302 



Mr. J. Brown. 



[Nov. 18, 



These are the only cases tried ; no exception has, therefore, been 

 found by me, nor, so far as I know, by others. 



17. One of the most ingenious explanations (from the pure contact 

 theory point of view) of the change of potentials near the copper- 

 iron or copper-nickel pair effected by hydrogen sulphide or chloride 

 is that of Gr. Wiedemann (' Lehre von der Electrikitat,' vol. 1, p. 205), 

 who suggests that in addition to the films of sulphide or chloride 

 formed on the metals, a film of hydrogen may be deposited, and the 

 contact effect of all these substances is added to that of the metals. 

 It may be pointed out that precisely the same explanation may be 

 given of the usual experiments made in presence of water- vapour 

 where the oxygen combining with the metal forms an oxide film, 

 which may again be covered with the hydrogen left free. 



18. Several other authors have urged as an objection to these expe- 

 riments, that the action of the gases causes the formation of coatings 

 of various compounds, and so the whole state of things is altered by 

 the introduction of additional bodies or substances assumed to intro- 

 duce new so-called " contact effects." (See Ayrton and Perry, ' Phil. 

 Mag.,' January, 1881, p. 48 ; Pellat, ' Journal de Physique,' vol. 10, 

 1881, or ' Theses presentees a la Faculte des Sciences de Paris,' 

 No. 461, p. 16.) 



Dr. Lodge also states ('Brit. Assoc. Report,' 1884, p. 50), "but satis- 

 factory observation in these gases is difficult, because they not only 

 tend to attack the plates, but they do attack them, and so a film is 

 formed and everything is rendered uncertain." 



But it is surely too much to say that no such film (of oxide) is 

 formed in the experiments as usually made in air. We are even jus- 

 tified in inferring that such a thing as a "contact" experiment on 

 clean metals has not yet been made, since they are invariably cleaned 

 w T hile exposed to the atmosphere, and therefore covered with a layer 

 or film of highly condensed water- vapour containing other dissolved 

 gases. 



Each abrading point of the polishing material (emery, glass-paper, 

 or whatever it be) may almost be said to be working under water on 

 the metal. 



It seems difficult to suppose that even for an instant after each 

 little abrading point has passed, the surface of the metal exposed in 

 its track remains unoxidised. All such critics assume quite unwar- 

 rantably that in the old contact experiments of Volta the surface of 

 the plates is not altered by the atmosphere, whereas the alteration is 

 not only after a time visible to the naked eye, but is accompanied by 

 a sequence of electrical effects in a quite similar way to that when 

 other gases are used, which attack chemically one or other of the 

 metals, so as to mask or overpower the action of the oxidising 

 atmosphere still present. On the other hand, when gases are added 



