SOME UNSOLVED SCIENTIFIC PROBLEMS 



matter in the universe. To make the hypothesis work- 

 able, so to say, it is necessary to assume that the " ultra- 

 mundane " particles are possessed of absolute elasticity, 

 so that they rebound from one another on collision 

 without loss of speed. It is also necessary to assume 

 that all tangible matter has to an almost unthinkable 

 degree a sievelike texture, so that the vast proportion 

 of the coercive particles pass entirely through the body 

 of any mass they encounter a star or world, for ex- 

 ample without really touching any part of its actual 

 substance. This assumption is necessary because grav- 

 itation takes no account of mere corporeal bulk, but 

 only of mass or ultimate solidarity. Thus a very 

 bulky object may be so closely meshed that it retards 

 relatively few of the corpuscles, and hence gravitates 

 with relative feebleness or, to adopt a more familiar 

 mode of expression, is light in weight. 



This is certainly heaping hypotheses together in a 

 reckless way, and it is perhaps not surprising that Le 

 Sage's conception did not at first arouse any very great 

 amount of interest. It was put forward about a cen- 

 tury ago, .but for two or three generations remained 

 practically unnoticed. The philosophers of the first 

 half of our century seem to have despaired of explaining 

 gravitation, though Faraday long experimented in the 

 hope of establishing a relation between gravitation 

 and electricity or magnetism. But not long after the 

 middle of the century, when a new science of dynamics 

 was claiming paramount importance, and physicists 

 were striving to express all tangible phenomena in 

 terms of matter in motion, the theory of Le Sage was 

 revived and given a large measure of attention. It 



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