A HISTORY OF SCIENCE 



of the tube may be altered by the proximity of a pow- 

 erful magnet. From this it would seem that the rays 

 are composed of particles charged with negative elec- 

 tricity, and Professor J. J. Thomson has modified the 

 experiment of Perrin to show that negative electricity 

 is actually associated with the rays. There is reason 

 for believing, therefore, that the cathode rays are rap- 

 idly moving charges of negative electricity. It is pos- 

 sible, also, to determine the velocity at which these par- 

 ticles are moving by measuring the deflection produced 

 by the magnetic field. 



From the fact that opaque substances cast a shadow 

 in these rays it was thought at first that all solids were 

 absolutely opaque to them. Hertz, however, discov- 

 ered that a small amount of phosphorescence occurred 

 on the glass even when such opaque substances as 

 gold-leaf or aluminium foil were interposed between 

 the cathode and the sides of the tube. Shortly after- 

 wards Lenard discovered that the cathode rays can be 

 made to pass from the inside of a discharge tube to the 

 outside air. For convenience these rays outside the 

 tube have since been known as "Lenard rays." 



In the closing days of December, 1895, Professor 

 Wilhelm Konrad Roentgen, of Wiirzburg, announced 

 that he had made the discovery of the remarkable ef- 

 fect arising from the cathode rays to which reference 

 was made above. He found that if a plate covered 

 with a phosphorescent substance is placed near a dis- 

 charge tube exhausted so highly that the cathode rays 

 produced a green phosphorescence, this plate is made 

 to glow in a peculiar manner. The rays producing 

 this glow were not the cathode rays, although ap- 



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