136 Dr. J. Erskine-Murray. 



Time. Contact -potential. 



Hrs. Mins. Volt. 



4 -t-0'060 



13 +0-035 



which shows that its immersion had raised its potential 0"12 volt. I 

 now warmed ifc with a soldering-bolt applied to its back. When 

 about 47 C. (see Chap. IX) it gave 



Time. Contact-potential. 

 Hrs. Mins. Yolt. 



16 +0-020 



It was again warmed slightly, and when at about 30 C. gave 



Time. Contact-potential. 



Hrs. Mins. Volt. 



42 -0-010 



When 16 C., 



4 48 -0-040 



its potential thus coming back to very nearly the original value. 



This experiment shows : (1) that the change requires considerable 

 time ; for even a fairly strong jet of oxygen playing on the plate for 

 2 or 3 minutes produces no appreciable effect, while 25 minutes 

 in still oxygen causes a rise of 0*20 volt; (2) that gentle heating 

 does not produce a rise, as it would do with clean unoxygenized copper 

 (see Chap. IX), but it must be remembered that the copper was, 

 when heat was applied, already above the potential to which heat 

 alone would have raised it; hence this experiment does not show any 

 connection between temperature- variation and density of oxygen 

 film, as might at first sight be supposed. 



60. A zinc plate which had been polished on glass-paper gave 

 with a standard copper plate 



0'81 volt, zinc positive. 



It was then put into oxygen and left for 15 minutes. After being 

 taken out, its potential was again measured with the same standard 

 plate, and was as follows : 



This shows that zinc also is more positive after immersion in oxygen. 

 61. The same zinc plate was again polished on glass-paper and 

 gave 



+073 volt 



