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, 5 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 As2Sa, 

 but retained colloidal Prussian blue, did not allow a clear mixture of 

 the two to pass through. The particles of As2S,3, 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- 

 tanteously 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 r61e 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. 



3 Ibid., 2, 219 (1906); 3, 109 (1906). 



4 Zeitschr. f . physik. Chem., 47, 343 (1904). 

 6 Ibid., 60, 299 (1907). 



Ibid., 33, 326 (1900). 



'Bioch. Zeitschr., 18, 367 (1909). 



8 Zeitschr. anorg. Chem., 13, 233 (1896); 20, 185 (1899). 



