PHYSICOCHEMICAL ORGANIZATION OF THE PLANT 15 



ruptured. Hence, in measuring these pressures, Pfeffer pro- 

 ceeded in the following way. He took a porous porcelain 

 cylinder such as is used in batteries, filled it with a solution of 

 copper sulphate, and submerged it in a solution of potassium 

 ferrocyanide. A membrane was precipitated in the small pores 

 of the porcelain, the walls of these pores providing a sufiiciently 

 strong support for the membranes to sustain a pressure of several 

 atmospheres (Fig. 5). 



Pfeffer's osmometer shows a certain similarity to the plant 

 cell. In the latter also is found a readily permeable yet solid 

 support in the form of the cell wall and the adjoining layer of 

 semipermeable protoplasm. Therefore, the osmometer has fre- 

 quently been called an artificial cell. However, in comparison 

 with a real cell capable of being distended, Pfeffer's osmometer 

 has rather rigid walls, and naturally it is unable to alter in volume. 



In studying the influence of different factors upon the mag- 

 nitude of osmotic pressure by means of his osmometer, Pfeffer 

 estabhshed that it increases in direct proportion to the concen- 

 tration of the solution and to the absolute temperature. Thus, 

 it is governed by the laws of Boyle and Gay-Lussac, established 

 for gaseous pressures. 



In spite of the unity of the laws governing osmotic and gaseous 

 pressures, these two pressures show certain differences, the chief 

 of which is that gases always develop a pressure on the walls of 

 the container enclosing them, while the osmotic pressure of 

 solutions shows itself only in case they are placed in a container 

 with a semipermeable membrane separating the solution from 

 water. When placed in a glass or even in an osmometer not 

 immersed in water, the most concentrated solution of any sub- 

 stance does not display any osmotic pressure on the walls. 

 Osmotic pressure arises only when the solution and solvent are 

 separated by a semipermeable membrane. 



The osmotic pressure produced when an osmometer is immersed 

 in water is explained by the attraction between the molecules 

 of the solvent and those of the solute. Water enters the osmom- 

 eter under the influence of this affinity and distends it, creating 

 pressure on the walls. 



If the water penetrating into the artificial cell is forced to Hft 

 a mercury column, as is the case in Pfeffer's osmometer, the rise 

 ceases when the pressure of the mercury column balances the 



