APPENDIX 203 



and the process is called diffusion. In a similar way diffusion will eflFect in 

 time a homogeneous naixture of two liquids or solutions. If water is carefully 

 poured on to the stu-face of a solution of salt, the salt or its ions will soon be 

 equally distributed throughout the whole. If a solution of albumin er any 

 other colloidal substance is used instead of salt in the experiment, diffusion 

 will be found to occur much more slowly. If, instead of pouring the water 

 on to the surface of a solution of salt or sugar, the two are separated by a 

 membrane made of such a material as parchment paper, a similar diffusion 

 will OCCIU-, though more slowly than in cases where the membrane is absent. 

 In time, the water on each side of the membrane wUl contain the same 

 quantity of sugar or salt. Substances which pass through such membranes 

 are called crystalloids. Substances which have such heavy molecules 

 (starch, proteid kc.) that they will not pass through such membranes are 

 called colloids. Diffusion of substances in solution in which we have to deal 

 with an intervening membrane is usually called dialysis. The process of 

 filtration (i.e. the passage of materials throogh the pores of a membrane 

 under the influence of mechanical pressure) may be excluded in such experi- 

 ments by placing the membrane (m) vertically as shown in the diagram 

 (fig. 83), 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 

 dissolved in the water. But very few or no membranes 

 are equally permeable to water and to molecules of the 

 substances dissolved in the water. If in the accompany- 

 ing diagram the compartment a is fiUed with pure water, 

 and B with a sodium chloride solution, the hquids in 

 the two compartments will ultimately be fomid to be 

 equal in bulk as they were at the start, and each will 

 be a solution of salt of half the original strength of 

 that in the compartment b. But at first the volvune 

 of the liquid in compartment b increases, because more 

 water molecules pass into it fi-om a than salt mole- 

 cules pass from b into a. The term osmosis is generally 

 limited to the stream of water molecules passing ^^^- ^• 



through a membrane, while the term dialysis is apphed 



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 

 next necessary to define 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 amoimt 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. 



