428 ON CONCRETIONS, SPICULES, ETC. [ch. 



this phenomenon, still somewhat problematic, the student must 

 consult the text-books of physical and colloid chemistry*. 



But, speaking very generally, we may say the appearance of 

 Liesegang's rings is but a particular and striking case of a more 

 general phenomenon, namely the influence on crystallisation of 

 the presence of foreign bodies or "impurities," represented in this 

 case by the "gel" or colloid matrix |. Faraday shewed long ago 

 that to the presence of slight impurities might be ascribed the 

 banded structure of ice, of banded quartz or agate, onyx, etc. ; 

 and Quincke and Tomlinson have added to our scanty knowledge 

 of the same phenomenon {. 



Fig. 205. Liesegang's Rings. (After Leduc.) 



Besides the tendency to rhythmic action, as manifested in 

 Liesegang's rings, the association of colloid matter with a crystal- 

 loid in solution may lead to other well-marked effects. These, 

 according to Professor J. H. Bowman §, may be grouped somewhat 

 as follows : (1) total prevention of crystallisation ; (2) suppression of 

 certain of the lines of crystalline growth ; (3) extension of the crystal 

 to abnormal proportions, with a tendency for it to become a com- 

 pound crystal ; (4) a curving or gyrating of the crystal or its parts. 



* Cf. Taylor's Chemistry of Colloids, pp. 146-148, 1915. 



t Cf. S. C. Bradford, The Liesegang Phenonemon and Concretionary Structure 

 in Rocks. Nature, xcvii, p. 80. 1916; cf. Sci. Progress, x, p. 369, 1916. 



X Cf. Faraday, On Ice of Irregular Fusibility, Phil. Trans., 1858, p. 228; 

 Researches in Chemistry, etc., 1859, p. 374; Tyndall, Forms of Water, p. 178, 

 1872; Tomlinson. C, On some effects of small Quantities of Foreign Matter on 

 Crystallisation, Phil. Mag. (5) xxxi, p. 393, 1891, and other papers. 



§ A Study in Crystallisation, /. of Soc. of Chem. Industry, xxv, p. 143, 1906. 



