THE PROBLEMS OF SOLUTION 97 



have already been shown to play important parts 

 in the processes of living structures. 



Attention was first called to the interest and 

 importance of osmotic pressure from a physical 

 standpoint by the distinguished Dutch chemist, 

 the late professor Van't Hoff. In 1885 Van't 

 Hoff pointed out that Pfeffer's numbers showed : 

 (i) that the osmotic pressure was inversely 

 proportional to the volume in which a given 

 mass of sugar was confined ; and (2) that the 

 absolute value of the pressure in the case of the 

 solution of sugar was the same as that which 

 would be exerted by an equal number of molecules 

 of a gas when placed in a vessel having a volume 

 equal to that of the solution. For instance, a 

 quantity of gas of the same molecular concentra- 

 tion as a I per cent, solution of sugar would, at 

 6°.S C, exert a pressure equivalent to that of 

 508 millimetres of mercury, a number identical, 

 within the limits of experimental error, with 

 Pfeffer's observed value for the osmotic pressure 

 quoted above. The first result is equivalent to 

 the extension to dilute solutions of Boyle's law 

 for gases, a law which states the experimental 

 result that the volume of a gas is inversely 

 proportional to its pressure. The second result 

 shows that, in a dilute solution, the pressure 

 depends only on the number of molecules present, 

 and not on their nature — a statement which, 

 applied to gases, is known as Avogadro's law. 



But Van't Hoff did not alone call attention to 

 the experimental basis of the new subject. He 

 also placed the theory of it on a sound footing. 

 The amount of a gas which dissolves in a given 

 quantity of water is proportional to the pressure, 



H 



