Electromotive Force of different Forms of the Clark Cell. 145 



Table III.— Change of E.M.F. with Strength of Solution at 15° C. 



j-jpt? Ul Cell. 



(w) 



gram per c.c. 



millivolts. 



dm 



calculated. 



Difference, 

 obs. — calc. 



r 



0*105 



33-2 



32-8 



+ 0-4 



B.O.T. J 



0-116 



31 -6 



31-8 



-0-2 



unsaturated j 



0-199 



24-6 



24-5 



+ 0-1 





0-263 



19 -1 



18-9 



+ 0-2 



Sat. at 0° C. . . 

 Sat. at 15° . . 



0-401 

 0-478 



6-5 



o-o 



6-7 



o-o 



-0-2 



o-o 



The differences given in the last column are of the same order as 

 the accidental changes of E.M.F. observed in the case of these weaker 

 cells. It would therefore appear probable that in this type of cell 

 the diminution of E.M.F. is simply proportional to the volume con- 

 centration of the salt. 



In comparing the above results, a curious point remains to be 

 noticed. Taking a cell saturated at 15° C, the increase of E.M.F., 

 on cooling down to 0° C, has been shown above to be 8'4 millivolts, if 

 there is no change of strength of the solution. The increase of E.M.F., 

 due to change of strength of solution from saturation at 15° C. to 

 saturation at 0° C, lias been found to be 6'5 millivolts. We might, 

 therefore, naturally expect the total effect due to both causes com- 

 bined to be 14*9 millivolts, whereas the saturated crystal cells, in 

 which both causes are operative, show an increase of E.M.F. of 

 16'6 millivolts. 



The explanation of this apparent discrepancy is to be found 

 probably in the lowering of E.M.F., due to the greater diffusivity of 

 the mercurous sulphate in the weaker solutions. In the saturated 

 crystal cells this diffusion is practically prevented by the dense layer 

 of crystals. In order to test this hypothesis, some weaker cells were 

 set up in the W form, in which the possibilities of diffusion are dimi- 

 nished by the smallness of the tube and the increased distance 

 between the electrodes. These cells showed, as was expected, 

 higher values of the E.M.F. than those given by the formula, the 

 difference amounting in some cases to between 2 and 3 millivolts. 



§ 23., On the Solubility of Zinc Sulphate. 



It is well known that zinc sulphate forms various hydrates, which 

 may be obtained by crystallisation at different temperatures. These 

 hydrates differ in point of solubility, and it is important for Clark 

 cells to employ the heptahydrate, which has the lowest solubility at 



