[From Encyclopaedia Britannica.] 



XCVII. Ether. 



ETHER, or ^ETHER (aWijp, probably from aW<a, I burn, though Plato in his 

 Cratilus (410, b) derives the name from its perpetual motion on del del irepl 

 rov depa peiav, aeiQerjp SIKCUWS av /caXoiro), a material substance of a more 

 subtle kind than visible bodies, supposed to exist in those parts of space which 

 are apparently empty. 



The hypothesis of an sether has been maintained by different speculators 

 for very different reasons. To those who maintained the existence of a plenum 

 as a philosophical principle, nature's abhorrence of a vacuum was a sufficient 

 reason for imagining an all-surrounding aether, even though every other 

 argument should be against it. To Descartes, who made extension the sole 

 essential property of matter, and matter a necessary condition of extension, 

 the bare existence of bodies apparently at a distance was a proof of the 

 existence of a continuous medium between them. 



But besides these high metaphysical necessities for a medium, there were 

 more mundane uses to be fulfilled by aethers. ^Ethers were invented for 

 the planets to swim in, to constitute electric atmospheres and magnetic effluvia, 

 to convey sensations from one part of our bodies to another, and so on, till 

 all space had been filled three or four times over with aethers. It is only 

 when we remember the extensive and mischievous influence on science which 

 hypotheses about aBthers used formerly to exercise, that we can appreciate the 

 horror of aethers which sober-minded men had during the 18th century, and 

 which, probably as a sort of hereditary prejudice, descended even to the late 

 Mr John Stuart Mill. 



The disciples of Newton maintained that in the fact of the mutual gravi- 

 tation of the heavenly bodies, according to Newton's law, they had a complete 

 quantitative account of their motions; and they endeavoured to follow out the 

 path which Newton had opened up by investigating and measuring the attrac- 



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