224 OSMOTIC PRESSURE 



any experiment will, when it has become constant, be found to vary 

 directly with the concentration and to increase with a rise of tempera- 

 ture. This osmotic pressure, as it is termed, has been shown to be 

 analogous to gaseous pressure and to be amenable to the laws of 

 Boyle and Charles. 



It is to be noted that osmotic pressure is only developed with the 

 aid of a more or less semi-permeable membrane and that the solvent 

 moves to the place where the osmotic pressure is greatest. A clearer 

 idea of this latter, apparently paradoxical, effect may be obtained by 

 the aid of the following analogy : 



Imagine a confined space to contain a large number of rapidly 

 moving solid bodies, e.gr., small shot, oscillating about and hitting and 

 rebounding from each other and from the walls. The impact of the 

 shot upon the walls would exert a certain pressure per unit area. Into 

 the same space let a number of oscillating plastic clay balls be intro- 

 duced and suppose that where a shot strikes a clay ball, the two adhere 

 and move about together. This would go on, until each clay ball 

 became attached to a certain number of pellets and complexes, con- 

 sisting of a central mass of clay, adhering to a certain number of shot, 

 and each complex moving about, comparatively slowly, would be 

 formed. Let the pressure on the walls be made equal to what it was 

 before. Next imagine a similar adjacent space to contain only small 

 shot in a sufficiently concentrated condition to give the same pressure, 

 per unit area, on the walls as in the first space. 



It is obvious that the concentration of the freely moving small shot 

 will be much greater than in the first chamber, for the impact of a 

 clay ball with its captive pellets will be equal to that of many separ- 

 ate pellets. But an impervious wall separating the two compart- 

 ments would not be subjected to any excess of pressure on either side. 

 Now let this dividing wall be replaced by a net with meshes large 

 enough to readily permit the passage of the free shot, but small enough 

 to entirely stop that of the clay complexes. It would now be found 

 that the crowdedness of the free shot, at first much greater in the 

 second compartment, would tend to equalise itself throughout the 

 whole space, but the clay complexes would still be confined to the 

 first compartment and the netting would, after a time, receive an equal 

 amount of bombardment from the shot on each side, but, in addition, 

 would have, on one side only, the heavier impact of the clay complexes. 

 The same would also apply to the walls of the first compartment. 

 Here, then, would be the production of a greater pressure on one side 

 of the net than on the other, although to start with, the pressures on 

 each side were equal. 



The process would go on until the pressure set up was sufficient 

 to expel, through the net, as many free pellets per unit time from the 

 side where the crowdedness of such free pellets was smaller, as had 

 entered, without the aid of the extra pressure, from the side where the 

 concentration of free pellets was greater. Equilibrium would then be 

 attained and no further change of pressure would be noted, except for 

 the gradual tendency to uniform distribution of pressure consequent 

 upon any imperfection of the net. 



