238 THE REALITIES OF MODERN SCIENCE 



bit less l than the value which pipz may have cor- 

 responding to each position. Then work will be done 

 by the system composed of the two solutions. Of 

 course, this work is at the expense of the molecular 

 energy of the system, and the solutions will be cooled. 



It is possible to obtain a semipermeable membrane 

 which allows water molecules to pass freely but re- 

 tains the solute. Of this character are the cell walls 

 of plant and animal organisms ; in life the phenomena of 

 osmosis are of great importance. Organic membranes 

 are not, however, suitable for a laboratory experiment 

 on osmotic pressures, but a convenient one may be ob- 

 tained by the precipitate of copper ferrocyanide which 

 results from the reaction of copper sulphate and potas- 

 sium ferrocyanide. Since this will not stand large 

 forces it is usual to cause the precipitation to take place 

 in the pores of an unglazed earthenware vessel or cell. 

 In this way a number of small membranes are obtained, 

 the effect of which is the same as that of a single large 

 surface. 



The cell is partially filled with the solution of which 

 we wish to measure the osmotic pressure and immersed 

 in a vessel of pure solvent. In other words, we make 

 the difference in concentration between the two solu- 

 tions equal to that of the solution actually under test 

 by making one of them pure solvent. Now we have 

 seen that the two solutions tend to come to the same 

 pressure for their dissolved molecules. The solvent, 

 e.g. water, enters the cell, making the solution weaker 



1 This "bit less" is of course negligible, for it need only be enough 

 to give a negligible acceleration to a weightless, that is, inertialess, 

 piston. 



