240 ESSENTIALS OF CHEMICAL PHYSIOLOGY 



instead. The reason for this is that it has been found difficult to construct 

 a membrane which is absolutely semi -permeable. 



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 mem- 

 brane 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 Hoff's 

 hypothesis). The nature of the substance makes no difference ; it is only the 

 number of molecules 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 partial pressure of gases 

 is complete, as may be seen from the following statements : 



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

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



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

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



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

 the sum of the pressures which the individual substances would exert if they 

 were alone in the solution (Henry-Dalton's law for partial pressure of gases). 



4. The osmotic pressure is independent of the nature of the substance in 

 solution, and depends only on the number of molecules or ions in solution 

 (Avogadro's law for gases). 



Calculation of Osmotic Pressure. We may best illustrate this by an example 

 and to simplify matters we will take an example in the case of a non-electro- 

 lyte like sugar. We shall then not have to take into account any electrolytic 



