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



when the contrary is the case, we may safely conclude that, in- 

 dependently of the primitive heterogeneity of the plates, and the 

 alterations which the liquids have in the mean time sustained, 

 the presence of oxygen upon a plate renders it negative. This 

 result will be still more evident if, by turning the apparatus B 

 so that the aerated water be to the left and the boiled water to 

 the right, a deviation of, for example, 40° to the left be observed ; 

 the deviation with the apparatus A and boiled water being, as 

 before, from 10° to 12° to the right. 



In the case of one of the most imperfect preparations, a de- 

 viation of 50° to the right was obtained with the apparatus A ; 

 with the apparatus B, when the aerated water was to the right, 

 a deviation of 80° in the same direction ; and with the apparatus 

 B, when the aerated water was to the left, a deviation of 10° still 

 to the right ; the right plate being thus always negative, what- 

 ever the position of the aerated water with respect to the boiled. 

 At first sight, or in an isolated series of experiments, this might 

 appear to throw some incertitude on the results of the first and 

 second experiments, for here we find a plate of copper in aerated 

 water positive with respect to another plate of the same nature 

 in boiled water ; the apparent contradiction is, however, readily 

 explained by the heterogeneity of the plates ; and in fact, by 

 comparing the deviations, it will at once be seen that these ex- 

 periments confirm the general principle, of which I have here 

 undertaken the demonstration ; that is to say, that when any 

 two plates are simultaneously immersed in the same electrolyte, 

 an increase of the quantity of oxygen on the side of one of them 

 tends to render that plate more negative or less positive. 



The above, however, is not the only disturbing cause to be 

 guarded against ; for when two metallic plates have been plunged 

 into either apparatus, the part of the plate which has not been 

 wet, and the part situated at the junction between air and liquid, 

 are not in the same condition as the parts which have been im- 

 mersed in the liquids. 



Experiment proves that in passing from one operation to 

 another, very different deviations may be obtained, according as 

 these parts are above or beneath the surface of the liquid ; for 

 we are never certain of operating always under the same circum- 

 stances, even when employing tubes of exactly the same length. 



This cause of error, encountered also by Du Bois Reymond, 

 disappears in a great measure by covering a certain length of the 

 wires or the plates with a non-conducting substance. I have 

 generally employed glue or gum-lac; and experiment having 

 proved that a copper wire entirely covered with glue or gum-lac, 

 and a zinc wire, when plunged together into acidulated water, 

 caused no deviation of the needles of one of my most sensitive 



