642 PROCEEDINGS OF THE AMERICAN ACADEMY. 



chromate. It is our intention to investigate further the possible influence 

 of impurities on the formation of these precipitates. 



It is of interest to know that gelatine is not necessary for the pro- 

 duction of rings, for when a very flue capillary tube filled with a water 

 solution of potassium chromate is carefully plunged into a silver nitrate 

 solution precisely similar phenomena may be observed. In this case, 

 however, the layer of precipitate exists for only a short time after its 

 formation. It breaks up under the influence of gravity and convection 

 and mingles with the mass of silver chromate below. Three or four 

 layers may exist at the same time, those of later formation being sharp, 

 and the older ones gradually disintegrating into a mass of precipitate. 



V. Conclusion. 



We have shown by quantitative measurement that there exists in 

 the case of the formation of silver chromate in gelatine solution a definite 

 constant product 



Ag a X Cr67= H 



which determines the limit of supersaturation with respect to silver 

 chromate in the absence of the solid £>hase. 



Incidentally we have obtained a value for the diffusion constant of 

 silver nitrate diffusing through gelatine containing solid silver chromate. 



Jefferson Physical Laboratory, 

 Harvard University, Cambridge, Mass., U. S. A., 

 March 1, 1903. 



