PASSAGE OF WATER AND DISSOLVED SUBSTANCES 1 ; ; 

 impermeable to dissolved substances. In this case the transference of water 

 from one side to the other depends entirely on the difference of osmotic 

 pressure between the two sides. 



If we suppose two vessels, A and B (Fig. 24), separated by such a mem- 

 brane, A containing a solution of a and B a solution of p, water will pass 

 from A to B so long as the osmotic pressure of p is greater than the osmotic 

 pressure of the solution of a. If B be subjected to a hydrostatic pressure 

 greater than the osmotic difference between the two fluids, water will pass 

 from B to A until the force causing filtration or transudation (the hydrostatic 

 pressure) is equal to the force causing 

 absorption into B (the difference of 

 osmotic pressures). Under no circum- 

 stance will there be any transference 

 of salt or dissolved substance between 

 the two sides. Such semi-permeable 



B 



membranes as this, however, rarely p IG 2 4. 



occur in the body over any extent of 



surface. The external layer of the cell protoplasm may resemble the 



protoplasmic pellicle of plant cells in possessing this ' semi-permeability * ; 



but in nearly all cases where we have a membrane made up of a number 



of cells, it can be shown to permit the free passage of at any rate a large 



number of dissolved substances. 



Let us now consider what will occur when the two solutions 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 experiment, 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 move- 

 ment 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 represent equimolecular and isotonic solutions of a and p. It is evident 

 that the movement of water into A will vary as Kp Bp* = 0. But 

 diffusion also occurs of oc into B and of p into A. Now the amount of sub- 

 stance diffusing from a solution is proportional to the concentration, and 

 therefore to its osmotic pressure, as well as to its diffusion coefficient. 



Hence the amount of a diffusing into B will vary as A p.ak (when k i 

 the diffusion coefficient). 



In the same way the amount of P diffusing into A will vary as Ep. [ 



Hence, if ocfc is greater than pk', i.e. if a is more diffusible than 

 initial result must be that a greater number of molecules of a will pass 

 than of p into A. The solutions on the two sides of the membrane will 

 be no longer equimolecular, but the total number of molecules 

 B will be greater than the number of molecules of a + p m A, a] 

 * Ap = osmotic pressure of A, &c. 



