HYDROPHILIC SOLS AND GELS 151 



known, but it is possible that so-called van der Waals forces — 

 forces that act at greater distances than usual intermolecular 

 ones and probably account for a number of phenomena involving 

 relatively loose bonds such as adsorption — may serve as attrac- 

 tional forces sufficient to pull together, as it were, the isolated 

 structural units of a collapsed gel. 



Thixotropic phenomena occur in protoplasm. The mere stir- 

 ring of highly viscous protoplasm will often reduce its consistency. 

 The elaborate mitotic figure of a dividing egg after fertilization, 

 with astral rays and spindle, maintained by structural properties 

 of the protoplasm may, when subjected to pressure, collapse with 

 great suddenness and reduce the egg to an undifferentiated mass 

 with no vestige of the former mitotic parts; mechanical disturb- 

 ance has caused instantaneous and complete collapse of the 

 internal arrangement of the cell material. 



The Liesegang Phenomenon. — The properties of gels are such 

 as to give them some of the characteristics of solids and some of 

 liquids. A gel may possess the rigidity of a solid and yet permit 

 soluble substances to diffuse through it as quickly as in liquids. 

 The rigid quality of a gel permits precipitates, when formed 

 within it, to be held in place. Substances that form precipitates 

 while diffusing through a gel leave in the gel striking formations 

 which closely parallel certain processes in nature, both in appear- 

 ance and very probably in manner of formation. The precipi- 

 tates may be crystals irregularly distributed, which, when at 

 their best, present one of the most beautiful of colloidal phe- 

 nomena (A and B, Fig. 91). Hatschek has made very attractive 

 crystals of gold scattered in sihca gel. Gold chloride is first 

 added to the gel. A reducing substance (sodium sulphide) is 

 allowed to diffuse in from the outside; the chloride is reduced, 

 and gold crystals result. In many instances, the crystal forma- 

 tion occurs rhythmically so as to produce bands {A, Fig. 91), 

 which, when glucose has been previously added, may number 

 more than a hundred within 8 cm. When precipitation in gels 

 is rhythmic, so as to produce a series of concentric rings or bands, 

 it is known as the Liesegang phenomenon (C and E, Fig. 91; 

 A, Fig. 92). 



Rhythmic precipitation was discovered by Liesegang in 1896. 

 His original experiment consisted in coating a glass plate with 

 a film of gelatin which contained a small amount of potassium 



