[255] The "Cassius' purple" test for gold is an instance where the colloidal condition is used in analysis for a positive test. See Wo. Ostwald, ‹loc. cit.›, p. 68, for other, similar applications for positive tests. [256] ‹Vide› Wo. Ostwald's ‹Kolloidchemie›, 1909, and the references given by Noyes, ‹loc. cit.›, p. 86. [257] A general discussion of the preparation and properties of colloidal mixtures is given by A. A. Noyes, ‹J. Am. Chem. Soc.›, «27», 86–104 (1905) («Stud.»). [258] Picton and Linder, ‹J. Chem. Soc.› (London), «61», 160 (1892), «67», 63 (1895), «71», 568 (1897), etc., and others. Wo. Ostwald, ‹loc. cit.›, p. 240, gives a list of references. [259] The arrangement of the experiment is described by Noyes, ‹loc. cit.›, p. 98. [260] ‹Cf.› Wo. Ostwald, ‹loc. cit.›, p. 108. Billitzer has found that gelatine is positive in acid solution, negative in alkaline, ‹Z. phys. Chem.›, «51», 147 (1905). The charges are, however, relatively small ones. [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). [271] Hardy, ‹Proc. Royal Soc.›, «66», 110 (1899); ‹Z. phys. Chem.›, «33», 391 (1900). [272] Picton and Linder, ‹J. Chem. Soc.› (London), «67», 63 (1895). [273] Whitney and Ober, ‹J. Am. Chem. Soc.›, «23», 852–856 (1901) (Stud.). [274] Picton and Linder, ‹loc. cit.› «71», 572 (1897); Lottermoser, ‹Anorganische Kolloide›, p. 76; Biltz, ‹Ber. d. chem. Ges.›, «37», 1095 (1904).