28 FORESHADOWING OF THE ELECTRON [CH. iv. 



now emitted from suitable substances, like alumina 

 and most earths ; and if the exhaustion is pressed 

 further still, the bombarded target emits no visible 

 light but only that higher kind of radiation known 

 as Rontgen or X-rays. It may be doubted, however, 

 whether the target itself emits these rays, whether its 

 function is not rather to stop the projectiles, as 

 suddenly as possible, by the massiveness of its atoms. 

 Thus the best target would be a substance with 

 the heaviest atoms. X-rays are emitted by the 

 suddenly stopped projectiles, in a manner which has 

 been investigated both by Sir G. Stokes and 

 Professor J. J. Thomson, and which is intelligible 

 to anyone who has studied the properties of moving 

 electric charges moving at or near the speed of light : 

 a matter on which Mr. Heaviside has written with 

 great clearness in his volume called Electromagnetic 

 Theory. 



Cathode rays have a remarkable penetrating 

 power; for Hertz found that a thin metal diaphragm, 

 especially if it were of aluminium, was powerless to 

 stop their passage completely ; as could be demon- 

 strated by the phosphorescence and other effects 

 appearing in the further half of the tube beyond the 

 diaphragm. 



The position of the anode in such experiments is of 

 small consequence. There must be one somewhere, 

 and the easiest plan is to make it a cylinder through 

 which the cathode ray bombardment goes. The 

 bombarding particles fly in straight lines and decline 

 to turn a corner, taking no apparent notice of the 

 position of the anode, and exhausting themselves by 

 bombarding the side of the glass opposed to them ; 

 as can be well shown by having the tube bent into 

 a V shape, for instance. 



