Electrodes^ and the Tlieory of Secondary Piles. 447 



may perhaps be not uninteresting to some of your numerous 

 readers. During a long series of experiments upon the 

 theory of secondary piles, I had occasion to obtain and to 

 verify the result announced by Mr. Golding Bird in the Phi- 

 losophical Magazine for November 1838, namely, that the 

 negative platina electrode of a voltaic arrangement which has 

 been used in the decomposition of water, will give out less gas 

 than the positive electrode under the influence of an equal 

 negative current. I afterwards found, 



1. That the negative platina electrode will in certain cases, 

 and often most unexpectedly, give out juore hydrogen than 

 the j^ositive electrode; that frequently when the two plates 

 have been together active for some time (both giving out hy- 

 drogen) in a simple zinc and platina circuit, such as that 

 described by Mr. Golding Bird, the (originally) negative elec- 

 trode will evolve least gas in the first half of the experiment 

 and 7nost in the last. 



2. That in this latter condition a delicate galvanometer will 

 show the negative electrode to be ihefi negative as it was be- 

 fore 'mva.r\ah\y jiositive to the other plate. 



3. That this condition depends upon the relative cle^nhness 

 of the two electrodes. After scraping bright the surface of 

 the negative electrode, or the one giving out least hydrogen 

 in most cases, it gave off still less gas. If a similar operation 

 were performed upon the positive electrode instead, the re- 

 lative quantities of hydrogen evolved at the two plates ap- 

 proached nearer to an equality; finally, by using a negative 

 electrode of which the surface was much discoloured, ^"<i a 

 very bright positive electrode, I obtained the effect of "^^^^^ 

 hydrogen evolved from the negative electrode, the latter being 

 at the same time actually negative to the other plate. Thus, 

 we may understand how it iiappens that the secondary cur- 

 rent from two platina plates which have been repeatedly used 

 to decompose water as negative and positive electrodes re- 

 spectively, becomes gradually feebler and feebler, and at last 

 altogether disappears. 



4. That the secondary currents between the two electrodes 

 is considerable in proportion as the water electrolysed by the 

 plates has been pure, that is, as the matter evolved upon them 

 has been purely gaseous. Thus a constant battery was con- 

 nected during ten minutes with a decomposing apparatus in 

 which stood the platina plates, and which was filled with a 

 quantity of acidulated distilled water. When the connexion 

 with the battery was broken the secondary current was 54°, 

 estimated by the deflection of the galvanometer needle. A 

 solution of sulphate of copper was then electrolysed for ten 



