238 SCIENCE AND THE HUMAN MIND 



observed phenomena ; the measured mass of the 

 corpuscle shows the calculated increase. 



Thus, at speeds such as these, the constancy of mass, 

 one of the fundamental axioms of the Newtonian 

 dynamics, fails us. A new dynamic must then come 

 into action. We shall be brought back to this re- 

 markable conclusion later by another road. 



Both cathode rays and Rontgen rays produce 

 luminosity when they strike phosphorescent screens. 



This phenomenon suggested the idea that 

 Radio-activity. , , i_. At. 



phosphorescent substances might them- 

 selves emit similar rays, and in 1896 led to the dis- 

 covery by Henri Becquerel that compounds of the 

 metal uranium, whether phosphorescent or not, 

 constantly emitted rays which affected a photographic 

 plate through opaque screens, and rendered gases 

 through which they passed conductors of electricity. 

 In 1898 M. and Mme. Curie, finding that the mineral 

 pitchblende was more radio-active than its contents 

 of uranium suggested, succeeded in isolating from it 

 compounds of an intensely active new element, to 

 which they gave the name of radium. 



Radio-active substances emit several types of radia- 

 tion. Two of these certainly consist of projected 

 electrified particles, which are deflected from their 

 straight paths by the action of electric and magnetic 

 forces, to an extent which shows that, while one type 

 is identical with the sub-atomic corpuscles of cathode 

 rays, the other consists of a stream of particles of 

 atomic dimensions, now known to be atoms of the 

 gas helium. 



The emission of rays is always accompanied by 



