MATTER AND ETHER THOMSON. 235 



that bodies when electrified are not subject to the third law, and that 

 therefore any mechanical explanation of the forces due to such 

 bodies is impossible? This would mean giving up the hope of 

 regarding electrical phenomena as arising from the properties of 

 matter in motion. Fortunately, however, it is not necessary. We can 

 follow a famous precedent and call into existence a new world to 

 supply the deficiencies of the old. We may suppose that connected 

 with A and B there is another system which, though invisible, pos- 

 sesses mass and is therefore able to store up momentum, so that when 

 the momentum of the A and B system alters, the momentum which has 

 been lost by A and has not gone to B has been stored up in the in- 

 visible system with which they are in connection, and that A and B 

 plus the invisible system together form a system which obeys the 

 ordinary laws of mechanics and whose momentum is constant. We 

 meet in our ordinary experience cases which are in all respects analo- 

 gous to the one just considered. Take for example the case of two 

 spheres, A and B, moving about in a tank of water. As A moves it 

 will displace the water around it and produce currents which will 

 wash against B and alter its motion ; thus, the moving spheres will 

 appear to exert forces on each other. These forces have been cal- 

 culated by Kirchhoff and resemble in man}^ respects the forces be- 

 tween moving electric charges; in particular unless the two spheres 

 are moving with the same speed and in the same direction the forces 

 between them are not equal and opposite, so that the momentum of 

 the two spheres is not constant. If, however, instead of confining 

 our attention to the spheres we include the water in which they are 

 moving, we find that the spheres plus the water form a system which 

 obeys the ordinary laws of dynamics and whose momentum is con- 

 stant ; the momentum lost or gained by the spheres is gained or lost 

 by the water. The case is quite parallel to that of the moving elec- 

 tric charges, and we may infer from it that when we have a system 

 whose momentum does not remain constant, the conclusion we should 

 draw is not that Xewton's third law fails, but that our S3'stem, in- 

 stead of being isolated as we had supposed, is connected with another 

 system which can store up the momentum lost by the primary, and 

 that the motion of the complete system is in accordance with the 

 ordinary laws of dynamics. 



Returning to the case of the electrified bodies we see then that these 

 must be connected Avith some invisible universe, which we may call 

 the ether, and that this ether must possess mass and be set in motion 

 when the electrified bodies are moved. We are thus surrounded by 

 an invisible universe with which we can get into touch by means of 

 electrified bodies; whether this universe can be set in motion by 

 bodies which are not electrified is a question on which we have as 

 yet no decisive evidence. 



