ELECTRONS 71 



photographic plates ; they can discharge insulated 

 electrical bodies ; they have a definite momentum, 

 and they carry a fixed electric charge. Finally, 

 they are exactly the same in size and character 

 whatever the nature of the gas in the vacuum and 

 whatever the metal used for the electrodes^ and they 

 have an apparent mass of only about yoqo ^f ^^^ 

 hydrogen atom. Since the particles are projected 

 from the negative electrode or " cathode," they 

 are often called " cathode " rays. 



In 1895 Rontgen's discovery of the X-rays 

 attracted still more attention to the phenomena of 

 vacuum tubes; and in 1896 the famous French 

 savant, Henri Becquerel, undertook a series of 

 experiments with the object of investigating the re- 

 lationship between the phosphorescence of vacuum 

 tubes under the influence of " cathode " rays, and 

 the phosphorescence of various substances under 

 the influence of light or pressure. 



In the course of his experiments Becquerel 

 made the striking discovery that salts of uranium 

 have the power of spontaneously emitting invisible 

 radiations, which affect photographic paper, which 

 pass through metals and other substances opaque 

 to ordinary light, and which discharge electrified 

 bodies. Obviously these radiations were closely 

 akin to the cathode rays produced in a vacuum 

 tube. 



