PROPERTIES CONFERRED BY CONSTITUENTS 



257 



for osmotic-pressure measurements is the membranous precipitate of 

 Copper Ferrocyanide which is formed when a solution of copper sulphate 

 comes into contact with a solution of potassium ferrocyanide. 



If the continuous entry of water into the right-hand chamber were 

 permitted and the level of fluid did not rise so as to create a pressure, 

 water would pass indefinitely from left to right until the sugar in 

 the right-hand compartment was infinitely diluted. In this way no 

 measurement of the attraction of the solution for water would be 

 possible, since, in theory, if no frictional forces or pressures interfered 

 with the free motion of water, every solution, concentrated or dilute, 

 would attract an infinite volume of water. We may, however, measure 

 the degree of attraction for water which is exerted by the dissolved 

 substance by determining the pressure or temperature necessary to 



100 H,0 



90 H,0 



10 SUGAR 



B 



FIG. 9 



increase the force or frequency of the impacts on the right-hand side 

 of the partition, until the greater speed of transit from right to left 

 compensates for the greater volume of transit from left to right. If 

 pressure be applied to the contents of the right-hand compartment, 

 the force of the impacts of the molecules upon the partition is increased 

 so that although only ninety water molecules collide with the right- 

 hand side of the partition for every hundred which collide with the 

 left-hand side, yet those colliding on the right do so more forcibly, 

 and thus a greater proportion succeed in penetrating the membrane, 

 until, when the pressure applied to the solution attains a certain 

 magnitude the greater proportion of collisions leading to penetration 

 of the membrane exactly balances the excess of the total number of 

 collisions on the side which is bathed by pure water, 

 17 



