572 The Photoelectric Fatigue of Metals. 



views of the character of the change may be summarized as 

 follows : — 



1. A chemical change such as oxidation of the surface. 



2. A physical change of the metal itself, as for example 



a roughening of the surface. 



3. An electrical change in the formation of an electrical 



double layer (Lenard *). 



4. A disintegration of the metal due to the expulsion of 



electrons by light (Ramsay and Spencer t). 



5. A change in the surface film of gas or in the gas 



occluded in the metal (Hallwachs). 



Hallwachs has shown from the behaviour of copper and 

 its oxides that oxidation cannot be the cause of fatigue, and 

 the results of the present paper confirming those of other 

 observers are inconsistent with the second, third, and fourth 

 views. We must therefore conclude with Hallwachs that 

 the main cause of photoelectric fatigue is to be found in the 

 condition of the gaseous layer at the surface of the plate. 

 This does not exclude the existence of secondary causes of 

 fatigue in particular cases. 



The fact that the activity at any instant can be expressed 

 by means of exponential terms is not inconsistent with the 

 theory put forward. It is sometimes assumed that equations 

 of the type here indicated necessarily refer to unimolecular 

 changes. But in certain cases reactions are met with which, 

 though really polymolecular, behave like unimolecular re- 

 actions %. Thus certain gaseous reactions take place on the 

 surface of the walls of the containing vessel, and the velocity 

 of the reaction is proportional to the pressure of the gas. 

 The chemical change then appears as a reaction of the first 

 order. Thus a purely surface action may simulate the 

 character of a unimolecular reaction. 



In conclusion I may be allowed a few words of personal 

 explanation. I have been represented as a supporter of the 

 theory that photoelectric fatigue is due to a kind of radio- 

 active change induced by light. Although when my first 

 paper was written I was prepared to recognise the 2^ossibilitt/ 

 of this explanation, I was careful to state that the nature of 

 the modifications § of the surface suggested was left an open 

 question. The view which I favoured for a long time was 

 that these modifications of the surface corresponded with the 



* P. Lenard, Ann. d. Phys. viii. p. 196 (1902). 



t Sir W. Eamsay and J. F. Spencer, Phil. Mag. [6] xii. p. 397 (1906). 



t H. M. Dawson, Nature, lxxi. p. 532 (1905). 



§ " Whether physical or chemical modifications of zinc." 



