296 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



have in every case been carefully determined. The chemical reaction 

 taking place in the cell may be represented by the following equation : — 



Zn + CuS0 4 . 200H 2 O = ZnS0 4 . 200H 2 O + Cu. 

 The change in the heat capacity of this system may be computed easily 

 by subtracting the total heat capacity of the factors from that of the 

 products. The " absolute " standard of heat capacity, the mayer* (or the 

 heat capacity which is raised 1°C by 1 joule of energy), is peculiarly 

 convenient for calculations of this kind ; hence it is used below. The 

 following table contains the data in question : — 



Factors. 



mayers. 

 CuS0 4 200H 2 O 



(0.951G) X (3762.7) X (4.181) = 14,970 



Zn = 26 



Total Factors 14,996 



Products. 



mayers. 

 ZnS0 4 200H 2 O 

 (0.9523) X (3764.5) X (4.181) = 14,989 



Cu = 24 



Total products 15,013 



Difference 17 



Thus the heat capacity of the system is increased by 0.11 percent 

 during the reaction. 



Since this is the case, we should expect to find that some of the heat 

 energy actually evolved in the reaction would be needed to "fill" the 

 extra capacity, and hence would not become manifest as rise of tempera- 

 ture. As a matter of fact, the observed evolution of heat energy in the 

 reaction (210 kilojoules) is less than the electromotive energy (1.10 X 

 96,580 X 2 = 213 kilojoules) by about 3 kilojoules. Hence the data 

 are consistent with the theory as to sign; and since both figures are 

 small in proportion to the totals, the order of magnitude might also be 

 consistent. 



A single case does of course suffice to prove a point of such impor- 

 tance, hence ten galvanic cells, comprising every combination of the five 

 metals before mentioned, are given in the following table. 



Wright and Thompson used amalgamated zinc, but Richards and 

 Lewis t have shown that amalgamation makes no essential difference in 

 the potential of this metal, because of a singular compensation of effects. 

 The table explains itself; the fourth and eighth columns contain the 

 significant results to be compared, namely the change in heat capacity 

 and the so-called " bound energy." 



* Richards, These Proceedings, 36, 327 (1900). 



t Richards and Lewis, Zeitsch. phys. Chem., 28, 1 (1899). 



