26 DIFFUSION AND OSMOTIC PRESSURE 



these are almost freely permeable to it, but a diffusion ten- 

 sion of water exists and can be demonstrated in other ways. 

 The internal pressure of dissolved sugar molecules forces the 

 walls of the bag outward through the water, just as a cloth 

 bag may be distended under water by the expansion of wire 

 springs inside of it. Of course, in such a process of dis- 

 tension water enters the bag from without, the bag being 

 permeable to that substance. If in either case the bag is 

 not strong enough to bear the pressure, it will burst when its 

 limit of extensibility is reached. If the parchment bag is 

 strong enough to withstand the pressure developed within, 

 an equilibrium will be established, just as in the case of the 

 expanding gas described in chap. ii. In this condition 

 of equilibrium the inward pressure (due to the resilience of 

 the walls of the bag) is just equaled by the outward pres- 

 sure (due to the bombardment of the walls by the solute 

 particles). 



Such a membrane as parchment paper, which allows the 

 solvent to pass, but greatly retards or prevents the passage 

 of solute particles, is said to be semi-permeable; and the 

 pressure which such a membrane makes evident under the 

 conditions just described is the osmotic pressure of .the 

 solute. This is merely the diffusion tension of the solute, 

 made evident by the opposition of the membrane. All pos- 

 sible gradations exist between membranes which are freely 

 permeable to solutes and those which retard them or are 

 impermeable to them. It needs to be noted here, however, 

 that a theoretically perfect semi-permeable membrane has 

 not been found. The best ones which have been tested 

 allow some passage of solute particles. Many animal mem- 

 branes are nearly semi-permeable in certain solutions, pig's 

 bladder being often used. A membrane of copper ferro- 

 cyanid is almost perfectly semi-permeable in aqueous sugar 

 solution, but it is permeable to certain salts, e. g., potassium 



