294 HERMANN VON HELMHOLTZ 



Faraday, on the ground of the experiments which he had under- 

 taken with the object of deciding for or against the theory 

 of action at a distance, he first endeavoured as a cautious 

 critic to include a series of other and apparently remote pheno- 

 mena in the circle of his considerations. 



The communications which he made to the Naturforscher- 

 Versammlung at Leipzig, August, 1872, 'On the Galvanic 

 Polarization of Platinum/ and to the Berlin Academy in the 

 following year, ' On Galvanic Polarization in Gas-free Liquids/ 

 which are of a purely experimental character, originated in 

 theoretical considerations, arising from the law of the conserva- 

 tion of energy. It was known that when a Daniell cell of zinc 

 and copper is connected to an electrolytic cell with platinum 

 electrodes, a polarizing current is set up which declines rapidly, 

 but does not entirely cease even after a long time. It was 

 further known that if, after removing the Daniell cell, the 

 platinum plates were connected with the galvanometer, the 

 depolarizing current in ordinary liquids, saturated with gas, 

 is initially strong, and then soon diminishes so as to be im- 

 perceptible. Helmholtz now asked upon what this apparently 

 unlimited duration of the polarizing current depended, and 

 found that the persistent current was in close relation with 

 the gases present in the liquid, or at the electrodes, before 

 the passage of the current. A portion of the electrolytic oxygen 

 is neutralized by the presence of hydrogen, which again sets 

 free some of the hydrogen at the other electrode, which then 

 dissolves in the liquid or penetrates the platinum, so that the 

 decomposition of a corresponding amount of water again occurs. 

 This process of conduction of electricity by the motion of its 

 material carriers is termed by Helmholtz electrical convection. 

 The motion of a gas enclosed in the electrodes ensues very 

 slowly when the liquid itself is free from gas, so that the 

 depolarization current in gas-free liquids may persist for a very 

 long time. By assuming that in galvanic polarization it was not 

 merely gas on the surface, but also that which had penetrated 

 deeper into the platinum that came into play, and that the same 

 laws held for the motions of the gases occluded in the metals as 

 for the conduction of heat, Helmholtz removed the contradic- 

 tion to the law of the conservation of energy. The products of 

 electrolysis need not make their appearance at all, nor need 



