IX 

 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 thirty years 

 ago by James Clerk-Maxwell, one of the very great- 

 est of nineteenth - century physicists, regarding the 

 existence of an all - pervading plenum in the uni- 

 verse, 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 "ponderable" matter or of 

 energy. It is true there are those among them who are 

 disposed to deny that this all-pervading plenum merits 

 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 nothing of 

 li^ht, of radiant heat, of electricity or magnetism; 

 without it there would probably be no such thing as 



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