56 PHYSICAL CHARACTERS OF ORGANIC SUBSTANCES 



will dialyse, and that colloids will not. But different 

 crystalloid solutes pass through a given membrane at 

 very different rates. The membrane may be likened 

 to a sieve with meshes of definite size, though varying 

 within limits. These meshes will let through molecules 

 up to that size, but not larger ones. The disperse 

 particles of a colloid sol are in general too large to pass 

 through the membrane, but in some cases they may 

 be just small enough to pass in small numbers. The 

 matter is further complicated, as we have seen, by the 

 reaction in some cases between the molecules of the 

 solute or disperse phase and those of the membrane. 



A membrane which will allow a solute to pass through 

 it is said to be permeable to the solute. But a mem- 

 brane which is permeable to the solvent and not to 

 the solute is said to be semi-permeable. Different 

 colloidal membranes show very different degrees of 

 permeability to different crystalloid solutes, some 

 letting through a large variety of solutes, others being 

 impermeable to many, while others are impermeable 

 to practically all solutes, though they allow water to 

 pass. This fact has a great importance in the living 

 cell because the gel membrane or film on the surface of the 

 layer of protoplasm which lines the cell wall is a semi- 

 permeable membrane, allowing certain solutes to pass 

 quickly, others slowly, and others, again, not at all. 



Osmosis. — If we , place some sugar solution in a 

 parchment bag (i.e. a colloidal membrane) open at 

 the top, and immerse the bag in water up to the level 

 of the sugar solution within the bag, we find that the 

 level of the liquid^within the bag gradually rises owing 

 to the passage of water from the beaker through the 

 membrane, while the sugar does not pass out into the 

 beaker. The membrane is said to be " semi-permeable 



