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30 GENERAL AND PHYSICO-CHEMICAL.
Tanning is also brought about by adsorption processes, as the prepared skins
adsorb the tanning substance. 1
The precipitation of portein by adding finely divided solids (carbon*
kaolin 2
) or by suspended solids (mastic 3
) precipitated in the liquid,
as well as the action of protective colloids as already mentioned are also
due to adsorption processes. The precipitation of protein, which occurs
on shaking the protein solution with liquids, in which the protein is
not soluble, is also to be considered as a surface tension action (Ramsden 4
).
Bechhold,3
in his above-mentioned experiments on the filtration of
colloids, has observed conditions which he considers as adsorption phe-
nomena. Under certain circumstances a colloid can prevent the filtra-
tion of another colloid. A filter which was permeable for colloidal AS2S3,
but retained colloidal Prussian blue, did not allow a clear mixture of
the two to pass through. The particles of AS2S3, were adsorbed by the
particles of Prussian blue, and could therefore not pass through the
filter.
Gels. We have often mentioned gels or jellies (page 14). Only
certain colloids can occur in the form of gels. Certain gels are spon-
taneously formed in sufficiently concentrated solutions (silicic acid,
certain metallic hydroxides) and these do not redissolve in water. Other
gels, like gelatin and agar, are formed on cooling of the hot, concentrated
solutions, and are again soluble in water.
According to Hardy 6
the gel formation of gelatin is to be considered
as a segregation process whereby a separation into two fluids occurs,
one of which solidifies. The two phases are only differentiated by the
microscope, and the chemical testing of the theory fails because of the cir-
cumstances that the two phases cannot be analyzed separately. In opposi-
tion to this Pauli claims that the gel passes through all of the intermediary
steps into the corresponding sol and is therefore homogenous in the same
sense as these.7
When gels are freed from water by evaporation or in other ways,
they show a special ability to take up water, which is brought about
by different processes which are included in the ordinary term imbibition.
The views on this imbibition are indefinite. Surface phenomena play
a role here. According to van Bemmelen 8 the water is not chemi-
1 See Zeitschr. f. Chem. u. Ind/d. Koll., 2, 257 (1908).
2
Bioch. Zeitschr., 5, 365, 1907.
• Ibid., 2, 219 (1906); 3, 109 (1906).
• Zeitschr. f. physik. Chem., 47, 343 (1904).
1
Ibid., 60, 299 (1907).
• Ibid., 33, 326 (1900).
>
Bioch. Zeitschr., 18, 367 (1909).
’Zeitschr. anorg. Chem., 13, 233 (1896); 20, 185 (1899).
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