RICHARDS. — CHANGING HEAT CAPACITY. 295 



the fact that it is an assumption, and being ready to abandon the position 

 when it has been proved untenable. 



The immediate problem under investigation then resolves itself into 

 the following question : lias the change of heat capacity any definite 

 causal relationship to the relative magnitudes of the free-energy change 

 and the heat of the reaction ? 



Throughout this paper the inductive method rather than the deductive 

 one is to be used in all the reasoning. Hence the first step is the 

 comparison of actual data concerning a number of carefully studied 

 reactions. 



Unfortunately the change in free energy can be computed from actual 

 data only in the case of easily reversible changes ; and these form but a 

 small minority of the cases of chemical reaction. The reversible gal- 

 vanic cell was shown by Helmholtz * to belong to this class, and may 

 therefore be used as an example. 



Among the many galvanic cells which have been measured, not all are 

 immediately available for the present jiurpose. In the first place, only 

 a few heat capacities of solutions have been accurately determined; and 

 in the next place, in some of the cells perceptible electromotive energy 

 is to be ascribed to the unequal balancing of osmotic pressures, which 

 has nothing to do with the affinities involved. We must then select cells 

 containing two liquids in which the extent of ionization and the migration 

 velocities are similar, the anion identical, and the electromotive force and 

 heat of reaction well known. The series of reversible cells which seems 

 to fulfil these conditions most satisfactorily is that composed of pairs of 

 the sulphates of copper, nickel, iron, zinc, and magnesium. The heat 

 capacities of these solutions have been accurately determined by iNIarig- 

 nac t and Thonisen t a'ld the electrical potentials liave been determined 

 by Wright and Thompson. § 



In order to illustrate more clearly the table containing the results 

 obtained from these cells, a single case may be given here in detail. The 

 common Daniell cell is chosen for this purpose, using solutions of the 

 strength MSO4 . 2OOH2O because the heat capacities of such solutions 



* Ilelmlioltz, Sitz. ber. Berlin Acad., 2 February and 7 July (lb82), Ges. Abh. 

 IM, II. 



t Marisnac, Ann. Chim. (5), 8, 410 (1870). 



t Tlioinson, Vogtx. Ann., 142, 337 (1871). 



§ Wriglit and Tlionipson, Phil. Mag. (5), 17, 288 (1884), 19, 1 (IbSu). This 

 work is revised in connection willi others by Wiismore, Zeitschr. phjs. Clieni., 35, 

 2in. (1900). 



