CH. XXIY.] OSMOTIC PRESSURE 325 



It is therefore necessary to use a membrane which will not allow salt to pass 

 out either by dialysis or filtration, though it will let the water pass in. Such 

 membranes are called semi-permeable membranes, and one of the best of these is 

 ferrocyanide of copper. This may be made by taking a cell of porous earthenware 

 and washing it out first with copper sulphate and then with potassium ferrocyanide. 

 An insoluble precipitate of copper ferrocyanide is thus deposited in the pores of the 

 earthenware. 



If such a cell is arranged as in fig. 276, and filled with a 1 per cent, solution of 

 sodium chloride, water diffuses in, till the pressure registered by the manometer 

 reaches the enormous height of 5000 mm. of mercury. If the pressure in the cell is 

 increased beyond this artificially, water will be pressed through the semi-permeable 

 walls of the cell and the solution will become more concentrated. 



In other words, in order to make a solution of sodium chloride of greater 

 concentration than 1 per cent., a pressure greater than 5000 mm. of mercury must 

 be employed. The osmotic pressure exerted by a 2 per cent, solution would be 

 twice as great. 



Though it is theoretically possible to measure osmotic pressure by a manometer 

 in this direct way, practically it is hardly ever done, and some of the indirect 

 methods of measurement described later are used instead. The reason for this is 

 that it has been found difficult to construct a membrane which is absolutely semi- 

 permeable ; they are nearly all permeable in some degree to the molecules of the 

 dissolved crystalloid. In course of time, therefore, the dissolved crystalloid will 

 be equally distributed on both sides of the membrane, and osmosis of water will 

 cease to be apparent, since it will be equal in both directions. 



Many explanations of the nature of osmotic pressure have been brought 

 forward, but none is perfectly satisfactory. The following simple explanation is 

 perhaps the best, and may be rendered most intelligible by an illustration. 

 Suppose we have a solution of sugar separated by a semi-permeable membrane 

 from water ; that is, the membrane is permeable to water molecules, but not to 

 sugar molecules. The streams of water from the two sides will then be unequal ; 

 on one side we have water molecules striking against the membrane in what we 

 may call normal numbers, while on the other side both water molecules and sugar 

 molecules are striking against it. On this side, therefore, the sugar molecules 

 take up a certain amount of room, and do not allow the water molecules to get 

 to the membrane ; the membrane is, as it were, screened against the water by 

 the sugar, therefore fewer water molecules will get through from the screened to 

 the unscreened side than vice versa. This comes to the same thing as saying that 

 the osmotic stream of water is greater from the unscreened water side to the 

 screened sugar side than it is in the reverse direction. The more sugar molecules 

 that are present, the greater will be their screening action, and thus we see that 

 the osmotic pressure is proportional to the number of sugar molecules in the 

 solution, that is, to the concentration of the solution. 



Osmotic pressure is, in fact, equal to that which the dissolved substance would 

 exert if it occupied the same space in the form of a gas (Van't HofTs hypothesis). 

 The nature of the substance makes no difference ; it is only the number of mole- 

 cules which causes osmotic pressure to vary. The osmotic pressure, however, of 

 substances like sodium chloride, which are electrolytes, is greater than what one 

 would expect from the number of molecules present. This is because the molecules 

 in solution are split into their constituent ions, and an ion plays the same part as 

 a molecule, in questions of osmotic pressure. In dilute solutions of sodium chloride 

 ionisation is more complete, and as the total number of ions is then nearly double 

 the number of original molecules, the osmotic pressure is nearly double what would 

 have been calculated from the number of molecules. 



The analogy between osmotic pressure and the pressure of gases is very com- 

 plete, as may be seen from the following statements : 



1. At a constant temperature osmotic pressure is proportional to the concentra- 

 tion of the solution (Boyle-Mariotte's law for gases). 



2. With constant concentration, the osmotic pressure rises with and is propor- 

 tional to the temperature (Gay-Lussac's law for gases). 



3. The osmotic pressure of a solution of different substances is equal to the sum 



