640 PROCEEDINGS OF THE AMERICAN ACADEMY. 



of Ag2Cr04 in the two cases are in the ratio _ This 



V5.1 X 10-13 

 means that the supersaturated gelatine at the concentration of precipita- 

 tion held in solution 145 times the amount of silver chromate required 

 to saturate it in the presence of the solid phase. 



Lobry de Bruyn * and Liesegang f have shown that in the ease of a 

 large number of substances, which, like the silver haloids, form amor- 

 phous precipitates, gelatine inhibits precipitation. This phenomenon is 

 especially easy to observe and demonstrate in the case of colored sub- 

 stances like silver chromate and lead iodide. It is well known that 

 electrolytes, whether or not they act chemically on the colloid, may cause 

 the precipitation of a colloid from solution, and it might be supposed 

 tliat the case we are discussing can be explained as such an action of 

 the diffusing silver nitrate in throwing down colloidal silver chromate. 

 We are of the opinion that this is not the case, because, first, precipita- 

 tion is here sudden and not slow, as is the case when colloids are pre- 

 cipitated by electrolytes, and, second, in the several cases of different 

 concentrations of the reacting substances, the precipitate is formed, not 

 for a constant concentration of the diffusing electrolyte, but for a con- 

 stant value of the product 



aV X Cr07. 



Purification of the Gelatine. The best commercial gelatine contains 

 usually about two per cent of ash, consisting of phosphates and car- 

 bonates, both of which form insoluble compounds with silver. Although 

 the silver salts of these radicals are more soluble than silver chromate, 

 it was thought necessary to remove them and other salts as thoroughly 

 as possible from the gelatine to avoid their possible influence on the 

 precipitation of the silver chromate. This removal was effected by 

 electrolyzing the gelatine between membranes of parchment paper at 

 five hundred volts continually for a week, during which time the mem- 

 branes and electrodes were frequently washed by a stream of distilled 

 water. Specimens of gelatine so prepared gave but a trace of ash on 

 io^nition. 



Liesegang in his original research on reactions in gelatine solutions 

 in capillary tubes found two classes of rmgs : the heavy widely separated 

 red deposits of silver chromate, and beyond these and between them 



* Rec. trav. cliim. Pays-bas, XIX. 236. 

 t Phot. Wochenblatt, p. 229 (1894). 



