PLANT LIFE. OSMOSIS. FERTILIZERS 



407 



The selective flow of certain components of a solution through 

 a membrane is known as osmosis. Osmosis and its consequence, 

 osmotic pressure, are phenomena which may be very clearly 

 illustrated in the laboratory by means of a solution of cane sugar 

 and an artificial membrane of precipitated cupric ferrocyanide 

 Cu 2 .Fe(CN) 6 . 



Osmotic Pressure. A suitable semi-permeable membrane 



may be obtained by soaking a clean porous pot in a solution of 

 potassium ferrocyanide K 4 .Fe(CN) 6 (p. 497), 

 rinsing in water, and then allowing to stand 

 in a solution of cupric sulphate. The diffusion 

 of the latter substance into the cell produces, 

 by double decomposition, a film of insoluble 

 copper ferrocyanide within its walls. This 

 film is freely permeable to water molecules, 

 but not to molecules of sugar dissolved in the 

 water. 



A simpler method is to use a diffusion shell 

 of specially treated parchment, of test-tube 

 form. This, however, is not entirely im- 

 permeable to sugar molecules. 



The porous pot or diffusion shell, filled 

 with sugar solution, is securely attached to 

 a long glass tube, and suspended in pure 

 water (Fig. 100). It is found that the level of 

 the liquid in the tube gradually rises until, if 

 the membrane remains intact and is truly im- 



^^^^^^^^^ permeable to sugar, a definite hydrostatic 

 FIG 100 pressure is established, the magnitude of this 



osmotic pressure depending only upon the 



temperature and upon the fraction of sugar molecules in the 



solution within the cell. 



