28 PHYSICAL CHEMISTRY IN BIOLOGY. 



From PFEFFER'S results of the osmotic pressure of cane-sugar solu- 

 tions VAN'T HOFF has calculated that it is the same as the pressure exerted 

 by any gas of the same molecular concentration and temperature. In 

 general the following is true : 



Dissolved bodies exert in solution the same osmotic pressure they 

 would exert if they were gases at the same temperature and in equal volume. 



As the gas pressure, according to the kinetic theory of gases, is caused 

 by the impact of the gas molecule upon the walls of the vessel, so also 

 the osmotic pressure is considered as a pressure exerted by the dissolved 

 substance toward the outside upon the confining walls and the free 

 surface. Ordinarily this pressure never becomes evident on the out- 

 side, because it is always much surpassed by the reverse action of the 

 surface tension of the fluid. When the surface tension is removed, as is 

 the case when the solution is separated from the pure solvent by a semi- 

 permeable membrane, then the osmotic pressure becomes evident, as either 

 the membrane is moved by the pressure, or in case the membrane 

 is not movable and is not ruptured, pure solvent enters into the solution. 

 The pressure of the surface tension, which is transferred through the 

 membrane, is less than the sum of the surface tension of the solution 

 and the reverse-acting osmotic pressure; and the solvent is, therefore, 

 pressed against the membrane, with a force equal to the osmotic pressure 

 of the solution. 



For physiological purposes it seems best to make use of the above 

 explanation, according to which the osmotic pressure is considered as 

 a measure of the force with which a solution attracts the solvent. 



PFEFFER'S above-described method of directly determining the 

 pressure can only be used in exceptional cases, first because the prepara- 

 tion of the semipermeable membrane is connected with difficulties, and 

 second because there are only a few bodies which have been found 

 impermeable to the membranes. There are other quicker and easier 

 ways of determining the osmotic pressure. 



Solutions of non-volatile substances boil at a higher temperature 

 than the pure solvent. This is due to the fact that the dissolved sub- 

 stance, because of the osmotic pressure, holds on to the solvent with 

 a certain force. As in boiling a part of the solvent is separated from 

 the dissolved body and as the osmotic pressure can be considered as a 

 measure of the attractive power between the solvent and the dissolved 

 substance, then it is clear that solutions which are prepared with the 

 same solvent and have the same osmotic pressure (isosmotic solutions) 

 must also boil at the same temperature. The rise in the boiling-point 

 of a solution above the boiling-point of the solvent (elevation of the 

 boiling-point) is also, like the osmotic pressure, for dilute solutions pro- 

 portional to the concentration. 



