^ectromotive Forces from Thernwchemical Data. 223 



values, I determined the heat of replacement of silver by 

 copper from a solution of AgNOs, 2OOH2O. The amount of 

 silver was accurately determined both by titration of the 

 solution and by calculation from the weight of the dis- 

 solved copper. 



The results of two exactly similar experiments gave 

 I. = 33,420 calories, 

 II. = 33,450 calories, 

 as the heat of replacement : — 



Cu + Agg (N03)2Aq= Cu (N03)2 Aq + Ag^. 



These numbers are slightly below those found by Thomson, 

 he giving 35,630 as a mean ; bnt they are sufficiently in 

 agreement to show that the actual E.M.F. furnished by a 

 copper-silver nitrate cell is considerably less than the calcu- 

 lated value. 



It has been above remarked that the " therm ovoltaic con- 

 stants " of silver vary greatly with the salt of the metal em- 

 ployed, the soluble salts having much larger constants than 

 the insoluble, and the latter diminishing to an insignificant 

 quantity in the case of the iodide, or of the chloride sus- 

 pended in cupric chloride. This would therefore show that 

 it probably does not depend in any w^ay upon the metal itself 

 acting as a thermoelectric junction, but upon the nature of 

 the chemical changes occurring in the cell. 



From these, and other considerations which are given later, 

 I conclude that the thermovoltaic constant depends upon two 

 prime causes. One, which operates with both soluble and in- 

 soluble salts of silver, is a tendency of that metal to form sub- 

 salts, e. g. subchloride AggCl, the heat of formation of the 

 equivalent of which is greater than that of the normal salt, so 

 that silver is more readily attacked superficially and acquires 

 a lower relative potential than would be predicted from the 

 heat of formation of its normal salt. 



Wetzlar has shown that silver-leaf innnersed in solutions 

 of cupric or ferric chloride becomes converted into AggCl, so 

 that when a silver plate is immersed in cupric-chloride solu- 

 tion its surface rapidly becomes covered with a film of the 

 subchloride of sufficient thickness to prevent the metallic 

 silver producing its usual effect, and thus the thermovoltaic 

 constant practically vanishes. 



The other cause of divergence is of much wider applica- 

 tion, and appears to me to depend upon the fact that with 

 any salt, such as silver nitrate, which does not form a hydrate 

 with water, but being soluble absorbs heat on dissolving, 

 this heat can be supplied by the thermometric heat of sur- 



