OSMOSIS 189 



is based on the kinetic theory of molecular activity. This 

 activity increases with rise in temperature. Molecules at the 

 surface of hot water are moving about so actively that some 

 escape into the air and form vapor. The hotter the water 

 the more active are the molecules, and the more escape from the 

 surface. The escaping tendency of molecules is a measure of the 

 ease with which they leave the surface. It depends upon tem- 

 perature and pressure. In order to escape from a liquid, mole- 

 cules must overcome the surface tension, and energy is required 

 to do this. Let us imagine two dishes, side by side, one filled 

 with water and the other with a sugar solution. We may assume 

 that there are at the surface of the water in the first dish 12 mole- 

 cules over a given area. There cannot be so many water mole- 

 cules over the same area of the sugar solution in the second dish, 

 because sugar molecules occupy part of the space, just as in our 

 previous example (Fig. 103). Here, the semipermeable ''mem- 

 brane" is the air between the two. It permits water molecules 

 to enter and pass through it (evaporate) but not sugar molecules. 

 As a result, there will be a greater vapor pressure above the 

 surface of the pure water than over the sugar solution; in other 

 words, a solution has a lower vapor pressure than its solvent; 

 and, obviously, the greater the concentration of the solution 

 the lower the vapor pressure. As the vapor pressure of a solvent 

 (water) is greater than that of its solution (water and sugar), 

 the solvent moves from a region where its escaping tendency 

 (vapor pressure) is high to one where its escaping tendency is low. 

 In time, the level of the solution in the one dish will rise, and the 

 level of the pure water will fall. If pressure is applied to the 

 surface of either, the corresponding vapor pressure will be 

 increased. Osmotic pressure may then be redefined as the 

 increase in pressure on a solution necessary to bring about 

 equilibrium. 



Methods of Determining Osmotic Pressure. — The direct 

 method of measuring osmotic pressure is difficult if it is to be 

 accurate. It is the one that was used by Pfeffer and consists in 

 determining the weight of that column of mercury which is just 

 sufficient to prevent osmosis. The osmotic pressure is then 

 expressed in centimeters of mercury, or atmospheres (e.g., a 

 O.l-M sugar solution has an osmotic pressure of 2^^ atmospheres). 

 A manometer (pressure gauge) may be connected directly to the 



