ETHER AND PONDERABLE MATTER 



according to Lord Kelvin, is furnished by a mould of 

 transparent jelly. It is a crude, inaccurate analogy, of 

 course, the density and resistance of jelly in particular 

 being utterly different from those of the ether; but the 

 quivers that run through the jelly when it is shaken, 

 and the elastic tension under which it is placed when 

 its mass is twisted about, furnish some analogy to the 

 quivers and strains in the ether, which are held to con- 

 stitute radiant energy, magnetism, and electricity. 



The great physicists of the day being at one regard- 

 ing the existence of this all-pervading ether, it would 

 be a manifest presumption for any one standing with- 

 out the pale to challenge so firmly rooted a belief. 

 And, indeed, in any event, there seems little ground on 

 which to base such a challenge. Yet it may not be al- 

 together, amiss to reflect that the physicist of to-day is 

 no more certain of his ether than was his predecessor 

 of the eighteenth century of the existence of certain 

 alleged substances which he called phlogiston, caloric, 

 corpuscles of light, and magnetic and electric fluids. 

 It would be but the repetition of history should it 

 chance that before the close of another century the 

 ether should have taken its place along with these dis- 

 carded creations of the scientific imagination of earlier 

 generations. The philosopher of to-day feels very sure 

 that an ether exists; but when he says there is 

 doubt" of its existence he sjuaks incautiously, and 

 1 the bounds < >f demonstration. He does 

 not know that action cannot take place at a distance ; 

 ;<>es not know that empty spare itself may not per- 

 form the funeti' ms whieh he ascribes to his space-filling 

 eth< 



289 



