Dr. Andrews on the Properties of Voltaic Circles. 



151 



solution of zinc, similar portions of connected and unconnected zinc were exposed 

 to the action of sulphuric acid of sp. gr. 1.845 at the same time and in the same 

 vessel. The platina was placed opposite to both surfaces of the zinc, and at the 

 distance of one-fourth of an inch ; it exposed to the liquid a surface which was 

 about one-third of that of the zinc. The connexion was made above the liquid. 

 The following table contains the results of a series of experiments made at diffe- 

 rent temperatures, in which the second column gives the ratio of the quantities 

 of zinc dissolved from equal weights of that metal, when alone and when united 

 to platina, assuming as unit the quantity dissolved in the latter case. 



Temperature. 



168° to 170° cent. 



203° to 206° „ 



221° to 233° „ 



238° to 240° „ 



242° „ 



250° to 270° „ 



265° „ 



Although the zinc was removed from the acid, and its loss ascertained, before 

 its surface had undergone much alteration, yet as the connected zinc diminished 

 less during the process of solution than the other, the surfaces became no longer 

 precisely similar ; andhence the differences exhibited by the table are less than they 

 ought to be. From an inspection of the table, it appears that the greatest diffe- 

 rence in the quantity of zinc dissolved occurs at the temperature of 242°, where 

 the action of the acid is reduced to less than one-third by the contact of the 

 platina ; while at higher and lower temperatures the difference is less consider- 

 able. This circumstance may, perhaps, be explained by the following conside- 

 rations. The rapidity of the solution of zinc, whether alone or connected, 

 increases at a much faster ratio than the temperature, till it reaches a maximum 

 point, when it can scarcely be augmented by farther increments of temperature. 

 Now the effect of the contact of platina being to reduce the rate of solution of 

 the zinc in the acid at a given temperature, (to what it is at 40° or 50° cent, lower 

 than when unconnected,) it is evident that the difference will increase till they 



TOL. XVIII. Y 



