RETARDING EFFECTS OP INTERPOSED PLATES. 461 



surface and hydrogen at the other (that is, if the decomposing current passes), may be 

 considered as the cause of any obstruction arising from the decomposition of water 

 by the electrolytic action of the current ; and I have usually called it the interposed 

 plate. 



1010. In order to simplify the conditions, dilute sulphuric acid was first used in 

 all the cells, and platina for the interposed plates ; for then the initial intensity of the 

 current which tends to be formed is constant, being due to the power which zinc has 

 of decomposing water ; and the opposing force of decomposition is also constant, 

 the elements of the water being unassisted in their separation at the interposed plates 

 by any affinity or secondary action at the electrodes (744.), arising either from the 

 nature of the plate itself or the surrounding fluid. 



1011. When only one voltaic pair of zinc and platina plates were used, the current 

 of electricity was entirely stopped to all practical purposes by interposing one platina 

 plate, fig. 20, i. e. by requiring of the current that it should decompose water, and 

 evolve both its elements, before it should pass. This consequence is in perfect ac- 

 cordance with the views before given (910. 917- 973.). For as the whole result de- 

 pends upon the opposition of forces at the places of electric excitement and electro- 

 decomposition, and as water is the substance to be decomposed at both before the 

 current can move, it is not to be expected that the zinc should have such powerful 

 attraction for the oxygen, as not only to be able to take it from its associated hy- 

 drogen, but leave such a surplus of force as, passing to the second place of decompo- 

 sition, should be there able to effect a second separation of the elements of water. 

 Such an effect would require that the force of attraction between zinc and oxygen 

 should under the circumstances be at least twice as great as the force of attraction 

 between the oxygen and hydrogen. 



1012. When two pairs of zinc and platina exciting plates were used, the current 

 was also practically stopped by one interposed platina plate, fig. 21. There was a 

 very feeble effect of a current at first, but it ceased almost immediately. It will be 

 referred to, with many other similar effects, hereafter (1017.). 



1013. Three pairs of zinc and platina plates, fig. 22, were able to produce a current 

 which could pass an interposed platina plate, and effect the electrolyzation of water 

 in cell iv. The current was evident, both by the continued deflexion of the galva- 

 nometer, and the production of bubbles of oxygen and hydrogen at the electrodes in 

 cell iv. Hence the accumulated surplus force, of these plates of zinc, which are active 

 in decomposing water, is more than equal, when added together, to the force with 

 which oxygen and hydrogen are combined in water, and is sufficient to cause the 

 separation of these elements from each other. 



1014. The three pairs of zinc and platina plates were now opposed by two inter- 

 vening platina plates, fig. 23. In this case the current was stopped. 



1015. Four pairs of zinc and platina plates were also neutralized by two interposed 

 platina plates, fig. 24. 



MDCCCXXXIV. 3 o 



