MECHANICS AND RADIUM. 201 



equality of action and reaction, or more briefly, the 

 principle of reaction. 



Astronomical observations, and the commonest 

 physical phenomena, seem to have afforded the most 

 complete, unvarying, and precise confirmation of these 

 principles. That is true, they tell us now, but only 

 because we have never dealt with any but low velo- 

 cities. Mercury, for instance, which moves faster than 

 any of the other planets, scarcely travels sixty miles a 

 second — Would it behave in the same way if it travelled 

 a thousand times as fast? It is clear that we have still 

 no cause for anxiety ; whatever may be the progress 

 of automobilism, it will be some time yet before we 

 have to give up applying the classical principles of 

 Dynamics to our machines. 



How is it then that we have succeeded in realizing 

 velocities a thousand times greater than that of 

 Mercury, equal, for instance, to a tenth or a third of 

 the velocity of light, or coming nearer to it even than 

 that? It is by the help of the cathode rays and 

 the rays of radium. 



We know that radium emits three kinds of rays, 

 which are designated by the three Greek letters a, ^, y. 

 In what follows, unless I specifically state the contrary, 

 I shall always speak of the ^ rays, which are analogous 

 to the cathode rays. 



After the discovery of the cathode rays, two opposite 

 theories were propounded. Crookes attributed the 

 phenomena to an actual molecular bombardment, 

 Hertz to peculiar undulations of the ether. It was a 

 repetition of the controversy that had divided physi- 

 cists a century before with regard to light. Crookes 

 returned to the emission theory, abandoned in the case 



