1879.] 



The Contact Theory of Voltaic Action. 



423 



must be kept as low as possible, conditions that were always carefully 

 observed in the experiments. 



The apparatus employed by the authors in the present investigation 

 is then explained in detail, and it is shown how, by improving on their 

 earlier form, they have removed a difficulty which formerly existed, 

 and which prevented their previously experimenting on pairs of sub- 

 stances having very different weights, such as a vessel of mercury and 

 a sheet of metal. The method of making the permanent and temporary 

 adjustments, the tests for leakage, the experimental mode adopted of 

 compensating the error arising from defects in parallelism of the 

 apparatus affecting the results obtained from two rigid surfaces (as 

 that of copper and zinc) differently from the result found with one or 

 with two liquid surfaces under test are then described, as well as the 

 details of a complete experiment and the precautions adopted to obtain 

 clean metallic surfaces. 



The authors explain that the results they have obtained in this 

 investigation have divided themselves into three groups : — 



1st. The contact difference of potentials of metals and liquids at 

 the same temperature. 



2nd. The contact difference of potentials of metals and liquids when 

 one of the substances is at a different temperature from the other in 

 contact with it, for example, mercury at 20° C. in contact with mercury 

 at 40° C. 



3rd. The contact difference of potentials of carbon and platinum 

 with water, and with weak and with strong sulphuric acid ; 

 but that they give only the results under head No. 1 in the present 

 communication, reserving those they have obtained under heads ISTos. 2 

 and 3 for a future occasion. 



Then follow arranged in the order in which they were obtained 

 from January to May, 1878, some 150 results of experiments (each 

 number being on the average the mean of eight observations), repre- 

 senting the contact differences of potential of nine solids and twenty- 

 one liquids. They explain that the numbers given are those to which 

 alone they attach importance ; but that in consequence of much time 

 having in such delicate experiments to be spent in obtaining measure- 

 ments, which are often found out to be wrong, a considerable number 

 of results have been rejected, and are not mentioned in the paper, and 

 that this, therefore, explains why in some cases one measurement only 

 is apparently the result of a whole day's work. These remarks 

 especially apply to the authors' attempts to measure the contact 

 difference of potentials between a liquid and a paste, for example, 

 mercury and mercurous sulphate paste ; great difficulty being intro- 

 duced by the extremely thin layer of water on the surface of the 

 paste acting inductively instead of the paste itself. They mention 

 that this difficulty is a very good example of the inaccuracies that 



