22 SCIENCE PROGRESS 



mental idea advanced by Tinker is that the solvent pressure in 



the solution is less than the solvent pressure in the pure solvent 



itself which lies on the other side of the membrane. To see 



how this might occur, let us think of equal volumes of two 



separate gases, A and B. If these are mixed, and the resulting 



volume is just double the original volume of either, it follows 



that the partial pressure of each constituent — say the substance 



A — in the resulting mixture is exactly half of its value in the 



original state. It is to be observed that this diminution in 



the pressure exerted by A is brought about, although the 



«effective space occupied by any single molecule is just the 



same before and after mixing, i.e. the space per molecule 



total number of molecules , . .. . . ' 



— z , , 1 , and in the case considered we 



total volume 



have doubled the number of gas molecules and doubled the 

 volume at the same time. To return to the case of the solu- 

 tion, the partial liquid pressure of the constituent, which we 

 call the solvent, is diminished by the presence of the solute, 

 for exactly the same reason that the partial pressure of the 

 gas A was diminished. 



If therefore the solvent and solution are separated by a 

 membrane, permeable to the solvent, impermeable to the 

 solute, the solvent pressure on the pure solvent side is greater 

 than the solvent pressure on the solution, and therefore some 

 solvent is forced into the solution. That is, the phenomenon 

 of osmosis occurs. Further, we can apply an external pressure 

 to the surface of the solution by means of a piston or an inert 

 gas, and so prevent osmosis taking place. This applied pressure 

 is numerically equal to the osmotic pressure of the solution, and, 

 in fact, the method is employed experimentally to determine 

 osmotic pressure. By compressing the solution in this way we 

 obviously diminish the free space in the solution, and, therefore, 

 cause the solvent pressure in the solution to rise, until it is 

 equal to the solvent pressure in the pure solvent itself, and no 

 further osmosis can take place, i.e. the solvent is now passing in 

 and out of the membrane at the same rate. 



Now we know that liquids are very incompressible. It 

 follows therefore that there must be very little free space in 

 liquids. If this were not the case, it would be quite inconceiv- 

 able that the application of an external pressure of a few 

 atmospheres by the piston could sensibly affect the free space, 



