300 Dr. Norman Campbell on the 



decrease; in the conditions of my work the layer would be 

 very quickly restored. It is true that I found the effect of 

 the discharge to be the same in whichever direction it passed; 

 but Seelio-er does not mention that he tried the effect of 

 making his surface the anode. 



However, there is one serious objection to this explanation. 

 The double layer is presumably material, and it is extremely 

 improbable that 11 volt rays could penetrate even a single 

 layer of atoms ; they would be absorbed. The ionization 

 would take place at the outer surface of the double layer 

 and be unaffected by the potential in it. Moreover, the 

 estimate of the minimum ionization potential would not 

 require a correction for the potential in the layer, so long as 

 it was made by the method of § 6. We may then suppose 

 that the effect of the discharge which removes the layer is 

 merely to allow the rays to act upon the metal underneath 

 instead of on the material forming the layer. This supposition 

 would account well enough for the consequent diminution 

 of Z, but it would make the explanation which has been 

 offered between states A and B somewhat doubtful. 



If the effect of the discharge can be explained on the 

 supposition of a double layer, a similar explanation might 

 possibly be offered of the complicated effects of heat. We 

 might suppose that heat temporarily changed the potential 

 of the layer. 



18. However this may be, the important result of the 

 research, to my mind, is the proof that the ionization pro- 

 duced at a metal surface by cathode rays can be decreased 

 very markedly by the same treatment which reduces so 

 greatly the thermionic and photoelectric effects. It hns been 

 suggested freely of late that the theories of these two actions 

 which were once accepted are quite erroneous ; that the 

 thermionic current is not due to an increase in the thermal 

 energy of the free electrons, and that the photoelectric effect 

 is not the analogue of the emission of secondary cathode 

 radiation due to X rays; it has been suggested that both are 

 quite subsidiary actions due to some form of circular chemical 

 action. The suggestion was doubtless right in its application 

 to some thermionic currents; the experiments of Richardson* 

 on tungsten prove conclusively that it cannot be applied to all. 

 It is rather the application to the photoelectric effect that I 

 wish to combat. I have never seen how an " indirect" 

 theory could explain the intimate relation between the speed 

 of the liberated electrons and the frequency of the incident 

 light ; but the experiments which have been described seem 

 * Richardson, Phil. Mag. xxvi. p. 345 (1913). 



