150 Dr. Helmholtz on Galvanic Polarization 



with platinum electrodes, maintain an unceasing feeble current 

 not merely when the liquid is in contact with air, but also in a 

 perfectly closed vessel, if its electrodes are saturated with oxygen 

 and its liquid holds oxygen in solution. 



The apparatus with which I experimented in this direction 

 was an hermetically closed voltameter connected with a mercu- 

 rial air-pump, containing two large platinum plates of approxi- 

 mately 180 and 300 sq. centims. surface cylindrically bent to- 

 wards each other, and having derivation outwards through pla- 

 tinum wires fused into them. The liquid in this voltameter 

 extended downwards to the mercury of the pump, with which it 

 was raised and lowered, while the gases which collected above 

 the liquid could always be drawn off through a separate cock. 

 It was thus possible at any time to restore above the liquid a 

 vacuum containing only aqueous vapour, and gradually to clear 

 the liquid of every trace of dissolved gas. 



Saturation of the plates with oxygen is attained thus : during 

 several days oxygen is developed on them by a feeble current 

 conducted to them through a platinum wire as hydrogen -elec- 

 trode. I have seen for weeks together a current maintained by 

 electrolytic convection alone, under the influence of a limited 

 stock of oxygen in hermetically isolated liquid. It is character- 

 istic of the influence of the liquid herein, that every mechanical 

 motion of it, but especially circulating motions which are called 

 forth by changes of temperature, adds considerably to the inten- 

 sity of the current. This as good as entirely fails in gas-free 

 liquids. 



Much more efficient than oxygen in this relation is hydrogen, 

 because it can collect in very great quantity in the plates. When 

 the plates and the liquid are copiously saturated with electroly- 

 tically developed hydrogen, such a decomposition-cell behaves 

 towards feebler currents, for hours or even for days, as a non- 

 polarizable element, similarly to a solution of silver between 

 silver electrodes. In spite of its being inserted, measurements 

 of resistance in its circuit can be effected with the most perfect 

 accuracy ; and it shows, after interruption of the battery-cur- 

 rent, scarcely a trace of polarization. Hitherto I have succeeded 

 better in producing this state of hydrogen saturation by em- 

 ploying dilute sulphuric acid as the electrolytic liquid than with 

 distilled water. 



The constancy of the current, however, comes to an end when, 

 through convection of the hydrogen, its store in one of the plates 

 begins to run short. 



Under these conditions, by employing only one (but a well- 

 conducting) Daniell element, development of hydrogen as a gas 

 can be observed at the plate to which it is carried, and therefore 



