660 ON CONCRETIONS, SPICULES, ETC. [ch. 



If we dissolve, for instance, a little potassium bichromate in 

 gelatine, pour it on to a glass plate, and after it is set pour upon it a 

 drop of silver nitrate solution, there appears in the course of a few 

 hours the phenomenon of Liesegang's rings. At first the silver 



Fig. 302. Conostats. * After Harting. 



forms a central patch of abundant reddish-brown chromate pre- 

 cipitate; but around this, as the silver nitrate diffuses slowly 

 through the gelatine, the precipitate no longer comes down con- 

 tinuously, but forms a series of concentric rings or zones, beautifully 



Fig. .303. Liesegang'.s rings. After Leduc. 



regular, which alternate with clear interspaces of jelly and stand 

 farther and farther apart in a definite ratio as they recede from 

 the centre*. For a discussion of the raison d'etre of this phenomenon, 

 the student will consult the textbooks of physical and colloid 

 chemistry. But, speaking generally, we may say that the appearance 



* It is now known that periodic precipitation may be exhibited even in aqueous 

 solutions, and that what the gel does is to enlarge the intervals, and to enhance 

 the phenomenon, by affecting the rate or relative rates of diffusion. Cf. H. W. 

 Morse, Journ. Phys. Chem. 1931. 



