Chemical Affinity in terms of Electromotive Force. 191 



longer periods, a slight alteration in the E.M.F., due to for- 

 mation of films of oxide on the surfaces of the plates, was 

 usually noticeable. As with the normal Daniells (§ 108), the 

 effect of the oxidation of the copper plate was to reduce the 

 E.M.F. by a few thousandths of a volt; on the other hand, the 

 formation of a film of oxide on the surface of the cadmium 

 plate produced an increase in the E.M.F. of from '001 to "004 

 volt ; so that in many cases the cell with partially oxidized 

 plates gave sensibly the same value as a newly set-up cell, the 

 diminishing effect of the oxidation of the copper being just 

 about counterbalanced by the increasing effect due to the oxi- 

 dation of the cadmium. In this respect cadmium behaves in 

 the opposite way to zinc (§ 108). 



Relations between the E.M.F. of Cadmium- Copper Cells and 

 that corresponding to the net Chemical Action taking place 

 therein. 



120. According to Julius Thomsen's determinations (Journ. 

 prak. Chem. ii. p. 233, and xi. p. 271), the heat of displace- 

 ment of copper from copper-sulphate solution (CuSO 4 ,400H 2 O) 

 by cadmium is as follows, in gramme-degrees per gramme- 

 molecule ; — 



Cd, 0,S0 3 aq. . . . 89,500 

 Cu,0,SO s aq. . . . 55,960 



Difference = 33,540 



the difference corresponding to 16,770 gramme-degrees per 

 gramme equivalent, or "740 volt*. As with normal Daniell 

 cells (§ 114), a small quantity (#) is to be added to this, repre- 

 senting a variable correction dependent on the physical con- 

 dition of the deposited copper. Evidently the average values 

 above cited (-7475 to "7525), obtained with bright and electro- 

 cadmium, are sensibly the same as the value -740-Kr, thus 

 deduced as representing the net chemical action taking place 

 in the cell; i. e., as with zinc-copper cells, the whole of the 

 energy developed in the cell is adjuvant under the conditions 

 obtaining in the above experiments. 



In order to compare the results obtained with the amalga- 

 mated-cadmium cells with Julius Thomsen's figures, the heat 

 of solution of cadmium (precipitated from the sulphate by 

 zinc, crystalline) in twenty-five times its weight of mercury 

 was determined by means of the calorimeter, 20 grains of 



* The value 4410, used in the former parts of these researches for the 

 factor for converting gramme-degrees into C.G.S. units, is employed 

 throughout the present paper : vide § 103.. footnote. 



