548 POPULAR LECTURES AND ADDRESSES. 



hypothesis to suppose that it is chiefly a motion of 

 ether with a comparatively small but not incon- 

 siderable loading by fringes of ponderable mole- 

 cules carried with it. This makes Maxwell's 

 " electric displacement " simply a to-and-fro 

 motion of ether across the line of propagation, 

 that is to say, precisely the vibrations in the un- 

 dulatory theory of light according to Fresnel. But 

 we have as yet absolutely no guidance towards 

 any understanding or imagining of the relation 

 between this simple and definite alternating motion, 

 or any other motion or displacement of the ether, 

 and the earliest known phenomena of electricity 

 and magnetism the electrification of matter, and 

 the attractions and repulsions of electrified bodies ; 

 the permanent magnetism of lodestone and steel, 

 and the attractions and repulsions due to it : and 

 certainly we are quite as far from the clue to 

 explaining, by ether or otherwise, the enormously 

 greater forces of attraction and repulsion now so 

 well known after the modern discovery of electro- 

 magnetism. 



Fifty years ago it became strongly impressed on 



