THE THEORY OF OSMOTIC PRESSURES. 153 



force causing absorption into B (the difference of osmotic 

 pressures). Under no circumstance will there be any trans- 

 ference of salt or dissolved substance between the two sides. 

 Such semi-permeable membranes as this, however, rarely 

 occur in the body. It is possible that the external layer of 

 the cell-protoplasm may in some cases resemble the proto- 

 plasmic pellicle of plant-cells in possessing this "semi-per- 

 meability " ; but in nearly all cases where we have a mem- 

 brane made up of a number of cells, it can be shown that 

 such a membrane permits the free passage of at any rate a 

 large number of dissolved substances. 



Let us now consider what will occur when the two solu- 

 tions A and B are separated by a membrane which permits the 

 free passage of salts and water. If the osmotic pressure of 

 B be higher than A at the commencement of the experi- 

 ment, the force tending to move water from A to B will be 

 equal to this osmotic difference. But there is at the same 

 time set up a diffusion of the dissolved substances from B 

 to A and from A to B. The result of this diffusion must 

 be that there is no longer a sudden drop of osmotic pressure 

 from B to A, and the result of the primary osmotic difference 

 on the movement of water will be minimised in proportion 

 to the freedom of diffusion which takes place through the 

 membrane. Now let us take a case in which A and B re- 

 present equimolecular and isotonic solutions of o and /3. 

 It is evident that the movement of water into A will vary 

 as Ap - Bp l = O. But diffusion also occurs of a into B and 

 of (3 into A. Now the amount of substance diffusing from 

 a solution is proportional to the concentration, and there- 

 fore to its osmotic pressure, as well as to its diffusion 

 coefficient. 



Hence the amount of a diffusing into B will vary as 

 Aft . ak (when k is the diffusion coefficient). 



In the same way the amount of (3 diffusing into A will 

 vary as Bp, (5k'. 



Hence if ak is greater than (3k', i.e., if a is more diffusible 

 than (3, the initial result must be that a greater number of 



1 Ap = osmotic pressure of A, etc. 



