ON COLLOID CHEMISTRY AND ITS INDUSTRIAL APPLICATIONS. 5 
alumina (Al,03;) up to the most highly hydrous form that can be 
obtained. Asa matter of fact, people have generally been satisfied 
with distinguishing only two sets of colloidal solutions, which they 
have called solutions of alumina and metalumina, stannic and 
metastannic acids, &@4® While adsorption will cause peptisation 
under suitable conditions, the disintegrating power of the adsorbed 
substance is relatively small and often is not sufficient to break up 
solid masses. A protecting colloid, for instance, will prevent the 
formation of a precipitate when it may not be able to disintegrate a 
massive precipitate. 
The preparation of colloidal solutions by peptisation is usually 
classed under the general head of dispersion methods. Five different 
types may be distinguished :— 
1. Removal of Agglomerating Agent.—lf a precipitate has settled 
from a colloidal solution owing to the addition of too much of an 
agglomerating agent, the precipitate may go back into apparent 
solution if the excess of agglomerating agent is washed out. No 
colloidal solution will be obtained if it is impossible to wash out the 
coagulating agent or if the agglomeration has gone too far.” Ifa 
silver halide precipitate is washed on a filter at once, the silver salt 
is apt to run through the filter when the excess-of potassium salt has 
been removed and there is present only the amount which would 
have kept the silver halide in suspension originally. When the rare 
earth nitrates are precipitated by ammonia and then washed, they 
are liable to stay suspended in the liquid when the ammonium 
nitrate is nearly all removed. In alloy work stannic oxide should be 
washed with dilute nitric acid and not with water. Zinc sulphide 
is apt to forma colloidal solution when the ammonium galt is washed 
out,** and copper ferrocyanide does the same thing if all the copper 
sulphate is removed by washing.*® In fact, Chautard*” claims that 
the quickest way to wash a gelatinous precipitate is to evaporate the 
solution to dryness and heat before trying to wash. Merely evapor- 
ating on a water-bath is not always sufficient.*! 
2. Addition of Peptising Agent.—Instead of washing out a pre- 
cipitating agent, a peptising agent may be added. Ammonia is very 
effective in suspending clay,” silicic acid is peptised readily by 
caustic soda, and Prussian blue is peptised by oxalic acid or by 
potassium oxalate. In some cases the concentration of the peptising 
agent has to be high, as when oxides are peptised by alkali, and 
people usually assume the formation of compounds. 
The action of soap on rouge or carbon black looks like a disin- 
tegration ; but itis not. Ifasuspension of carbon black in water 
46 Hantzsch, Zeit. anorg. Chem. 30, 338 (1902) ; Bancroft, Jour. Phys. Chem. 19, 
232 (1915). 
47 Cf, Abegg and Schroeder, Zeit. Kolloidechemie, 2, 85 (1907). 
48 Donnini, Jowr. Chem. Soc. §§ U1. 318 (1894). 
49 Berkeley and Hartley, Phil. Trans. 206 A, 486 (1906). 
50 Jour. Chem. Soc. 26, 527 (1873). 
51 Wright, Zbid. 43, 156 (1883) ; Kratz: Jour. Phys. Chem. 16, 121 (1912). 
58 Skey, Chem. News, 17, 164 (1868) ; 22, 236 (1870) ; 34, 142 (1876) ; see also 
Doelter, Handbuch der Mineralehemie, 2, 122 (1912). 
53 Graham, Jour. Chem. Soc. 1'7, 324 (1864). 
“4 Spring, Zit. Kolloidchemie, 4, 161 (1909) ; 6, 11, 109, 164 (1902). 
Bancroft, Jowr Phys. Chem. 20, 107 (1916). 
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