464 DR. FARADAY'S EXPERIMENTAL RESEARCHES IN ELECTRICITV. 



exciting cells (908.), or cause the current produced to traverse the decomposing cells 

 more freely. But if to very weak sulphuric acid a few drops of nitric acid were added, 

 then either one or other of those effects could be produced ; and, as might be expected 

 in a case like this, where the exciting or conducting action bore a direct reference 

 to the acid itself, increasing the strength of this (the nitric acid), also increased its 

 powers. 



1027. The nature of the interposed plate was now varied to show its relation to the 

 phenomena either of excitation or retardation, and amalgamated zinc was first sub- 

 stituted for platina. On employing one voltaic pair and one interposed zinc plate, 

 fig. 28, there was as powerful a current, apparently, as if the interposed zinc plate 

 was away. Hydrogen was evolved against P in cell ii, and against the side of the 

 second zinc in cell i ; but no gas appeared against the side of the zinc in cell ii, nor 

 against the zinc in cell i. 



1028. On interposing two amalgamated zinc plates, fig. 29, instead of one, there 

 was still a powerful current, but interference had taken place. On using three in- 

 termediate zinc plates, fig. 30, there was still further retardation, though a good 

 current of electricity passed. 



1029. Considering the retardation as due to the inaction of the amalgamated zinc 

 upon the dilute acid, in consequence of the slight though general effect of diminished 

 chemical power produced by the mercury on the surface, and viewing this inaction 

 as the circumstance which rendered it necessary that each plate should have its ten- 

 dency to decompose water assisted slightly by the electric current, it W^<s expected 

 that plates of the metal in the unamalgamated state would probably not require sucH 

 assistance, and would offer no sensible impediment to the passing of the current. This 

 expectation was fully realized in the use of two and three interposed unamalgamated 

 plates. The electric current passed through them as freely as if there had been no 

 such plates in the way. They offered no obstacle, because they could decompose 

 water without the current ; and the latter had only to give direction to a part of the 

 forces, which would have been active whether it had passed or not. 



1030. Interposed plates of copper were then employed. These seemed at first to 

 occasion no obstruction, but after a few minutes the current almost entirely ceased. 

 This effect appears due to the surfaces taking up that peculiar condition (1040.) by 

 which they tend to produce a reverse current ; for when one or more of the plates 

 were turned round, which could easily be effected with the couronne des tasses form 

 of experiment, fig. 18, then the current was powerfully renewed for a few moments, 

 and then again ceased. Plates of platina and copper, arranged as a voltaic pile with 

 dilute sulphuric acid, could not form a voltaic trough competent to act for more than 

 a few minutes, because of this peculiar counteracting effect. 



1031. All these effects of retardation, exhibited by decomposition against surfaces 

 for which the evolved elements have more or less affinity, or are altogether deficient 

 in attraction, show generally, though beautifully, the chemical relations and source 



