TENDENCIES OF MODERN PHYSICS 47 



tempts to reconcile the antagonistic ideas of continuity 

 and atomicity by placing himself squarely on the side 

 of Descartes. He does not regard space as mere 

 empty geometrical continuity. According to his no- 

 tion, the universe is a plenum or ether that is, a con- 

 tinuous, frictionless fluid, everywhere uniform and 

 quiescent. This plenum constitutes what he calls true 

 matter. It is entirely unrecognizable by our senses 

 and cannot be brought to them by any experience. By 

 such a metaphysical hypothesis we may account for 

 the aberration of light and many other actions occur- 

 ring in free space. To provide for ordinary or sensible 

 matter,, making it at the same time discontinuous in 

 character, he supposes there exist in the plenum in- 

 numerable places of variation, which are uncreatable, 

 indestructible, and humanly indivisible, and by their 

 combination present to our senses all the phenomena 

 making up for us the material universe. These dis- 

 continuities are free to move without disturbing the 

 quiescence of the continuous medium, much as wind 

 blows through a forest. By this supposition we ac- 

 count for the experimental fact that motion of matter 

 does not affect the velocity of light in a vacuum. If, 

 however, the discontinuities show unbalanced electrical 

 force, then their motion causes real, although tempo- 

 rary, variations in the medium. 



The next requirement in his cosmogony is to specify 

 what these variations in the supposititious plenum may 



