230 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



The cathode rays pass quite readily through thin metal plates 

 especially through thin plates of aluminum. By using a Crookes 

 tube of which a portion of the wall is made of thin sheet aluminum, 

 the cathode rays may be made to pass through into the outside 

 air. The properties of cathode rays in the air were first studied 

 by Lenard who found that they produce a very high degree of 

 ionization ol" the air making it a fairly good electrical conductor. 

 Lenard found the cathode rays capable of traversing from 10 to 

 20 centimeters of air at atmospheric pressure, he found them 

 capable of exciting luminescence, and he found them capable of 

 affecting a sensitive photographic plate. 



130. The Roentgen rays. Objects upon which the cathode 

 rays impinge, not only become heated and luminescent as de- 

 scribed above, but they emit a type of radiant energy which was 

 discovered by Roentgen in 1894. Roentgen rays are of the same 

 physical nature as light rays, that is, they consist of waves in the 

 ether, and they are related to light waves very much as a sharp 

 " razor" wave on the surface of water would be related to a long 

 ocean swell, as shown in Fig. 161. Helmholtz pointed out in 



"razor wave" ocean swell 



Fig. 161. 



1891 that abrupt wave pulses of this kind in the ether would have 

 certain properties, the properties, in fact, which are exhibited by 

 Roentgen rays, as follows : These rays are not reflected in a reg- 

 ular way by the surface of a mirror, and they are not refracted by 

 a lens. They pass through all substances, subject to a certain 

 amount of absorption which is greater the greater the density of 

 the substance, and subject to a certain amount of diffused scatter- 

 ing. The Roentgen rays affect an ordinary photographic plate 

 and they have a powerful ionizing effect on gases. 



The fluoroscope. Many substances such as barium platinocy- 



