ROENTGEN RAYS 



119 



some reason the motion of an electron becomes centrifugal, then, 

 if sufficient speed be developed, it will tend to fly off. The 

 atom will be ruptured and a new atom will be formed. 



TABLE XIX. 



In 1878, Crookes found that the passage of an intermittent 

 high-tension current of electricity through a tube from which air 

 had been so thoroughly withdrawn that only about 10 ~ 7 of atmo- 

 spheric pressure was present, produced the so-called Kathode Rays. 

 These rays originate at the kathode at right angles to its surface 

 and proceed in straight lines like light independent of the position 

 of the anode. Whatever comes in the path of the rays is caused 

 to fluoresce, e.g. the walls of the tube. They heat the object 

 struck. By using a concave kathode, they may be focussed on 

 a piece of platinum which soon becomes red hot, and may even 

 be fused. Mechanical pressure is exerted by the rays. If directed 

 on to light vanes attached to an axle they may be made to turn 

 little mills, or in the " railway tube " they drive a wheel along 

 glass rails. The stream of rays is deflected by a magnet as if it 

 were a stream of negatively charged particles. In 1893, Lenard, 

 following up Hertz's discovery that metal was transparent to the 

 kathode rays, made a small window of aluminium foil in the end 

 of the vacuum tube and so brought the kathode rays through the 

 foil into the open air. 



In 1895, Roentgen, repeating Lenard's work, accidentally dis- 

 covered the X-rays. He had covered the vacuum tube with a 

 black paper case to shield the eyes from the kathode fluorescence, 

 so that the effect of the rays outside the tube might be more easily 

 observed. He thus noticed that a barium-platinocyanide screen 

 which happened to be near became fluorescent whenever the tube 

 was working though no visible rays could reach it. On placing 



