MORSE AND PIERCE. — SUPERSATURATION IN GELATINE. 629 



advancing front of the diffusing substance began to appear in layers, or 

 discs, widely separated in comparison with their thickness. These discs 

 appeared suddenly as a sharp, thin film, so that the time of their appear- 

 ance could be determined with accuracy. The distance of each disc from 

 the bottom of the tube was read off on the cathetometer. Measurements 

 so obtained are recorded in the appended tables, IV to XXIV". The 

 experiment was conducted in a constant-temperature room, which, except 

 for the short time necessary for the taking of readings, was kept dark, 

 or dimly lighted, to exclude the possible action of light in hardening the 

 gelatine. 



The reaction is in accordance with the formula : 



2AgN03 + K^CrOi ±^ Ag.CrO, + 2KNO3 



The advanee of the reaction in the one sense or the other is conditioned 

 upon the concentration of the active substances. For equilibrium the 

 concentrations must satisfy the quantitative relation : 



Ag2 X Cr07= ^. Ag2Cr04 (a) 



+ — 



where Ag and Cr04 are the concentrations of the silver ion and the 



chromate ion respectively, Ag2Cr04 the concentration of the undissociated 



silver chromate in solution, and K the equilibrium constant. When the 



solid phase is present the undissociated silver chromate in solution must 



be in equilibrium with the solid phase, and must, therefore, be present in 



constant amount. In saturated solution, therefore, 



Ag2 X Cr07= k, (1) 



where k is the solubility product. 



Now in supersaturated solution the mass law (Eq. a) is probably still 

 true, and the question arises; is there a relation similar to (1), with, 

 however, a different constant product that defines the limit of super- 

 saturation ? 



Is there a formula of the form 



Ag2 X Cr07= H (2) 



for the condition that the concentrations must satisfy when the precipitate 

 just begins to form in the absence of the solid phase ? 



