76 THE THEORY OF IONIZATION OF GASES 



the positive electrode. It would appear from this that 

 some new process of ionization is called into play which 

 acts in addition to the effects of collisions. The force in 

 the neighbourhood of the positive electrode affects prin- 

 cipally the negative ions, all of which pass through that 

 region. The potentials which are obtained experimentally 

 can therefore be explained on the hypothesis that some 

 form of radiation is emitted from the anode when the 

 negative ions impinge on it. Some such effect might 

 be expected, as it is known that Eontgen rays are pro- 

 duced by the same cause when the pressure is very low. 

 The pressures under consideration are, however, large 

 compared with any pressure in a vacuum tube at which 

 Eontgen rays can be detected outside the tube by 

 ordinary methods, but it is possible that non-penetrating 

 rays of a similar kind may be produced, capable of 

 ionizing the gas inside the tube, at these comparatively 

 high pressures. The experimental results may thus 

 be explained in a general way, but it is impossible 

 at this stage to form a complete theory of the sparking 

 potentials at pressures below the critical pressure, as 

 they have not yet been examined very extensively 

 with a view to obtaining evidence as to the relative 

 importance of the processes of ionization that take place. 

 It may be pointed out, in connection with the possible 

 existence of radiations of a non-penetrating character 

 inside a discharge tube, that E. Wiedemann 1 has found 

 that, when a current is established in a gas, easily absorbed 

 rays (termed Entladungstrahleri) are emitted by the 

 luminous portions of the discharge, and Professor Sir 

 J. J. Thomson has shown that these rays have the 



1 Wied. Ann., lx., p. 269. 



