on the Mechanical Function of an JEther. 419 



Either (1) because the inertia of aether is zero, or 



(2) because the inertia of aether is infinite, or 



(3) because aether is not disturbed by motion of plain 



matter, for want of any frictional connexion be- 

 tween them. 



Hypothesis 1 is apparently inconsistent with electromag- 

 netic experiments, with the hypothetical continuance of 

 electric currents inside molecules, and with the existence of 

 oscillations like light in free space. 



If the aether is of finite density and of infinite extent, of 

 course the total mass is infinite, and in that sense hypothesis 

 2 may be true : but infinitude of total mass, even combined 

 with incompressibility, would not prevent the aether from 

 affecting by its inertia the motion of bodies through it if it 

 flowed round them like a fluid. Non-disturbance must mean 

 that in some sense it flows straight through them, and its unre- 

 sisting immobility must signify an entire absence of viscosity. 



Direct optical experiments confirm this suspicion of zero 

 viscosity, and show that matter and aether are mechanically 

 disconnected ; we have no mechanical method at present 

 known for moving aether, i. e. for affecting the speed of light 

 through free or " unloaded " aether. The Fizeau experi- 

 ment, which shows that moving matter does affect light- 

 velocity, though it is sometimes misapprehended as meaning 

 that moving matter carries aether with it, really implies, and 

 was so understood by Fresnel who suggested it and predicted 

 its actual result, that the aether is not moved at all, but that 

 the extra speed of light is to be accounted for by an affection 

 or modification, or say " loading," of the aether by matter in 

 situ, and by a motion of the load (see Glazebrook, Phil. Mag. 

 Dec. 1888 ; also Phil. Trans. (1893) p. 731). Hence I may 

 say at once that the third of the above hypotheses is the one 

 which commends itself to me. 



But now plainly arises a question about this same " loading." 

 If matter is able to load aether, is not that ipso facto a 

 mechanical connexion between them ? And even the bare fact 

 of radiation and absorption, does not that represent mechanical 

 connexion between aether and matter ? 



I reply, if light is a mechanical oscillation, yes ; if it is an 

 electrical oscillation, no. For what is it that is moved when 

 light- waves are absorbed or refracted, and what is it whose 

 motion excites radiation ? In all probability not the atoms of 

 matter themselves, but their electric or ionic charges. An 

 electric charge has inertia of its own (as was first shown by 

 J. J. Thomson), and that inertia is sufficient to account for 

 the facts without the necessity for postulating any motion of 

 the material substratum, if there be any such substratum. 



