THE ELECTRON THEORY. 283 



the negative glow ceases to exist because ionization is no longer 

 produced there. That is to say, the obstacle casts a shadow on 

 the cathode and it also casts a shadow into the negative glow. 



The electric field intensity in the Crookes dark space, being 

 necessarily sufficient to enable the positive ions to produce ioniza- 

 tion at the surface of the cathode, is able to impart very much 

 greater velocity to the electrons than is necessary to enable them 

 to produce ionization. The result is that the electrons which are 

 thrown off from the cathode travel in straight lines through a long 

 portion of the tube. These high velocity electrons constitute 

 what are called cathode rays. The cathode rays are faintly 

 visible throughout the tube because of occasional collisions with 

 the molecules of the gas. 



When the cathode has a small hole through it, the positive ions 

 which move towards the cathode from the negative glow pass 



Fig. 207. 



through this hole in the form of a stream of rays which is made 

 visible by the luminosity which accompanies the collisions of the 

 positive ions with the molecules of the gas. Such a stream of 

 positive ions constitutes what has been called the canal rays. 



An object of any kind placed in the Crookes tube casts a sharp 

 shadow upon the wall of the tube, as shown in Fig. 207. The 

 wall of the tube shows a brilliant luminescence everywhere except 

 where it is screened by the obstacle from bombardment by the 

 cathode rays. 



