570 Dr. H. Stanley Allen on the 



passing over solid caustic potash*. The fatigue in nydrogen 

 was found to be very similar to that in air. 



Some results as to th9 fatigue of other metals are here 

 briefly summarized. 



Silver. — One of the most interesting cases examined was 

 a plate of pure silver supplied by Messrs. Johnson, 

 Matthey & Co. The plate was polished with rouge paper 

 and put in position in the brass testing vessel. After two 

 hours' continuous exposure to the light of the mercury-vapour 

 lamp the activity remained unaltered. 



It is remarkable that Ladenburg |, who carried out fatigue 

 experiments in a vacuum, mentions silver as one of the metals 

 showing marked fatigue, but it must be remembered that his 

 surfaces were " polished once with emery and oil." 



When the air in the testing vessel was replaced by hydro- 

 gen, the same result was obtained as in air; that is, no 

 fatigue could be detected after two hours' exposure to the 

 source of light. 



The same plate tested later in the air of the room showed 

 fatigue effects, the activity falling to half its initial value in 

 two hours. We have here an example of the influence of 

 the size of the vessel to be discussed later. 



Aluminium. — This metal was found to behave in much the 

 same way as zinc. Its activity can be represented by the 

 sum of two exponential terms. When examined in air in 

 the closed testing vessel, using the mercury-vapour lamp 

 as the source of light, the first term fell to half value in 

 6 minutes, the second in J 90 minutes. In hydrogen the 

 values were not very different, being 7 minutes and 165 

 minutes, respectively. 



Copper. — When a copper plate was examined in the 

 testing vessel, the fatigue proceeded slowly from the outset ; 

 about 3 hours would be required for the activity to fall to 

 one-half of the initial value. In hydrogen the fatigue took 

 place at about the same rate as in air. In the air of the 

 room more rapid fatigue was observed. 



* It should be noted that the metal plate was polished and put in 

 position in the testing vessel, the air was displaced by a current of 

 hydrogen, and readings of the activity were commenced a few minutes 

 after polishing. It is not probable that the gas was entirely free from 

 water-vapour, nor is it likely that the air-film on the surface of the plate 

 was immediately changed. It is to be wished that experiments could be 

 carried out in which the plate should be polished after being placed in a 

 good vacuum or in an atmosphere of the gas to be employed in the 

 investigation. 



t E. Ladenburg, Ann. d. Thysik. xii. p. 558 (1903). 



