628 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



This phenomenon of the formation of a precipitate in rings on a plate 

 or in discs in a tube is not confined to the case of a reaction between 

 silver nitrate and potassium chromate. Liesegang found such a forma- 

 tion of a precipitate in the case of mercurous chromate, lead chromate, 

 and Prussian blue, in the familiar reactions which give these precipitates. 

 We have found that with a proper choice of concentrations rings are also 

 formed in the following cases : — 



TABLE I. 



It is our intention to give a more extended description of the various 

 reactions in this table at some future time. At present, however, in 

 order to discuss the quantitative aspect of the problem, we shall confine 

 our attention to a single case, — the formation of supersaturation rings 

 of silver chromate. 



The experiment was as follows : A capillary tube of diameter .5 to 

 1 mm. was filled with a y'j or a y^jf normal solution of potassium chro- 

 mate in gelatine. After the gelatine had set, a piece of the tube about 

 5 cm. long, obtained by breaking the tube under water so as to expose a 

 fresh moist surface of the gelatine, vpas plunged vertically into a water 

 solution of 2N, N or | AgNOg contained in a flat-walled glass vessel. 

 By means of a cathetometer carrying a microscope of low power we 

 watched the formation of the precipitate. After a certain period of time, 

 depending on the concentration of the solution, the precipitate near the 



