344 Prof. A. LI. Hughes on the Contact 



Air was then admitted to the re 

 molecular pump to a pressure of 1 cm. 



Air was then admitted to the rough vacuum of the 



f Zu electropositive 



to Pt by 

 J 5 mins. later ,, 



1 R 



}, 



20 volt. 



•61 „ 



I s ;; „ „ -64 „ 



15 „ „ „ -64 „ 



The admission of air at atmospheric pressure reduced the 

 contact potential to '30 volt. 



About twelve separate experiments were made on the 

 contact difference of potential between platinum and newly 

 distilled zinc, and results of the same character were always 

 obtained. The newly distilled zinc was usually electro- 

 positive to the platinum film by •■! or *2 volt, but on a few 

 occasions it was slightly electronegative. Even in the best 

 vacuum obtainable in these experiments, the electropositive 

 character of zinc invariably increased slowly, and this 

 increase was always greatly accelerated by the admission of 

 a trace of air. The electropositive character of zinc does 

 not increase indefinitely ; after a certain point, it decreases 

 and reaches a steady value at which it remains and after- 

 wards is practically independent of the pressure of the air 

 about it. These results suggest that there is an intimate 

 relation between the electropositive character of zinc and the 

 amount of gas absorbed in the surface. At first, immediately 

 after distillation, the zinc surface is almost free from absorbed 

 gases and is strongly electronegative. Then, as it absorbs 

 gas, it becomes more and more electropositive up to a certain 

 point. Beyond this point, the absorption of still more gas is 

 accompanied by a decrease in the electropositive character 

 of the zinc until a fairly constant end value is finally obtained. 

 The principal features of the relation between the amount of 

 gas absorbed and the electropositive character of zinc 

 (relative to platinum) may be represented graphically as in 

 fig. 2. 



There is a close similarity between the alteration in the 

 electropositive character of zinc on the one hand and the 

 shifting of the long wave-length limit of its photo-electric 

 effect on the other, as the amount of gas absorbed in the 

 surface increases. Kustner's result that a new zinc surface, 

 prepared in a vacuum from which all reacting gases have 

 been excluded with the utmost care, has no photo-electric 

 effect, has been interpreted as indicating that its long wave- 

 length limit under these conditions is beyond X 1850. With 

 this result we may link an investigation by Pohl and 



