MORSE AND PIERCE. — SUPERSATURATION IN GELATINE. 639 



In view of this sensitive dependence of the diffusion constant and the 



x 



metastable product on the value of — ■_, we consider the results to be 



Vt 



satisfactorily consistent. 



We have not been able to find elsewhere any determination of the 

 value of the diffusion constant of silver nitrate in gelatine. F. Voigt- 

 laender* has found that the diffusion constants of salts in agar from 

 his own measurements are in some cases smaller, in others equal to or 

 larger than the corresponding constants for water solutions, the solutions 

 employed having varying concentrations. He says that the question 

 whether the diffusion in agar and water is identical can only be answered 

 by further observations on pure water and other gelatinous solvents, 

 such as water glass. Voigtlaender's measurements point to the con- 

 clusion, however, that the diffusion in agar and that in water are not 

 very different, which makes our result for silver nitrate, 1.54, seem 

 rather high, as the values obtained by Kawalki f would be for silver 

 nitrate at 16° C. 1.13 to 1.20. Calculation of the diffusion constant 

 for silver nitrate by Nernst's formula 



a 2 = 0.0477 x 10 7 -^- [1 + 0.0034 (f — 18)1 

 u -f- v /J 



gives the value 1.29. 



It should be noted that the value we have obtained is for diffusion 

 through gelatine containing a rather heavy solid precipitate of silver 

 chromate, and should not, therefore, be expected to agree with diffusion 

 in water or pure gelatine. The important fact for this discussion is 

 that a 2 is a constant and that there is hence no inconsistency in the 

 conclusion that the metastable solubility product of silver chromate in 

 gelatine is a definite constant quantity satisfying the relation 



H = A + g 2 X Cr6 4 ~. 



As to the value of U, there are no other determinations of this quan- 

 tity with which we might compare our results. As far as we know 

 there have not been published any numerical data as to the value of the 

 metastable product in any case. The ordinary solubility product of 

 silver chromate in saturated water solution in the presence of the solid 

 phase is 5.1 X 10~ 13 as determined by Kohlrausch.$ The concentrations 



* Zeit. Phys. Cliem., III. 317 (1889). 



t W. A., LII. 300 (1894). 



t Leitver. d. Elektrolyteu, p. 202. 



