M. Viard on the Electro-chemical Deportment of Oxygen, 243 



spheric air which it holds in solution. Two plates of the same 

 nature, connected with a galvanometer, are plunged into the two 

 tubes, and then the deviation of the needle observed. 



Unfortunately, by employing the same electrolyte, deviations 

 in the same direction are not always obtained with plates of the 

 same nature. The experiments appeared generally to indicate 

 that the presence of oxygen upon a plate renders it negative ; 

 but contradictory experiments, although few in number, throw 

 some doubt upon the conclusions one is here tempted to draw. 



Let us therefore examine the causes capable of producing such 

 a variation in the results. 



However carefully the plates may be prepared, whether by 

 cleansing them chemically, or by exposing a fresh metallic sur- 

 face by means of friction with glass paper, emery paper, silica, 

 or lamp-black, all which methods were successively tested, it 

 appeared to me impossible to obtain plates which, when plunged 

 simultaneously into the same liquid, would produce no movement 

 of the needle in the galvanometer. By using pure acids, and by 

 purifying the materials from which the plates are mechanically 

 prepared, this desideratum may be sometimes approached; but 

 there are many cases where the plates are strongly heterogeneous. 



Poggendorff, Lenz, Saweljew, Schroder, Du Bois Reymond, 

 and many others have encountered a like difficulty. 



This heterogeneity, almost always present, was evidently one 

 of the most disturbing causes in my first experiments. They 

 were eliminated from all such, however, by uniting to the first 

 apparatus a second one, of as similar a construction as possible, 

 and containing in its two tubes the same electrolyte, but at the 

 same degree of oxidation ; for by this means it is evident that 

 the heterogeneity of the plates could at any time be measured 

 (fig. 2). 



Let A represent the apparatus whose two tubes contain the 

 same liquid at the same degree of oxidation, for example, boiled 

 water; and B the apparatus whose two tubes contain the same 

 liquid at two different degrees of oxidation, for instance, boiled 

 water and aerated water. 



If now, by plunging two plates of copper into the apparatus 

 A, the needle of the galvanometer deviate 10° to the right, 

 thereby announcing that the plate to the right is negative with 

 respect to the plate to the left ; if, by taking the two plates from 

 the apparatus A, and plunging them into the apparatus B, which 

 differs from the former in no other respect than that one tube 

 now contains aerated water instead of boiled, a deviation of 56° 

 again to the right be obtained ; if, by alternately repeating these 

 two experiments, the deviation always decrease when passing 

 from the apparatus B to the apparatus A, and always increase 



R2 



