610 The Earl of Berkeley and Dr. C. V. Burton on 



Fidr the aggregate downwardly-measured force on the 

 solvent molecules contained in dr, and F 2 dr the downwardly- 

 measured force on the solute molecules ; then F 1 H-F 2 = 0. 



The forces, F : and F 2 , reckoned per unit volume of solu- 

 tion, we shall call the " driving forces" on solvent and solute 

 respectively at the level under consideration. 



Now imagine the downwardly-measured driving forces F x 

 and — Fj to be balanced by an ideal field of force, whose 

 action at each level amounts to a force — F a on the solvent 

 molecules per unit volume of solution and + F ± on the 

 solute molecules per unit volume. The total driving force is 

 thus reduced to zero throughout the solution, and the con- 

 centration retains its distribution unaltered, although the 

 mobility of individual molecules remains unimpared. 



Consider now the small portion of the column represented 

 by figure 2. At the level h let the concentration be c 2 

 grams of solvent per gram of solution, and the hydrostatic 



Fig. 2. 



a 



\-({ h 



b' 



a,' 



T 



-h+Sh 



pressure be p. At the neighbouring level h + Bh the values 

 of these quantities will be, to a first order, 



c 2 + (dc 2 /dli) x Bh and p + (ff/w) X Sh respectively, 



where w is the specific volume of the solution at the 

 level A. 



At h, in the wall of the containing vessel, let q be a mem- 

 brane permeable to the solvent only, and q' a like membrane 

 at level 7i + $7i. Let the cylinders b and V (furnished with 



