176 DISCOVERY CH. 



The law of gravitation has proved to be a universal 

 key which has opened the door of many secret places 

 in Nature, yet what the key itself is or wherein lies 

 the cause of the attraction of gravitation is still to 

 seek. Now that the interstellar ether is becoming 

 almost as familiar to us as a household word, and its 

 recondite properties are being unravelled, there is, 

 perhaps, some prospect of the problem being solved. 

 Efforts have been made to discover whether gravita- 

 tional force is propagated instantaneously through space. 

 If we could suddenly create or destroy a centre of 

 attraction, as we can a beam of light, the answer would 

 be fairly easy. But as it is, we can only watch whether 

 changes of position among the heavenly bodies produce 

 their proper effect upon other bodies at once or after 

 a measurable lapse of time. So far, no evidence of 

 such a lapse of time has been forthcoming, although 

 the most favoured theories demand it. 



One of those theories, originally due to Le Sage, sup- 

 poses that an infinite number of very small " ultra- 

 mundane " particles is constantly traversing space in 

 all directions. They are partly intercepted by ponderable 

 matter. Hence two bodies will shield each other from 

 the particles on one side, and the bombardment on the 

 outside will drive them together, producing an apparent 

 " attraction " between them. Reasons have been 

 found for the conclusion that at a certain great distance 

 gravitational attraction is intercepted altogether. If 

 this were true, Newton's grand conception of universal 

 gravitation would have to be modified, and we should 

 have to consider the visible universe as held together 

 by beams of light. 



Next to the law of gravitation, the greatest generalisa- 

 tion established bv science, and the one to which likewise 



