4 THE PROPERTIES OF SOLUTIONS. [CH. I. 



called " semi-permeable." If a dissolved substance, like 

 glucose, be separated from water by means of a semi- 

 permeable membrane, the sugar solution is diluted by 

 water passing through the membrane. This process, the 

 passage of water through a membrane into a solution, is 

 known as <( osmosis," and is to be carefully distinguished 

 from " diffusion," the passage of a dissolved substance 

 through a membrane. If the sugar solution be contained in 

 a vessel connected to a manometer, and arrangements are 

 made to keep the volume of the solution constant, it is 

 found that the water passing into the vessel causes a rise of 

 pressure. The final pressure reached is known as " the 

 osmotic pressure " of the solution. 



It is not necessary here to enter into the theories of 

 osmotic pressure, but it is important to note that the 

 osmotic pressure of a solution depends on the number of 

 particles in a given volume, no matter whether these 

 particles be ions, molecules, or aggregates of molecules. 

 Thus the osmotic pressure of a dilute solution of sodium 

 chloride is nearly double that of an equimolecular solution 

 of glucose, because in dilute solution the sodium chloride is 

 almost completely dissociated into its constituent ions. 

 From these considerations it follows that the osmotic 

 pressure of a colloidal solution is extremely low in com- 

 parison with that of a crystalloid of the same percentage, 

 for the number of particles in a given volume of the colloidal 

 solution is very small compared with that in the crystalloid 

 solution. 



For non-electrolytes it has been shewn by Van 't Hoff that Boyle's law 

 for gases can be applied to solutions, if we substitute osmotic pressure for gas 

 pressure. It has also been found that the Law of Charles is obeyed, namely, 

 that at constant volume the pressure varies as the absolute temperature. 



It follows that in dilute solute solution 



V.P. = R.T. ; where V = Volume ; P = Osmotic Pressure ; 

 T = Temperature (absolute), and R = a Constant. 



Moreover, it has been shewn by Van 't Hoff that R in the case of osmotic 

 pressure has the same value as in the case of gases, that is, the solution exerts 

 the same osmotic pressure as the pressure that the dissolved substance would 

 exert if it were gasified at the same temperature and confined in the same 

 volume as that of the solution. It follows that one gramme-molecule of a 

 non-electrolyte will exert an osmotic pressure at o C. of 760 mm. of mercury 

 when the volume of the solution is 22-4 litres. 



