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ON COLLOID CHEMISTRY AND ITS INDUSTRIAL APPLICATIONS. 13 
each other.** Neither of these statements is as accurate as it ~ 
should be. I shall take up first the case of sols having opposite 
eharges. Positive and negative colloids will precipitate each 
other when in proper proportions and provided adsorption takes 
place.”° I see no theoretical reason why we should not have a 
positively charged and a negatively charged sol, neither of which 
adsorbed the other to any appreciable extent. In that case these 
two sols would not precipitate each other. Since complete 
neutralisation takes place only when one sol has adsorbed the 
amount of the sol carrying an equivalent amount of the ion 
having the opposite charge, it follows that the amount of one sol 
necessary to precipitate a given amount of another sol will vary 
with the degree of adsorption; it will therefore be a specific 
property and not an additive one. This can be tested experi- 
mentally on data by Biltz given in Table III.*° 
Taste III. 
1:4 mg. gold completely precipitated by 
CeQ, Fe:,03 Tho, ZrO» Cr2O3 Al,03; 
4 3 2°5 1°6 0°3 0-1 -0°2 mg. 
28 mg. Sb2O3 completely precipitated by 
Fe,03 ThO; CeO, ZrOz CrO; Al,03 
32 20 11 6°5 3°0 2°0 mg. 
24 mg. Ag.S; completely precipitated by 
Fe,03 ThO, CeOs ZrOs A1,03 Cr203 
13 6 4 2 2 0°5 mg. 
Alumina is more effective than chromic oxide in precipitating 
antimony sulphide, and much less effective in precipitating 
arsenic sulphide. ‘The alumina must therefore be adsorbed more 
by antimony sulphide than chromic oxide, while the reverse must 
be true for arsenic sulphide. Cerium oxide is less effective than 
ferric oxide and thorium oxide in precipitating gold, but is more 
effective than either of these in precipitating the sulphides of: 
antimony and arsenic. The phenomenon is thus specific, varying 
with the nature of the two colloids. This seems not to have been 
realised before. In fact, Freundlich says definitely*' that ‘ one 
seems to find approximately the same order regardless of what sol 
is to be precipitated.’ This statement is true, but it missed the 
important thing in the experiments, which was that the order 
was not always the same. 
We can now take up the case of sols having the same charge, 
The statement that neither has any perceptible effect on the other 
is based solely on the fact that no precipitation occurs. | We 
know, however, that cases of adsorption are not limited to colloids 
or electrolytes having opposite signs. Charcoal adsorbs both 
bases and acids. Silver bromide adsorbs silver ions or bromine 
78 Freundlich, Kapillarchemie, 444 (1909). Zsigmondy, Kolloidehemie, 56 
(1912). Hoéber, Physikalische Chemie der Zelle und Zewebe, 294 (1914). 
7 Bancroft, Jowr. Phys. Chem., 18, 555 (1914). 
%0 Freundlich, Kapillarchemie, 445 (1909). 
81 Freundlich, Kapillarchemie, 445 (1909). 
