[261] Billitzer, Z. f. Elektrochem., 8, 638 (1902). This is probably true of all amphoteric colloids (Chapter X); it is also true of many other substances, which are not pronouncedly amphoteric. (Cf. Perrin, Comp. rend., 136, 1388 (1903); Billitzer, Z. phys. Chem., 51, 157 (1905).)

[262] Hardy, J. of Physiology, 24, 288 (1899); Z. phys. Chem., 33, 387 (1900).

[263] Billitzer, loc. cit., p. 159; Müller's Allgemeine Chemie der Kolloide, 1907, p. 79.

[264] See below, p. [134].

[265] In a slightly acid solution colloidal silicic acid is negatively charged; in a strong acid solution, positively—a relation which agrees with its predominantly acid character.

[266] The general class of substances, showing both basic and acid properties, of which albumen is a derivative, is described in a footnote on glycocoll, Chapter X, p. [188].

[267] Hardy, loc. cit.

[268] J. Loeb, University of California Publications, Physiology, 2, 149 (1904).

[269] Very little is known about the nature of contact electricity. It is even doubtful whether it is different, in principle, from ionization.

[270] W. Ostwald, Lehrbuch der Chem., 2, (1) 553 (1903).