[ 188 ] 



XXII. On the Determination of Chemical Affinity in terms of 

 Electromotive Force. — Part VI. By C. B. Alder "Wright, 

 D.Sc. (Lond.), F.R.S., Lecturer on Cliemistry and Physic* 

 in St. Mary's Hospital Medical School*. 



On the Relation* between the Electromotive Forces of various 

 hinds of Cells analogous to Darnell's Cell but differing there- 

 from in the nature of the Metals used, and the Chemical Affi- 

 nities involved in the Action of these Cells. 



I. Cells containing Cadmium as one of the Metals, the Scdts used 

 being Sulphates. 



118. mHE experiments described in Part V. (§§ 106-109) 

 -L were repeated, using, instead of normal Daniell 

 cells, analogous arrangements containing plates of cadmium, 

 opposed in some instances to copper, in others to zinc, solu- 

 tions of the respective sulphates being employed to surround 

 the various plates used. With each of these two classes of 

 cells (cadmium-copper and zinc-cadmium cells) the same 

 result was obtained as that already recorded in the case of 

 Daniell cells containing zinc- and copper-sulphate solutions — 

 viz. that so long as the two solutions are of the same strengthf 

 the actual state of concentration of the fluids does not 

 exert any appreciable influence on the E.M.F. generated with 

 given plate-surfaces : at least the influence exerted is consi- 

 derably less than the errors of observation and the variations 

 due to unavoidable variations in the nature of the plate-sur- 

 faces, and does not amount to as much as + '0015 volt even 



* Communicated by the Physical .Society, haying been read at the 

 Meeting on June 24, 1882. 



t It is convenient to define solutions "of the same strength'" not as 

 solutions of the kind usually spoken of by chemists as "equivalent" to 

 one another, i. e. containing in a given volume quantities of dissolved 

 matter in the ratio of the chemical equivalents of the substances dissolved 

 (e. g. 159-5, 161, and 208 parts of anhydrous copper, zinc, and cadmium 

 sulphate respectively), but as solutions in which the dissolved matter and 

 the water -present are in the same molecular ratio, i.e. which are expressible 

 by parallel formulas, such as CuS0 4 50 H 2 O, ZnSO 4 50H 2 O, and 

 CdS0 4 50 H 2 O. With weak solutions the two definitions are practically 

 the same — but not so with more concentrated fluids, especially when the 

 molecular weights of the dissolved matters are considerably different (like 

 CuS0 4 and CdS0 4 ). Solutions of zinc and copper sulphate of the same 

 molecular strength are practically identical in specific gravity ; but a solu- 

 tion of cadmium sulphate is considerably more dense than one of either 

 zinc or copper sulphate of the same molecular strength. Thus solutions 

 of the strengths ZnSO 4 50H 2 O, CuS0 4 50H 2 O, CdSO,50H 2 O have at 

 18° specific gravities respectively close to 1T70, 1T67, and 1-208; with 

 stronger solutions the excess of density of the cadmium solution is still 

 more apparent. 



