324 



PHYSICAL CHEMISTRY 



[CH. XXIV. 



FIG. 



M 



is usually called dialysis. The process of filtration (i. e. , the passage of materials 

 through the pores of a membrane under the influence of mechanical pressure) may 

 be excluded in such experiments by placing the membrane 

 (M) vertically as shown in the diagram (fig. 275), and the 

 two fluids A and B on each side of it. Diffusion through 

 a membrane is not limited to the molecules of water, but it 

 may occur also in the molecules of certain substances dis- 

 solved in the water. But very few or no membranes are 

 equally permeable to water and to molecules of the sub- 

 stances dissolved in the water. If in the accompanying 

 diagram the compartment A is filled with pure water, and 

 B with a sodium chloride solution, the liquids in the two 

 compartments will ultimately be found to be equal in bulk 

 as they were at the start, and each will be a solution of salt 

 of half the strength of that originally in the compartment 

 B. But at first the volume of the liquid in compartment B 

 increases, because more water molecules pass into it from 



A than salt molecules pass from B to A. The term osmosis is generally limited 

 to the stream of water molecules passing through a membrane, while the term 

 dialysis is applied to the passage of the molecules in solution in the water. The 

 osmotic stream of water is especially important, and in connection with this it is 



necessary to explain the term osmotic pressure. At 

 first, then, osmosis (the diffusion of water) is more 

 rapid than the dialysis (the diffusion of the salt 

 molecules or ions). The older explanation of this 

 was that salt attracted the water, but we now 

 express the fact differently by saying that the salt in 

 solution exerts a certain osmotic pressure : the result 

 of the osmotic pressure is that more water flows from 

 the water side to the side of the solution than in the 

 contrary direction. The osmotic pressure varies with 

 the amount of substance in solution, and is also 

 altered by variations of temperature occurring more 

 rapidly at high than at low temperatures. 



If we imagine two masses of water separated by 

 a permeable membrane, as many water molecules 

 will pass through from one side as from the other, 

 and so the volumes of the two masses of water will 

 remain unchanged. If now we imagine the mem- 

 brane M is not permeable except to water, and the 

 compartment A contains water, and the compart- 

 ment B contains a solution of salt or sugar ; under 

 these circumstances water will pass through into B, 

 and the volume of B will increase in proportion to 

 the osmotic pressure of the sugar or salt in solution 

 in B, but no molecules of sugar or salt can get through 

 into A from B, so the volume of fluid in A will con- 

 tinue to decrease, until at last a limit is reached. The 

 determination of this limit, as measured by the height 

 of a column of fluid or mercury which it will support, 

 will give us a measurement of the osmotic pressure. 



If a bladder containing strong salt solution is 

 placed in a vessel of distilled water, water passes into 

 the bladder by osmosis, so that the bladder is swollen, 

 and a manometer connected with its interior will show 

 a rise of pressure (osmotic pressure). But the total 

 rise of pressure cannot be measured in this way for 

 two reasons : (1) because the salt diffuses out as the 



water diffuses in ; and (2) because the membrane of the bladder leaks ; that is, 

 permits of filtration when the pressure within it has attained a certain height. 



B 



FIG. 27(5. A, outer vessel, con- 

 taining distilled water ; B, 

 inner semi-permeable vessel, 

 containing 1 per cent, salt 

 solution ; M, mercurial 

 manometer. (After Star- 

 ling.) 



