130 Dr. J. Erskine-Muiray. 



when wet. Thus copper and zinc which had been dry-polished gave 

 with one another 



077 volt, 

 zinc positive. 



With the zinc wet with turpentine, 



0-96 volt. 



And with both zinc and copper wet about 



1-20 volts. 



Their potential difference increased somewhat as the turpentine 

 dried up ; and when apparently quite dry its value was still at least 



1-20 volts, 



the plates, however, smelt strongly of turpentine, showing that an 

 invisible layer remained on their surfaces. 



50. A crystal of copper sulphate gave, with a standard gold 



plate 



+ 0-02 volt, 



copper sulphate positive. This result was obtained in connection 

 with some experiments on the contact-potential of an aqueous solu- 

 tion of copper sulphate. It was found that such a solution in a 

 copper dish gave about + 0'07 volt with a dry standard gold plate, 

 the solution being positive. Blotting-paper saturated with copper 

 sulphate gave about +0*10 volt with the same standard plate. As 

 no special precautions were taken in these experiments to obtain 

 a perfectly pure liquid surface, one cannot, especially in view of the 

 changes described in 46, be quite certain that they represent the 

 true potential of copper coated with a solution of its sulphate. They 

 agree, however, with Professors Ayrton and Perry's values for satu- 

 rated and non-saturated solutions. 



51. One of the standard gold plates which had been polished 

 with Hollis's plate powder used dry gave, with a similar plate which 

 had a layer of alcohol on it, 



-0-13 volt. 



When the alcohol had dried up the value was 



-0-05 volt, 



the polished plate being negative as before. 



5*2. The conclusion which I draw from the experiments described 

 in this and the preceding chapter and in the tables is, that a layer of 

 liquid on a metallic surface does not give, with a metal separated 

 from it by air, a definite potential-difference of its own, as in similar 

 circumstances a solid conducting film would do, but merely adds a 



