20 COLLOIDS IN BIOLOGY AND MEDICINE 



dark brown. The more bromin we add, the darker both the water 

 and the carbon disulphid become; the latter, however, is always 

 more darkly colored than the former. If we study the process 

 quantitatively, the following becomes evident: if in a given case the 

 concentration of the bromin in the carbon disulphid is c (carbon 



,. , ,. IN ,1 / c (carbon disulphid) 



disulphid), in the water c (water), then - -. - ^- - = n, 



that is, the relative distribution of the bromin in a given case is n. 



If we double the quantity of both carbon disulphid and of water 

 and then test the quantity of bromin in the fluids, we shall find that 

 in both the concentration has fallen to half and that the distribution 

 continues to be n. If we double the quantity of water, its color is 

 only slightly less intense, because bromin enters the water from the 

 carbon disulphid. If we now measure the bromin content of the two 

 fluids, we shall find ac (carbon disulphid) and in the water be (water); 

 that is, ac (carbon disulphid) 



be (water) 



No matter how we vary the quantity of solvent or of bromin, the 

 apportionment of bromin is always n. We may, therefore, say that 



^i 



n is a constant and express it = k. 



Ci 



This equation is characteristic for the distribution of a substance 

 between two phases in which it is soluble. The process is reversible; 

 it is in labile balance. The law of distribution was formulated by 

 M. BERTHELOT and JUNGFLEISCH in 1872, though we still frequently 

 refer to HENRY'S Law of Distribution. Strictly speaking, this ex- 

 pression applies only to the distribution of a gas between a fluid 

 and a gaseous phase (proportionately to the pressure). 



/ 



The distribution = k applies only in case the molecular weight 



Ci 



of the dissolved substance is the same in both solvents. If this is 



not the case, the equation becomes = k, in which a and b express 



c& 



the difference in the molecular weight in the two solvents. 1 W. 

 NERNST has formulated the law of distribution in this way. For 

 example, benzoic acid in water has a simple molecular weight, but 

 in benzol it exists mostly in double molecules. In order to ex- 



c (water) , 



press this, the equation of distribution becomes . = = k. 



Vc (benzol) 



1 [Prof. J. L. R. MORGAN suggests that a better form would be the following: 



sff-* 



c 2 " k ' 

 where x is the ratio of the molecular weight in solvent (2) to that in solvent (1). 



Sx ^ xs ai , d -a, TV.] 



(2) (1) m in (1) 



