The Evolution of the Sciences 



numerous minerals, flames, metals exposed to the 

 sun's light, even snow, falling rain and leaves of 

 trees, are emitting them incessantly. The ex- 

 periments which reveal this emission are ex- 

 tremely simple; so it is a subject of surprise that 

 all these things should have escaped our atten- 

 tion so long. But such is the eternal history of 

 the progress of science, and the world is still full 

 of mysterious things invisible to us but which 

 the eyes of our successors will see as our own 

 distinguish the light of day. 



Physicists possess to-day numerous means 

 of producing corpuscles, and are able to define 

 their trajectory and determine their speed. 

 They dispose of an artillery whose projectiles, 

 all of the same mass, make up for their minute 

 size by their enormous speed. Thus the electrons 

 discharged from a metal plate exposed to ultra- 

 violet light have a speed of a thousand kilo- 

 metres per second; the electrons propagated in 

 Rontgen tubes and forming kathode rays may, 

 according to circumstances, reach speeds of from 

 20,000 to 100,000 kilometres, while the rays 

 emitted by radium are still more rapid. Their 

 speed ranges between 100,000 and 297,000 



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