jelly, and which stand farther and farther apart, in logarithmic ratio, as they recede from the centre. For a discussion of the raison dãûˆtre of {428} this phenomenon, still somewhat problematic, the student must consult the text-books of physical and colloid chemistry31.
But, speaking very generally, we may say the appearance of Liesegangãs rings is but a particular and striking case of a more general phenomenon, namely the influence on cryôÙstalôÙliôÙsaôÙtion of the presence of foreign bodies or ãimpurities,ã represented in this case by the ãgelã or colloid matrix32. 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 phenomenon33.
Besides the tendency to rhythmic action, as manifested in Liesegangãs rings, the association of colloid matter with a crystalloid in solution may lead to other well-marked effects. These, according to Professor J. H. Bowman34, may be grouped somewhat as follows: (1) total prevention of cryôÙstalôÙliôÙsaôÙtion; (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 compound crystal; (4) a curving or gyrating of