62 ULTIMATE SCIENTIFIC IDEAS. 



So that for the assumption of a luminif erous ether, there is 

 the. defence, not only that the exercise of force through 

 95,000,000 of miles of absolute vacuum is inconceivable, 

 but also that it is impossible to conceive motion in the ab- 

 sence of something moved. Similarly in the case of gravi- 

 tation. Newton described himself as unable to think that 

 the attraction of one body for another at a distance, could be 

 exerted in the absence of an intervening medium. But now 

 let us ask how much the forwarder we are if an intervening 

 medium be assumed. This ether whose undulations ac- 

 cording to the received hypothesis constitute heat and light, 

 and which is the vehicle of gravitation — how is it consti- 

 tuted ? We must regard it, in the way that physicists do re- 

 gard it, as composed of atoms which attract and repel each 

 other — infinitesimal it may be in comparison with those of 

 ordinary matter, but still atoms. And remembering that 

 this ether is imponderable, we are obliged to conclude that 

 the ratio between the interspaces of these atoms and the 

 atoms themselves is incommensurably greater than the like 

 ratio in ponderable matter; else the densities could not be 

 incommensurable. Instead then of a direct action by the 

 Sun upon the Earth without anything intervening, we have 

 to conceive the Sun's action propagated through a medium 

 whose molecules are probably as small relatively to their in- 

 terspaces as are the Sun and Earth compared with the space 

 between them: we have to conceive these infinitesimal 

 molecules acting on each other through absolutely vacant 

 spaces which are immense in comparison with their own di- 

 mensions. How is this conception easier than the other? 

 "We still have mentally to represent a body as acting where 

 it is not, and in the absence of anything by which its action 

 may be transferred ; and what matters it whether this takes 

 place on a large or a small scale ? We see therefore that 



the exercise of Eorce is altogether unintelligible. We can- 

 not imagine it except through the instrumentality of some- 

 thing having extension; and yet when we have assumed 



