On Contact Electricity of Metals. 129 



tube which dipped under the surface of a layer of distilled water on a 

 gold plate, the potential was changed from 



O'lO volt 

 with the standard plate to 



+ 0-16 volt, 



and remained at this latter value for more than half an hour, showing 

 that the change was not due to a rise in temperature. The curious 

 point is that the change seems to be only in the surface of the liquid ; 

 for when most of it was shaken off the potential fell to 



-0-05 volt 



although the plate was still wet all over. This appears as if the 

 alteration were due to a surface film of oily or dusty matter on the 

 water. It was found that distilled water which had been exposed to 

 the air of the room for some time gave the same value as water which 

 had been breathed through. 



47. A lead plate and a flat circular leaden dish were scraped 

 clean with a knife, and next morning, when compared with each 

 other, they gave 



+ 0-10 volt, 



the plate being positive to the dish. Water was now poured into the 

 dish until it was nearly full, 0'4 cm. deep, the potential remaining 

 the same; i.e., the lead plate gave with the water in the leaden dish, 



+ 010 volt, 



water in leaden dish being negative. 



48. The plate and dish were again scraped, and this time they 

 gave 



+ 0-05 volt, 



the plate being positive. Water was next poured into the dish, 

 which gave, with th.e dry lead plate, 



+ 0-09 volt, 



the water in the dish being negative as before. The dish was now 

 removed, and a standard gold plate put in its place j with this the 

 lead plate gave 



+ 0-52 volt, 



the lead being positive. Thus water in contact with lead is about 

 0*9 volt negative to dry lead. 



49. Turpentine seems to produce opposite effects on zinc and 

 copper, i.e., copper becomes more negative, and zinc more positive, 



VOL. LXIII. L 



