120 life's beginning on the earth 



permeable membrane in a cup of porous clay (Fig. 35). 

 This cup is not semi-permeable as it stands. A solution of 

 sugar passes through it, since the pores in the cup are too 

 big to hold back the sugar. But we can plug these pores 

 by forming in them a semi-permeable membrane, through 

 precipitation. To this end we allow two different solutions 

 which interact with each other to penetrate the cup, one 

 from the inside, the other from the outside. The detailed 

 technique of this procedure is described in the legend of 

 Figure 35. We mount the cup as shown in that figure, fill 

 it with a sugar solution, and immerse it in water. A mano- 

 meter attached to it measures the water-attracting force of 

 the solution — in other words, the osmotic pressure. We 

 find that the osmotic pressure is very great and can well 

 understand how it cracks iron pipes. The osmotic pressure 

 of a 4 per cent cane sugar solution is nearly equal to that of 

 a water column standing in a tube 100 feet high. For 

 higher sugar concentrations it is correspondingly higher. 



Owing to the magnitude of the osmotic pressure it is very 

 difficult to carry out such measurements. If the set-up has 

 the slightest leak, the sugar solution will pass through it 

 instead of pushing up the mercury column. If the whole 

 apparatus is tight, the osmotic pressure of the sugar solu- 

 tion equals that registered by the mercury column. If, on 

 the other hand, the pressure of the manometer is increased 

 by the addition of more mercury to the mercury column, 

 water will be driven out from the sugar solution against the 

 osmotic pressure. 



(). HISTORICAL REMARKS! THE LIFE OF MOR1TZ TRAUBB AND 

 THE FURTHER DEVELOPMENT OF THE WORK ON 



OSMOSIS 



We have made a great step forward by observing the ex- 

 panding forces of osmotic pressure and its influence upon 

 vital growth, as well as upon water exchange between sap 



