ROENTGEN RAYS 157 



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 tin-ii 

 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 

 his hand between the screen and the tube, he saw, for the first 

 time, the now familiar sciagraph of the bones of the hand. The 

 X- or Roentgen rays originate from the place where a kathode 

 ray strikes, from the walls of the tube, in the first instance, or in a 

 focus tube from the piece of platinum (anti-kathode) upon which 

 the kathode rays are focussed. They issue equally in all directions 

 and travel in straight lines. For any tube, the power of penetra- 

 tion of the X-rays is inversely proportional to the density of the 

 substance penetrated. The higher the degree of exhaustion of 

 the tube the greater the penetrating power of the rays produced. 

 In a " hard " tube the vacuum is so good that a very great differ- 

 ence in potential between the electrodes is necessary to force the 

 discharge through. The kathode rays therefore attain a high 

 velocity and the X-rays they produce on impact with the anti- 

 kathode have a high penetrating power. On the other hand, if 

 the tube is not well " exhausted," the X-rays evolved are easily 

 absorbed. Such a tube is termed " soft." Unlike the kathode 

 rays, they are not affected by the most powerful magnetic field. 

 Like the kathode rays, they excite fluorescence, act on sensitised 

 photographic plates and ionise gases, i.e. they make air, or other 

 gas through which they pass and which under ordinary circum- 



