THE SIZE OF ATOMS. 177 



oxide tints begin to appear, commencing with 

 buff, and going on through a ruddier colour to a 

 dark-blue slate colour, when no further heating 

 seems to augment the effect. The greatest con- 

 tact-electricity effect which I thus obtained between 

 a bright freshly polished copper surface and an 

 opposing face of copper, rendered almost black by 

 oxidation, was such as to require for the neutra- 

 lising potential in my mode of experimenting l 

 about one-half of the potential of a Daniell's 

 cell. 



Some not hitherto published experiments with 

 polished silver plates, which I made fifteen years 

 ago, showed me very startlingly an electric in- 

 fluence from a quite infinitesimal whiff of iodine 

 vapour. The effect on the contact-electricity 

 quality of the surface seems to go on continuously 

 from the first lodgment, to all other tests quite 



1 First described in a letter to Joule, published in the Proceedings 

 of the Literary and Philosophical Society of Manchester, of Jan. 

 21, 1862, where also I first pointed out the demonstration of a 

 limit to the size of molecules from measurements of contact- 

 electricity. The mode of measurement is more fully described in 

 the article of Nature (vol. xxiii. p. 567) referred to above. 



N 



