148 Dr. Helmholtz on Galvanic Polarization 



Indeed the phenomena which arise when a pair of polarizable 

 plates are inserted in a circuit correspond in their main features 

 with those which would be presented by a condenser of very- 

 great capacity. The polarizing current is the current which 

 charges the condenser, the depolarizing that which discharges 

 it. The capacity of the condenser has only to be imagined so 

 great that its charging and discharge require perceptible periods 

 of time (seconds or minutes). Mr. Varley has tried to measure 

 the capacity of such a condenser ; but the sequel will show that, 

 unless extraordinary measures of precaution be used in the ex- 

 periments, other occurrences may play an important part and in- 

 fluence in a high degree the final result. 



The occurrences, then, in actual experiments with polarizable 

 electrodes are distinguished from what happens in a well-insu- 

 lated condenser by this — that the charging current has a much 

 longer duration than the discharging one, diminishes more 

 slowly, and never entirely ceases. In this respect a cell with 

 polarizable platinum plates appears to resemble a condenser with 

 an imperfectly insulating partition ; and even the phenomena 

 of residual electricity find their analogue in the reinforcement of 

 polarization which occurs after each interruption of the current. 



It would lie near to assume the same reason for the continu- 

 ance' of the charging current with a polarized decomposition-cell 

 as for the imperfect insulation of a condenser, viz. the existence 

 of a slight metallic conducting-power in the electrolyzable 

 liquids ; and this would include a limitation of the validity of 

 Faraday's law. Before we draw such a conclusion, however, we 

 must inquire whether other changes, which might have similar 

 results, do not also occur in the liquid and the electrodes. And 

 indeed it would be necessary here to think chiefly of the part 

 which may be played by gases held in solution in the liquid, or, 

 according to Graham's discovery, occluded in the metal of the 

 electrodes. 



It is well known that the' galvanic polarization of a platinum 

 plate which serves as the hydrogen electrode in a decomposition- 

 cell is diminished or destroyed by direct contact with the oxygen 

 of the air, by access of water containing air, and by the contact 

 of liquids which contain chemically combined oxygen, but may 

 give it up to the separating hydrogen. 



The same holds for the oxygen polarization of a platinum plate 

 when it is in contact with hydrogen dissolved in water or other 

 chemical combinations which can take up oxygen. 



Besides, we know, as discovered by Graham, that platinum is 

 capable, though in a less degree than palladium, of taking hy- 

 drogen into its own mass. The reception of oxygen, which we 

 know takes place in silver in a state of fusion, could not, it is 



