MORSE AND PIERCE. — SUPERSATURATION IN GELATINE. 



627 



therefore, a metastable solution goes over into a labile condition. The 

 concentration at which the transition occurs is called the metastable 

 limit. 



" The metastable limit is, in the first place, dependent on the nature 

 of the substances, on the temperature, and on the pressure. In addition 

 it is influenced by various other circumstances not yet explained. At 

 the present time (1899) it is hardly possible to speak more definitely 

 concerning the value of the metastable limit and the methods of deter- 

 mining it." 



It is in connection with this discussion in the Lehrbuch that Ostwald 

 cites the sharpness and regularity of Liesegang's rings as evidence that 

 the metastable limit is something definite. But so far as we 

 are aware, no attempt has hitherto been made to study Liese- 

 gang's rings quantitatively, and to obtain from them the actual 

 numerical value of the metastable limit. In fact, so far as we 

 are aware, there has not been published a determination of 

 the metastable limit in any case. We quote further Ostwald's 

 explanation* of the formation of Liesegang's ring-system: — 



" By the diffusion of the silver salt into gelatine containing 

 chromate a solution is formed which is supersaturated with 

 respect to silver chromate ; precipitation does not take place 

 until the metastable limit has been exceeded. This naturally 

 happens simultaneously in a circle concentric with the drop. 

 Silver chromate, in relation to which the neighborhood of the 

 ring is supersaturated, deposits on the precipitate already 

 formed and strengthens it ; this continues until the soluble 

 chromate has been removed from the neighborhood and de- 

 posited on the precipitate. The silver salt, diffusing on far- 

 ther, supersaturates a new circular region, and the process 

 repeats itself. Since the silver nitrate becomes more dilute 

 by diffusion, the critical concentration, at which precipitation 

 begins, is reached later and later, and the rings form farther and farther 

 apart." 



Liesegang has also shown that if the potassium chromate in gelatine is 

 put into a capillary tube instead of being spread upon a plate, analogous 

 phenomena occur when one end of the tube is dipped into a solution of 

 silver nitrate. The precipitate in the tube is formed in layers, or discs, 

 perpendicular to the axis of the tube. (Figure 2.) 



Figure 2. 



* Lehrbuch, II. 2, 778. 



