On Contact Electricity of Metals. 1 2 7 



40. A plate of thickly oxidized copper giving, with, the standard 

 plate, 



-0-24 volt 



gave, after it had been washed with alcohol, and allowed to dry, 



O'OO volt, 



a rise of 0'24 volt. 



41. A zinc plate, when cleaned on glass paper, gave 



+ 0-61 volt. 



It was then washed with alcohol, and when apparently quite dry 

 gave 



+ 0-94 volt, 



remaining constant at this value for many hours. The rise in this 

 case is 0*33 volt. 



42. The results of many similar experiments made with .alcohol 

 and other liquids on gold, silver, zinc, copper, iron, tin, lead, and 

 aluminium will be found in Table II. It is notable that in almost 

 every case the contact-potential of a metal which has been polished 

 on a hard dry material rises at least O'l5 volt in consequence of 

 alcohol washing, and that this change is nearly permanent for many 

 hours after the plate appears to be quite dry. 



43. It may be remarked that, as M. Pellat appears to have; 

 invariably washed the metal with alcohol before making a measure- 

 ment of its potential, his results do not apply directly to metals which 

 have been cleaned by means of a hard dry agent only. Even in the 

 experiment which he gives on pp. 79 80 of his paper as a proof that 

 alcohol-washing does not permanently alter the potential of a clean 

 gold plate, he appears to have used a plate which had already been 

 washed with alcohol ; his result, therefore, does not prove that 

 alcohol-washing makes no change in the contact-potential of a clean 

 plate, but only that subsequent washings do not alter the state of 

 surface, i.e., that the effect is nearly permanent. This agrees with 

 the results given above and in Table II. 



44. The alcohol film, whether in combination with the metal or 

 not, must in any case displace the air from the surface of the plate. 

 Other liquids seem to leave films in much the same way, though the 

 change of potential is different for each liquid ; in some cases, e.g. 

 turpentine on zinc and copper, it is in the positive direction for the 

 positive metal and in the negative for the other. 



VII. Thick Liquid Films on Metals. 



45. In experimenting with films of sensible thickness the liquid 

 was usually placed on a flat metal plate, its surface-tension being 



