COLLOIDS. 51 



The precipitation of protein by adding finely divided solids (carbon, 

 kaolin J ) or by suspended solids (mastic 2 ) precipitated in the liquid,, is 

 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) . 3 



BECHOLD, 4 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 As2Ss, 

 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. 



Especially with regard to the precipitation of hydrophile colloids we must 

 mention another theory suggested by SPIRO. S According to this the precipita- 

 tion depends upon a division of the colloid between two phases, . one of which 

 contains considerable water and salt and little colloid and the other much colloid, 

 but little water and salt. 



Gels. We have already often mentioned gels or jellies (page 36). 

 Only certain colloids can occur in the form of gels. Certain gels are 

 spontaneously 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, concen- 

 trated 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 

 circumstances that the two phases cannot be analyzed separately. 



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 7 the water is not chemi- 

 cally combined in definite proportions, but the quantity continually 

 changes with the temperature and the vapor pressure. On the other 



1 Bioch. Zeitschr., 5, 365, 1907. 



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



3 Zeitschr. f. physik. Chem., 47, 343, 1904. 



4 Ibid., 60, 299, 1907. 



5 Hofmeister's Beitrage, 4, 300, 1903. 



c Zeitschr. f . physik. Chem., 33, 326, 1900. 

 'Zeitschr. anorg. Chem., 13, 233, 1896; 20, 185, 1899. 



