256 M. Viard on the Electro-chemical Deportment of Oxygen. 



Experimentally, it is easy to prove that the presence of oxygen 

 upon the negative plate augments the current ; but when it is 

 required to show that its presence upon the positive plate dimi- 

 nishes the same, the operation becomes very delicate. In most 

 of the experiments here made, the intensity of the current could 

 only be reduced from 1 to 2 degrees, and sometimes only the 

 fraction of a degree ; and then, in order to distinguish the direc- 

 tion of the current, it was often necessary, on account of the slow 

 modifications of the plates, to compare the deviations of each 

 experiment with the mean of the deviations in the foregoing 

 and in the following experiments. 



As the chemical actions at the contact of the metals with the 

 liquids change the composition and aeration of the latter, it is 

 always necessary, after verifying the influence of oxygen upon 

 one plate, to renew these liquids in each apparatus before seek- 

 ing to verify its influence upon the other. This renewal is indi- 

 cated in the table by the letter R. 



Lastly, we may remark, that, when passing from one series of 

 experiments to another upon different metals or electrolytes, a 

 change of glasses and corks is indispensable ; in short, the appa- 

 ratus must at such times be reconstructed. 



The theory of the phenomena just examined may be deduced 

 from the one before given for currents produced by two plates 

 of the same nature when placed in liquids differently aerated. 



Let us consider a plate of copper and a plate of silver in non- 

 aerated water. Under the influence of the copper, the liquid 

 will polarize itself, but there will be no decomposition and no 

 current ; but if the silver be covered with oxygen, this oxygen 

 will assist the copper in the polarization of the liquid; and 

 under the united influence of copper and oxygen, a current will 

 be produced accompanied with a formation of oxide of copper 

 and water. The introduction of oxygen upon the copper would 

 evidently tend to produce a contrary state of polarization to the 

 one which copper alone tends to produce ; hence the current 

 would be thereby diminished. 



The theory of a zinc and silver battery in water would again 

 be nearly the same ; with this difference, however, that without 

 oxygen a current would be produced. 



Some have maintained, that in these experiments, or in ana- 

 logous ones with a gaseous battery, an oxidation of the positive 

 plate at first takes place, and at the same time a deposit of 

 hydrogen upon the negative plate ; that free oxygen does not 

 interfere except to depolarize the negative plate. 



Others, that the negative plate oxidizes under the influence of 

 the free oxygen present, and that a current is thus produced 

 between the positive plate and the oxide of the negative plate. 



