THE CENTURY'S PROGRESS IN PHYSICS 



ifestations of motion among particles of matter. At 

 first glimpse that seems an enormous change of view. 

 And yet, when closely considered, that change in 

 thought is not so radical as the change in phrase might 

 seem to imply. For the nineteenth-century physicist, in 

 displacing the "imponderable fluids" of many kinds 

 one each for light, heat, electricity, magnetism has 

 been obliged to substitute for them one all-pervading 

 fluid, whose various quivers, waves, ripples, whirls, or 

 strains produce the manifestations which in popular 

 parlance are termed forms of force. This all-pervading 

 fluid the physicist terms the ether, and he thinks of it 

 as having no weight. In effect, then, the physicist has 

 dispossessed the many imponderables in favor of a single 

 imponderable though the word imponderable has been 

 banished from his vocabulary. In this view the ether 

 which, considered as a recognized scientific verity, is es- 

 sentially a nineteenth-century discovery is about the 

 most interesting thing in the universe. Something more 

 as to its properties, real or assumed, we shall have oc- 

 casion to examine as we turn to the obverse side of 

 physics, which demands our attention in the next chap- 

 ter. 



