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COLLOIDS. 23
which do not settle after months are precipitated in 24-48 hours by a minimum
quantity of lime or magnesia. Be also calls attention to the essential role which
the salts of sea water must play in the sedimentation of the cloudy fresh water
flowing into the sea (delta formation).
In consideration of the conditions just mentioned, under which the
suspension colloids are precipitated by electrolytes, the mutual precipita-
tion ability of suspension colloids is of considerable interest. Accord-
ing to what lias been stated previously, the colloids are considered as carriers
of electricity, and it has been proved that the oppositely charged col-
loids can act precipitatingly upon each other. This rule was first pro-
posed by Linder and Picton, 1
and has subsequently been substantiated
by many investigators. Biltz 2
has made especially systematic investiga-
tions on this subject and finds that colloids carrying ’the same kind of
charge do not precipitate each other. For the mutual complete precipita-
tion of opposed electrically charged colloids, a certain quantitative rela-
tion is necessary. On the action of two colloids with opposite charges in
variable quantities an optimum of the precipitation action is noticed; while
on overstepping the desirable precipitation conditions in both directions
no precipitation occurs at all.
In analogy with the mutual precipitation ability of the colloids, Biltz believes
that the especial great ability of most salts of the heavy metals to precipitate
colloids lies in the hydrolytically split and colloid-dissolving metallic hydroxides.
Protective Colloids. Certain hydrophile colloids, which are precip-
itated with difficulty by electrolytes, have the power of protecting
suspension colloids against the precipitating action of electrolytes. Meyer
and Lottermosser 3
have found with silver hydrosol that the presence
of protein prevented the flocking out by electrolytes. Zsigmondy 4
has investigated the relative action of the protective colloids and has
found considerable differences. The figure in milligrams of colloid which
is just insufficient to protect 10 cc. of gold solution (0.0053-0.0058
per cent) against the action of 1 cc. 10 per cent NaCl solution is called the
gold equivalent for the respective colloid. Gelatin offers the best pro-
tection, then comes isinglass, casein, ovalbumin, gum arabic, Irish moss,
dextrin, starch. The colloidal sulphides (As2 ^3. Sb2S3, CdS) are also
protected in the same manner against the influence of electrolytes (A.
Miller and Artmaxx 5
). Inorganic colloids may also act as protective
1
Journ. chem. Soc, 71, 572 (1897).
2
Ber. d. d. chem. Gesellsch., 37, 1095 (1904).
3
Journ. prakt. Chem. (2), 56, 241 (1897).
4
Zeitschr. analyt. Chem., 40, 697 (1901).
5
Oester. Chem. Ztg., 7, 149 (1904).
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