4 REPORTS ON THE STATE OF SCIENCE.—1918. 
evidence seems to be that alumina is not peptised appreciably by 
alkali and that it goes intv solution as sodium aluminate,*? though 
the other view has been supported.** Adsorption of hydroxyl ion 
accounts for the peptisation of silicic acid** and caseine by alkalies. 
Caseine can also be peptised by acids. A. Miiller*® has prepared 
colloidal solutions of aluminum, iron, cobalt, thorium, and yttrium 
oxides by peptisation with dilute hydrochloric acid, and Bentley and 
Rose** have peptised freshly precipitated alumina with 8 per cent. 
acetic acid. It is possible, but not probable, that the peptisation is 
done by a trace of metallic salt formed by the acid and not by the 
hydrogen ion. 
There are no cases where it has been shown conclusively that 
peptisation is due chiefly to adsorption of undissociated salt, but 
undoubtedly such instances; will be found. Water-peptisable colloids 
like gelatin,®?” gum arabic,*® dextrine,*® soap,*? or saponine,*! will 
peptise many precipitates, and they are often called protecting 
colloids, because they prevent the agglomeration and consequent 
settling of finely divided precipitates. Caseine is not peptised by water. 
but acts a protecting colloid when peptised by acids or alkalies; 
Hydrous chromic oxide when peptised by caustic potash can then 
prevent the precipitation of hydrous ferric oxide &c. If too much 
ferric oxide is present, all the chromic oxide is carried down by it.” 
Solutions of copper oxide in ammonia will peptise chromic oxide. 
*Molybdic acid is not precipitated from its salts by uranyl salts, but 
tungstic acid is. In presence of tungstic acid, practically all the 
molybdic acid is precipitated. This is obviously a case of adsorption 
and the converse is undoubtedly true that no tungstic acid would 
be precipitated in presence of a sufhcient excess of a molybdate.*# 
Aniline dyes, which are insoluble in benzene, can be peptised by a 
benzene-soluble colloid such as the so-called zinc or magnesium 
resinate.* 
Since a colloidal solution is one in which a finely divided phase 
is kept from coalescing in some way, it is clear that there may be any 
number of colloida! aluminas, for instance, varying from anhydrous 
33 Herz, Zeit. anorg. Chem. 25, 155 (1900). Hantzsch, Zeit. anorg. Chem. 30, 289 
(1902). Rubenbauer, Zeit. anvrg. Chem. 30, 331 (1902). Fischer and Herz, Zeit. 
anorg. Chem. 31, 355 (1902). Slade, Jour. Chem. Soc. 93, 421 (1908) ; Zeit. anorg. 
Chem..'7'7, 457 (1912); Trans. Faraday Soc., 10, 150 (1914). Blum, Jour. Am. 
Chem. Soc. 35, 1499 (1913). 
33 Mahin, Ingraham and Stewart, Jour. Am. Chem. Soc. 35, 30 (1913). 
34 Graham, Jour. Chem. Soc. 1'7, 324 (1864). 
35 Svedberg, Die Methuden zur Herstellung kolloider Lisungen anorganischer Stoffe, 
400 (1909). 
36 Jour. Am. Chem. Soc. 35, 1490 (1913). 
37 Eder’s Handbuch der Photographie, 5th Hd. 8, I, 28 (1902), Liippo-Cramer. 
Phot. Correspondenz, 4.4. 578 (1907). 
38 Lefort and Thibault, Jowr. Chem. Soc. 42, 1322 (1882). 
39 Lachaud, Bull. Soc. Chim. Paris (3) 15, 1105 (1896). 
40 Spring, Zeit. Kolloidehemie 4, 161 (1909) ; 6, 11, 109, 164 (1910). 
41 Schiaparelli, Jour. Chem. Soc. 46, 333 (1884). 
42 Northcote and Church, Jow'. Chem. Soc. §, 54 (1854) ; Nagel, Jour. Phys. Chem. 
19, 331 (1915). 
43 Prud’homme, Jour. Chem. Soc. 25, 672 (1872). 
44 Miss Hitchcock, Jour. Am. Chem. Soc. 1'7, 483, 520 (1895) ; Wohler. Zeit. Hlektro- 
chemie, 16, 693 (1910). 
4 Soxhlet, Art of Dyeing and Staining Marble, Jc. 76 (1902). 
