RICHARDS. — CHANGING HEAT CAPACITY. 311 



already quoted. In this work the electromotive force due to the con- 

 centratiou effect between zinc and a one per cent solution of zinc in 

 mercury was found to be 0.0585, at 20° C ; and of cadmium in the same 

 way 0.0615. On diluting the amalgams to the concentration ratio of 

 400:1 to bring them into comparison with the calculation .given above 

 about 0.025 would be added to the electromotive forces, corresponding to 

 a total of about 0.085 volt for this dilution. The electromotive energy 

 is then 2 X 96,580 X 0.085 = 16.4 kilojoules, or an amount quite close 

 to the above values (14 to 15 kilojoules) calculated theoretically. 

 More extended data of this kind would obviously be highly instructive ; 

 and it is our intention to study such cells experimentally in this labora- 

 tory in the near future. 



Obviously then the electromotive energy of most reversible elec- 

 trodes arise chiefly from the energy of the aflfinity, only about fif- 

 teen kilojoules per gram atom being probably due to the difference in 

 cpncentration between the pure metal and ion. Since, moreover, this 

 concentration effect cannot be very different with different metals, no 

 great error will result if the appropriate multiple of this value is sub- 

 tracted from each, in order to find the free energy due to the affinity 

 proper. Evidently this procedure would leave the results on page 

 297 unchanged,^ since the same value would be subtracted from each 

 electrode. 



In the light of these facts, we should expect to find an approximate 

 parallelism between electromotive force of a single metal in one of its 

 salt solutions and the heat of ionization. Moreover, since the affinity of 

 the metal for the water seems to be the cause of ionization, the heat of 

 formation of the hydroxide and the contraction occurring during its for- 

 mation should each exhibit a parallelism with each of these quantities. 

 Of course considerable irregularities due as has been shown to changing 

 heat capacity and varying compressibility would be expected, and all 

 that can be done with the present data is to reduce the figures to compar- 

 able magnitude by adding or subtracting a constant from each series. This 

 has been done in the following table. It is assumed that the heats of 

 hydration of cupric, argentic, and mercuric oxides are zero. 



It is clear that in general the expected parallelism actually exists. 

 The deviations are not frequent enough to obliterate the meaning of the 

 facts. This is especially so, since many of the deviations can be explained. 

 The great compressibility of potassium and sodium and their compara- 

 tively expanded condition in the elementary state account for the rela- 

 tively larger contraction to be observed in their cases. On the other 



