Electromotive Force of different Forms of the Clark Cell. 141 



other reasons, we thought it would be desirable to repeat the deter- 

 minations. The observations of Lannoy,* though evidently under- 

 taken with great care, did not extend to the case of solutions as 

 dense as those occurring in Clark cells. Other observers appear to 

 have confined themselves chiefly to the case of very dilute solu- 

 tions. 



According to the views of Valson and Bender, which are quoted 

 by Ostwald and other authorities, the density of a salt solution, such 

 as zinc sulphate, may be additively deduced from the observed den- 

 sities in the case of some standard solution {e.g., a solution of 

 ammonium chloride), by means of two moduli representing the acid, 

 and the base respectively. We have calculated the densities accord- 

 ing to the values which they give for the moduli at 18° C, but it 

 appears that the results are only a rough approximation, and miss 

 what seems to be the most characteristic feature of the change of 

 density. 



In determining the relation between density and strength of 

 solution, the chief difficulties to be encountered are in the exact 

 measurement of the strength. If the composition of the solution is 

 determined by weighing out known quantities of the hydrated salt 

 into a litre flask, it is very possible that errors may arise from 

 evaporation or efflorescence, or from the presence of other hydrates. 

 In order to avoid these possible errors, we adopted the much more 

 laborious method of evaporating a known w r eight of solution to 

 dryness at 100° C, assuming that the residue was the monohydrate. 

 Two determinations were made in this manner for each solution 

 tested, and in addition, two control experiments were made in 

 which the strength of the solution was measured by estimating the 

 sulphate by means of barium chloride. The following table contains 

 the results of these determinations for seven different solutions. 



Table II. — Density of Zinc Sulphate Solutions. 



ZnS0 4 per 

 cent, of 

 solution, 



CpO 



Density 

 of solution 

 (d). 



ZnS0 4 

 gram per c.c. 

 pdj 100. 



Difference 

 -9982 

 + w — d. 



Calculated 



by (D) 



formula. 



6-35 



1 -0653 



-0677 



-0006 



-oooo 



8*46 



1 -0896 



-0923 



-0009 



-oooo 



13*49 



1 -1522 



'1557 



-0017 



-0006 



17-69 



1 -2020 



-2130 



-0092 



-0070 



23 -75 



1 -2872 



'3062 



-0172 



-0174 



27 '27 



1 3418 



-3667 



-0231 



'0242 



33-21 



1 -4400 



-4790 



0-0372 



-0367 



* Ostwald, ' Zeit, Phys. Chem.,' Nov., 1895. 



