1877.] 



Contact of different Substances. 



305 



so far as it extends, the phenomenon of two metals being at the same 

 potential when immersed in a liquid is peculiar to zinc and copper, and 

 only takes place in the case of these metals when the liquid is either 

 water or dilute sulphuric acid, and when the metals have been recently 

 placed in it. 



For instance, when zinc and copper are immersed in a solution of 

 caustic potash, there is a small but distinct difference of potential be- 

 tween the zinc and copper, copper being positive to zinc. Again, iron 

 and copper in distilled water exhibit a marked difference of potential, 

 the copper plate being positive to the iron plate ; this is, of course, a 

 necessary consequence of the fact, previously stated, that copper is posi- 

 tive to water, while iron is negative to water, and it is confirmed by the 

 experiments with a copner-water-iron element. 



Similar observations were made with zinc and iron in distilled water, 

 with the result that zinc in water is positive to iron in water. 



Connected with the experiment by which I have attempted to show 

 that well-cleaned zinc and copper in water are at the same potential, 

 there are some points which seem worthy of mention. When the con- 

 denser-plate of zinc is thoroughly cleaned, but the zinc plate in the 

 water is not cleaned with the same care, the copper plate at first appears 

 to be slightly negative to the zinc plate ; but after a short time (the im- 

 mersion of the plates being continued) this difference of potential dis- 

 appears, and the two plates are, as before mentioned, exactly at the 

 same potential. If the plates continue immersed a small difference 

 of potential again appears between them ; but now the copper appears to 

 be positive to the zinc, and this difference of potential slowly increases 

 until it attains, after some hours, a considerable magnitude. 



A possible explanation of these changes seems to be that at first the 

 zinc oxide has not been entirely removed, and, being strongly negative to 

 the zinc, it causes the copper to appear negative to the zinc ; but as the 

 zinc becomes acted upon by the water*, its surface becomes, partially 

 at least, coated with hydrogen, which is positive to zinc, and hence 

 the copper plate, assuming the same potential, or nearly so, as the 

 altered zinc surface, becomes positive to the zinc plate. As the time 

 of immersion is prolonged the coating of hydrogen on the zinc be- 

 comes more complete, and the apparent positive potential of the copper 

 with respect to the zinc increases, the zinc plate becoming less and less 

 operative in the water, and serving more and more as a mere support for 

 the hydrogen. 



With amalgamated zinc the coating of hydrogen is formed much more 

 evenly and rapidly ; and thus, in spite of the presence on the zinc plate of 



the mercury, which is considerably negative to copper, an amalgamated 

 zinc plate and a copper plate in water, or in dilute sulphuric acid, exhibit, 

 when first immersed, a larger difference of potential than zinc and copper 

 * The zinc used in these experiments was commercial, not pure zinc. 



T 2 



