7] DIALYSIS STUDIES 107 



the subtilin. A series of such cells might be employed in countercurrent 

 manner as in countercurrent distribution. At each stage the dialysate could 

 be evaporated and put inside the sac of the next member of the series. Thus 

 from each cell 80% of the bacitracin would be moved forward at each stage, 

 but only 20% of the subtilin. The familiar mathematics of countercurrent 

 distribution (the binomial theorem) is directly applicable and will tell us 

 that the bacitracin-subtilin mixture is an easy separation requiring only a 

 few stages. Useful separations with peptides and derivatives have been 

 accomplished this way.^-® 



The straightness of the line with a supposedly pure substance affords a 

 measure of adherence to ideality in the solvent used and of the homogeneity 

 with respect to size and shape. In the countercurrent process a straight line 

 represents ideahty and is desirable just as a linear partition isotherm repre- 

 sents ideality in countercurrent distribution. An unfavorable solvent can 

 produce molecules of different sizes from an otherwise pure solute by pro- 

 moting association or theoretically by causing a fraction of the molecules 

 to assume a shape different from the others but perhaps not instantaneously 

 interconvertible. 



JOO 



80 

 70 

 60 

 50 



40 



en 30 

 C 



20- 



10 h 



8 

 7 

 6 

 5 



4- 



100 



400 



200 300 

 Minutes 

 Fig. 3. Escape rate curves obtained with a sample of protamine. 



Where a mixture of sizes is present the deviation from a straight line may 

 take the form of distinct breaks in the curve as shown in Fig. 3 which was 

 obtained with a 20 mg. sample of a crude protamine. Curve 1 was obtained 

 by weight analysis of the dialysate fractions, but Curve 2 was obtained by 



