Ionization of Platinum by Cathode Rays. 301 



to afford new evidence against that theory. Whatever 

 evidence there is that the photoelectric effect is the result of 

 some indirect action in which the gas plays a part, is now 

 available to prove the same proposition o£ the process of 

 ionization ; it clearly behoves anyone who supports the theory 

 in one of its applications to support it in the other. No 

 one can seriously doubt the truth of the main features of 

 the theory of ionization by charged particles ; there is no 

 reason to have any more doubt of the main features of the older 

 theory of photoelectric action. It appears that a much greater 

 energy is needed to eject electrons from a " pure " than a 

 " contaminated " surface of a metal by the agency of cathode 

 rays ; if the surface were perfectly "pure," it appears doubtful 

 whether an energy of 100 volts would be sufficient. Surely 

 it is likely then that a greater energy will be needed to eject 

 them by the agency of electromagnetic vibrations. The 

 reason why the photoelectric effect ceases when the surface 

 is perfectly " pure " is not that the process which gives rise 

 to the effect ceases in the absence of "contamination," but 

 simply that the pure surface holds its electrons much more 

 firmly. 



There is, of course, one objection. Towards the cathode 

 rays the " contaminated " surface behaves as if were a surface 

 of hydrogen ; it is characterized by the value of W for that 

 gas. But towards light it does not ; light which will cause 

 the photoelectric effect in a " contaminated " surface would 

 not cause it in hydrogen. But I have purposely written 

 " pure " and " contaminated " in inverted commas; it does 

 not seem at all certain that the words describe correctly the 

 relation between the two kinds of surface. It is true that on 

 the assumption that they do, the experiments which have 

 been described can be explained very simply; but there are 

 other possible assumptions. It may be thought indubitable 

 that the process which results in the change from state A 

 to state B consists in the removal of hydrogen from the 

 surface, but it does not follow that, because it is accompanied 

 by a decrease in ionization, the ionization takes place in the 

 hydrogen. The ionization may always take place in the 

 metal, the function of the hydrogen being only to facilitate 

 the escape of the liberated electrons. If this were so, state 

 A rather than state B should be termed "pure," for it would 

 be that state in which the properties of the metal were dis- 

 played most prominently. It is hoped by the continuation 

 of the work of which this paper is a preliminary account 

 to throw more light on these questions. 



