HYDROPHILIC SOLS AND GELS 153 



ance, but instead, the chromate is deposited in concentric rings 

 at ever increasing distances {A and D, Fig. 92). To explain this 

 peculiar phenomenon several theories have been advanced, one 

 of which invokes supersaturation. The silver chromate is 

 presumably carried along in the process of diffusion until a 

 point of supersaturation is reached, when the whole of the 

 precipitate is deposited and a ring is formed. The increase in 

 distance between rings is due to depletion of the silver as it 

 diffuses outward and unites with the dichromate. As the process 

 proceeds, the precipitation may become less and less periodic, 

 until at the end it is scattered uniformly through the gel. In D, 

 Fig. 91, small granules of the precipitate are to be seen scattered 

 near the bottom of the tube where the bands are becoming less 

 and less clear-cut. Perhaps no one theory will explain all forms 

 of precipitation in gels. Subsequent solubility of the precipitate 

 may be a factor. 



One occasionally chances upon variations in Liesegang 

 phenomena which are very difficult of interpretation. Double 

 banding is one of these. A strong solution of potassium citrate 

 in a 1 per cent agar gel containing potassium dichromate will, 

 upon the addition of silver nitrate to the surface of the gel, 

 produce a band of white silver citrate, followed by a neutral 

 zone; then a reddish brown band of potassium dichromate, 

 followed again by a neutral zone, a band of silver citrate, and 

 so on. But most remarkable is the rare occurrence of a continu- 

 ous spiral {E, Fig. 91), even though no change in technique has 

 wittingly taken place. For this extraordinary behavior no 

 explanation is known. Spiral structures often occur in nature 

 (Fig. 14). 



Another neat example of rhythmic precipitation is the rings 

 formed in fine capillaries (B, Fig. 92) ; indeed, where the capillaries 

 are very small, no gel is necessary to hold the rhythmically 

 produced precipitate in place. 



Liesegang pointed out the possible relationship of rhythmic 

 banding in gels to similar phenomena in the mineralogical and 

 geological worlds. Laboratory productions of banding may be 

 precise reproductions of natural agates (F, Fig. 91). 



Lloyd has observed the formation of Liesegang rings in the 

 vacuoles of living plant cells when stained (with neutral red and 

 other reagents). Either the contents of the vacuoles are a gel, 



