50 PHYSICAL CHEMISTRY IN BIOLOGY. 



That proteins, on precipitation, carry down other bodies with avidity 

 is well known; inorganic hydrogels also take up dissolved substances 

 with energy. The curves obtained for the latter process by VAN BEM- 

 MELEN l show a close analogy with the characteristic curves for the 

 adsorption compounds. It often occurs that the body taken up homo- 

 geneously saturates the hydrogel, in which case ^fc, and a sort of 



solid solution is the result. In this manner KC1 is taken up by colloidal 

 silicic acid. 2 In certain cases undoubtedly chemical combinations 

 with quite positive conditions are formed. 3 



The precipitation of colloids by electrolytes has also been discussed 

 by FREUNDLiCH 4 from the standpoint of the adsorption hypothesis. 

 Thus, for the precipitation ability of an electrolyte, the electric charge 

 of the precipitating ion comes first in consideration and secondly, 

 the ability of the precipitating colloid r to adsorb the same. According 

 to MOORE and ROAF 5 the salts of the red corpuscles are retained as 

 adsorption compounds (adsorpates) by the proteins. 



Thus far only the adsorption of crystalloids has been considered. 

 Colloids are also taken up by solid substances or by other colloids. Still 

 in these cases the conditions are more complicated than in the above-men- 

 tioned adsorption phenomena, as the combinations formed are in special 

 cases irreversible or gradually irreversible. It is well known that car- 

 bon takes up colloidal colored substances, and we have numerous examples 

 of the combination of dissolved colloids with solid colloids hi technology. 

 BILTZ 6 has been able to show that many dyeing processes are to be 

 considered as adsorption phenomena, and later FREUNDLICH and LOSEV 7 

 have measured the adsorption of basic and acid pigments by carbon 

 and also by fibers (wool, silk, cotton) , and have shown the correspondence 

 of the two processes. With the basic pigments, which were used as 

 salts, a splitting occurred into a pigment base, which was taken up by 

 the fibers as well as by carbon, and an acid which quantitatively remained 

 behind. This is similar to the cleavage which precipitating electrolytes 

 undergo in the precipitation of the suspension colloids (see page 47). 



Tanning is also brought about by adsorption processes as the prepared skins 

 adsorb the tanning substance. 8 



1 Zeitschr. anorg. Chem., 23, 111, 321, 1900. 



2 Schmidt, Zeitschr. f. physik. Chem., 15, 56, 1894. 



3 v. Bemmelen, Journ. prakt. Chem. (2), 23, 324 and 379, 1880. 



4 Zeitschr. f. Chem. u. Ind. d. Koll., 1, 321, 1907. 



5 Bioch. Journ., 3, 55, 1908. 



6 Ber. d. d. chem. Gesellsch., 37, 1766, 1904; 38, 2963, 2973, 4143, 1905. 



7 Zeitschr. f . physik. Chem., 59, 284, 1907. 



8 See Zeitschr. f. Chem. u. Ind. d. Koll., 2, 257, 1908. 



