CHAPTER VII 

 THE ETHER AND PONDERABLE MATTER 



" WHATEVER difficulties we may have in forming a 

 consistent idea of the constitution of the ether, there 

 can be no doubt that the interplanetary and interstellar 

 spaces are not empty, but are occupied by a material 

 substance or body which is certainly the largest and 

 probably the most uniform body of which we have any 

 knowledge." 



Such was the verdict pronounced some twenty years 

 ago by James Clerk Maxwell, one of the very greatest 

 of nineteenth-century physicists, regarding the existence 

 of an all-pervading plenum in the universe, in which 

 every particle of tangible matter is immersed. And this 

 verdict may be said to express the attitude of the entire 

 philosophical world of our day. Without exception, the 

 authoritative physicists of our time accept this plenum 

 as a verity, and reason about it with something of the 

 same confidence they manifest in speaking of " pondera- 

 ble " matter or of energy. It is true there are those among 

 them who are disposed to deny that this all-pervading 

 plenum merits the name of matter. But that it is a 

 something, and a vastly important something at that, all 

 are agreed. Without it, they allege, we should know 



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