C. Edmund Marshall 59 



positive values of a c , a E is always greater than a B . Let a E = qa B . Then 

 by substitution it is easy to show that 



a Ja B = q 2 — 1 (2) 



giving a well-defined value of q or a E /a B for each value of a c /a B . The 

 curve is a parabola, and we can see the situation at a glance (Figure 1). 

 Under the above assumptions the curve will apply to any electrolyte 



°z/°i 



°C/°B 



Figure i. The Donnan equilibrium between soil colloids 

 and expressed solution. For symbols see Table I. 



whose anion and cation have the same valency, and if several such are 

 present simultaneously each can be dealt with. Thus, if we know a c , 

 the cationic activity of the soil colloid, we can calculate how large the 

 salt content will be for any chosen ratio a E /a B . Alternatively, for any 

 value of a B , corresponding to the "Burdian" soil solution, we can de- 

 termine how large the cation activity of the colloid needs to be for q to 

 assume any value greater than 1. 



Some interesting limiting cases arise. Obviously, if a c is very much 

 less than a B , the "Burdian" soil solution becomes practically identical 

 with the expressed solution and both are indistinguishable from the 



