROUS MEDIUM. 
207 
the key-stone of electrodynamics will be found in an action propagated in time from 
one ‘‘ electric particle ” to another. The abstract philosophical distinction between 
actions at a distance and contact actions, which dates for modern science from 
Gilbert’s adoption of the scholastic axiom^"" Nulla actio fieri potest nisi per contactum, 
can have on an atomic theory of matter no meaning other than in the present sense. 
The question is simply whether a wider and more consistent view of the actions 
between the molecules of matter is obtained when we picture them as transmitted by 
the elasticity and inertia of a medium by which the molecules are environed, or when 
we merely describe them as forces obeying definite laws. But tins medium itself, as 
being entirely supersensual, we must refrain from attempting to analyse further. It 
would be possible [cf. § 6) even to ignore tbe existence of an aether altogether, and 
simply hold that actions are propagated in time and space from one molecule of 
matter to the surrounding ones in accordance with the system of mathematical 
equations which are usually associated with that medium ; in strictness nothing could 
be urged against such a procedure, though, in the light of our familiarity with the 
transmission of stress and motion by elastic continuous material media such as the 
atmosphere, the idea of an sethereal medium supplies so overwhelmingly natural and 
powerful an analogy as for purposes of practical reason to demonstrate the existence 
of the aether. The aim of a theory of the aether is not the impossible one of setting 
down a system of properties in which everything that may hereafter be discovered in 
physics shall be virtually included, but rather the practical one of simplifying and 
grouping relations and of reconciling apparent discrepancies in existing knowledge. 
2. It would be an unwarranted restriction to assume that the properties of the 
aether must be the same as belong to material media. The modes of transmission of 
remarks on the indirect manner in which dynamical equations had to he obtained, mainly on account of 
the absence of any notion as to the nature of the connexion between the stagnant aether and the 
molecules that are moving through it. “ Dans le chemin qui nous a conduit a ces equations nous avons 
rencontre plus d’une difficulte serieuse, et on sera probablement pen satisfait d’une tbeorie qui, loin de 
devoiler le mecanisme des phenomenes, nous laisse tout au plus I’espoir de le decouvrir un jour ” (§ 91). 
In the following year (1893) similar general ideas were introduced by vox Helmholtz, in his now well- 
known memoir on the electrical theory of optical dispersion, in which currents of conduction are 
included : hut his argument is very difficult, and the results are in discrepancy with those of Lorentz 
and the present writer in various respects in which the latter agree ; moreover they are not consistent 
with the optical properties of moving material media. Both these discussions, of Lorentz and of 
VON Helmholtz, are in the main confined to electromotive phenomena ; the treatment of the mechanical 
forces acting on matter in bulk would require for basis a theory of the mechanical relations of molecular 
media such as is developed in this paper. The results in the paper by Zeeman, above referred to, “ On 
the Influence of Magnetism on the Light emitted by a substance,” Verslagen Akad. Amsterdam, 
Nov. 28, 1896, have an important bearing on the view of the dynamical constitution of a molecule that 
has been advanced in these papers, and illustrated by calculation in an ideal simple case in Part I., 
§§ 114-8; cf. ‘Roy. Soc. Proc.,’ 60, 1897, p. 514. [See ‘Phil. Mag.,’ Dec. 1897 : where the loss of 
energy by radiatior ^rom the moving ions is also examined.] 
* Gilbert, de Magnete, 1600, 
