26 COLLOIDS IN BIOLOGY AND MEDICINE 



statement is too dogmatic; in dealing with colloidal particles which 

 approach molecular dimensions no sharp line can be drawn between 

 physical and chemical forces. Tr.] We shall, therefore, with Wo. 

 OSTWALD, call these purely physical phenomena mechanical adsorp- 

 tion. 



Most of the investigations on adsorption have been conducted 

 with pulverized solids, hydrophobe colloids, and gels as adsorbents. 

 From a biological standpoint studies of adsorption by hydrophile 

 sols are of especial importance, when we consider for example what 

 occurs in the blood. The only investigations of this character that I 

 am acquainted with are those of H. BECHHOLD* 4 on the distribution 

 of methylene blue between water and serum albumin. The volume 

 in solution is directly obtained by ultrafiltration and the amount of 

 methylene blue distributed in the aqueous nitrate thus obtained is 

 measured. It was shown that in very dilute solutions of methylene 

 blue the major part is held firmly by the albumin; whereas in greater 

 concentrations the distribution is displaced in the direction of the 

 water. The curve is similar to that of an adsorption curve. [The 

 work of A. B. MACALLUM, " Surface Tension and Vital Phenomena," 

 University of Toronto Studies. Physiol. Series No. 8 (1912) should 

 be read in this connection as it involves the adsorption of potassium 

 and its concentration at surfaces. Tr.] 



Based on what has been said hitherto, we might believe that 

 nothing is easier than to determine by the curves of distribution 

 whether we are dealing with chemical combination, distribution 

 between two solvents or adsorption. If, however, we examine the 

 experimental data, we see that there is rarely close agreement 

 between observation and the calculated results. 



These divergences led to the derivation of other formulas deter- 

 mined by the following considerations. According to the formula 

 on page 21 the more concentrated a solution, the more is there 

 adsorbed from it. Actually, saturation limits are reached in many 

 cases. This may be explained as follows: Each interface can adsorb 

 only a layer of a definite thickness, and saturation is reached when 

 this layer is filled with adsorbed molecules. The formulas of 

 ARRHENIUS,* ROB MARC,* and C. G. SCHMIDT* were made to be in 

 conformity with the observed facts. 



The question, whether we are dealing with adsorption or solution, 

 is frequently difficult to decide when the dissolved substance has a 

 different molecular weight in the dispersed phase than in the solvent 

 (see p. 20). The curve of distribution may then assume the form 

 of an adsorption curve and in solving the problem all the incidental 

 circumstances must be considered; for instance, W. BiLTZ* 1 found 



