SALT AND WATER IN LIFE AND GROWTH 



121 



direction of ' hyelrostcxttc 

 pressure , 



direct, on of osmotic 



pressure 

 —7 



semipermeable 



membrane 



e">be</e'ea' /n 



porous c/ay. 



Fig. 35. Diagram of Set-up for Measuring Osmotic Pressure 



With an apparatus of this type we can demonstrate the extraordinary 

 force of the osmotic pressure. A porous cup with clogged pores is the 

 essential part of this set-up. The pores are clogged with an insoluble 

 substance, copper ferrocyanide, which is generated in the pores by pouring 

 a copper sulfate solution into a moistened porous cup and immersing the 

 cup, with the solution in it, in a yellow prussiate solution. The two solu- 

 tions diffuse into the clay from opposite sides and produce a semi-permeable 

 membrane in the inner layers of the clay where they meet. 



The top of the cup is then fitted to a glas tube attached to a manometer. 

 The entire inside of the cup and the connecting tube is filled with sugar 

 solution. The cup is immersed in a larger beaker containing pure water. 

 Water will now be drawn into the cup and the manometer will rise as shown 

 in the figure. 



