CHANGE OF VAPOUR PRESSURE 



43 



lowering of vapour pressure produced by the dissolved 

 body. 



The vapour pressure of a solution is immediately related 

 to its osmotic pressure. Obviously two solutions in the 

 same solvent, if they have the same osmotic pressure, i. e. 

 are isotonic, must show the same 

 saturation pressure. Otherwise a per- 

 petuum mobile in the meaning of 

 Fig. 7 would be inevitable. The two 

 isotonic solutions A and B are sup- 

 posed separated by a semipermeable 

 membrane ; then if there is a difference 

 between their saturation vapour pres- 

 sures, a current of vapour will flow 

 in the upper part of the vessel, say 



from left to right. Then the decrease and increase of 

 concentration produced in B and A respectively will cause 

 a movement of the solvent through a b from right to left, 

 and so the conditions for a perpetual motion are given, 

 which can only be avoided if isotony goes with equality 

 of vapour pressure. 



But further, the magnitude of the change in vapour 

 pressure which a solvent suffers in taking up a dissolved 

 body may be calculated, as will be clear from what 

 follows. 



Deduction of the law of diminution of vapour pressure 

 without thermo-dynamics. We will first give the simple 

 proof, due to Arrhenius, which does not 

 involve thermo-dynamics. Suppose os- 

 motic equilibrium to be established by 

 means of the semipermeable membrane 

 a b (Fig. 8), above which the solution rises 

 to the height AB. Now the equilibrium 

 implies that the solvent should not pass 

 by distillation into or out of the solution. 

 But the pressure, which at B equals the saturation pressure p 

 of the solvent, is less at A by the pressure exerted by the 



