IX] OF LIESEGANG'S RINGS 661 



of Liesegang's rings is but a particular case of a more general 

 phenomenon, namely the influence on crystalhsation of the presence 

 of foreign bodies or "impurities," represented in this case by the 

 gel or colloid matrix. F. S. Beudant had shewn in a fine paper, 

 more than a hundred years ago, that impurities were the chief cause 

 of variation of crystal habit*. Faraday proved that to diffusion 

 in presence of slight impurities, not to actual stratification or 

 altei'nate deposition, could be ascribed the banded structure of ice, 



Fig. 304. The Liesegang phenomena. After Emil Hatschek. 



of agate or of onyx; and Quincke and Tomhnson added to our 

 scanty knowledge of this remarkable phenomenon f. Ruskin, who 

 knew a great deal about agates, spoke of the perpetual difficulty of 

 distinguishing "between concretionary separation and successive 

 deposition." And Rayleigh shewed how to such a periodic, but 



* F. S. Beudant, Recherehes sur les causes qui peuvent varier les formes crystal- 

 lines d'une meme substance minerale, Ann. de Chimie, viii, pp. 5-52, 1818. See 

 also his Memoire sur les parties solides des MoUusques, Mem. du Museum, xv, 

 pp. 66-75, 1810. 



t Cf. Faraday, On ice of irregular fusibility, Phil. Trans. 1858, p. 228; Researches 

 in Chemistry, etc., 1859, p. 374; Canon Moseley, On the veined structure of the 

 ice of glaciers, Phil. Mag. (4), xxxix, p. 241, 1870; R. Weber, in Poggend. Ann. 

 cix, p. 379, 1860; Tyndall, Forms of Water, 1872, p. 178; C. Tomlinson, On some 

 eflFects of small quantities of foreign matter on crystallisation, Phil. Mag. (5) 

 XXXI, p. 393, 1891, and other papers. Cf. Liesegang, Centralbl. f. Mineralogie, 

 XVI, p. 497, 1911; E. S. Hedges and J. E. Myers, The ^problem, of physico-chemical 

 periodicity, London, 1926; W. F. Berg, Crystal growth from solutions, Proc. R.S. (A), 

 CLxi\', pp. 79-95, 1938. 



