THE ELECTRON THEORY. 285 



tube of which a portion of the wall is made of thin sheet alumi- 

 num, the cathode rays may be made to pass through into the 

 outside air. The properties of cathode rays thus obtained in the 

 air were first studied by Lenard who found that they produce a 

 very high degree of ionization of 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 pres- 

 sure, he found them capable of exciting luminescence, and he 

 found them capable of affecting a sensitive photographic plate. 

 143. 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. 208.* Helmholtz pointed out 



"razor wave 9 * ocean swell 



Fig. 208. 



in 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 regular way by the surface of a mirror, and they are 

 not refracted by a lens. They pass through all substances, sub- 

 jected to a certain amount of absorption which is greater the 

 greater the density of the substance, and subject to a certain 

 amount of diffused scattering. The Roentgen rays affect an 

 ordinary photographic plate and they have a powerful ionizing 

 effect on gases. 



*Very recent investigations seem to show that the Roentgen rays are not 

 isolated abrupt rays but trains of waves of very short wave length. 



