190 SURFACES AND MEMBRANES 



where x was the change of volume during time t, c the original volume, 

 and k the velocity constant. It was also found that the speed of osmosis 

 was greatly increased as the temperature increased. The speed of pene- 

 tration was found to be much greater than if simple diffusion through a 

 membrane took place. 



Osmotic Pressure 



The osmotic pressure of a solution can be defined as the maximum 

 hydrostatic pressure (P = hdg) produced when a solution and solvent 

 are separated by a perfect semi-permeable membrane. It may also be 

 defined as the equivalent of the external pressure which must be applied 

 to a solution in order to prevent the passage of the solvent into it through 

 a perfect semi-permeable membrane. 



Measurements of Osmotic Pressure 



In the search for an ideal semi-permeable partition, Traube, as early 

 as 1867, discovered that a flexible film of cupric ferrocyanide has semi- 

 permeable properties. He found that, when a dilute solution of copper 

 sulphate is mixed with a dilute solution of potassium ferrocyanide, a 

 brown precipitate of cupric ferrocyanide is formed. This precipitate, if 

 used as a partition, will permit the passage of water, but will prevent the 

 passage of both copper sulphate and potassium ferrocyanide, as well as 

 of many other crystalloids. 



A very realistic model of a growing cell may be constructed by intro- 

 ducing a moderately concentrated solution of potassium ferrocyanide, 

 in the form of a drop, below the surface of a dilute copper sulphate solu- 

 tion. A brown precipitated film of cupric ferrocyanide is formed around 

 the drop, which subsequently ruptures at some point as the result of the 

 increasing osmotic pressure caused by osmotic flow of water through the 

 membrane. The rupture, however, is rapidly mended by a new patch 

 of precipitated cupric ferrocyanide. The osmotic pressure again enlarges 

 the drop until by a succession of ruptures and subsequent repairs the drop 

 has grown until its internal and external pressures have attained 

 equilibrium. 



Since this type of unsupported membrane is rather fragile, the botanist 

 Pfeffer (1877) supported the gelatinous cupric ferrocyanide film by the 

 framework of a porous earthenware pot. He used an unglazed porcelain 

 cylindrical vessel from which he had removed the air occluded in the 

 pores and then allowed them to fill with water. By placing this water- 

 saturated pot in a 3 per cent solution of copper sulphate and pouring into 

 the interior a 3 per cent solution of potassium ferrocyanide, he succeeded 



