OSMOTIC PRESSURE. 27 



concentration of the two solutions is great enough, the membrane is 

 ruptured by the pressure. 



In order to give the copper-ferrocyanide membrane a greater rigidity, 

 PFEFFER has suggested forming the precipitate on a porous, rigid wall. 1 

 For this purpose he makes use of a small, porous earthenware cell which, 

 after careful cleaning, is treated with copper sulphate and potassium 

 ferrocyanide so that the membrane is precipitated on the inner wall 

 of the cell. The membrane thus obtained is impervious to the cane 

 sugar. If the cell is filled with a cane-sugar solution and then placed 

 in pure water, no sugar leaves the cell, while water passes into the cell, 

 and this continues until the opposite pressure produced prevents the 

 further passage of water. If the cell is completely closed and in con- 

 nection with a manometer, then on the establishment of an equilibrium 

 the manometer indicates the force with which the inclosed solution 

 attracts water, or the osmotic pressure of the solution. For dilute cane- 

 sugar solutions the osmotic pressure is approximately proportional to 

 the concentration, which is shown by the following figures. 



Cone, (c) Osmotic Pressure (p) 



1 per cent 53 . 5 cm. Hg. 53 . 5 



2 " 101.6 " 50.8 

 4 " 208.2 " 52.1 

 6 " 307.5 " 51.3 



The osmotic pressure rises slowly with the temperature. 



Experiments with other semipermeable membranes have also been 

 carried out by DE VRIES, and these will be discussed on page 30. DE 

 VRIES' experiments have led to the following result : Solutions of analo- 

 gously constructed bodies having the same molecular concentration give the 

 same osmotic pressure. 



VAN'T HOFF first called attention to the analogy which exists between 

 the laws of osmotic pressure of a dissolved substance and of gases, 2 

 namely, that the osmotic pressure is proportional (or inversely propor- 

 tional to the volume of the solution) to the concentration, and corre- 

 sponds completely with BOYLE-MARIOTTE'S law on the relation between 

 the volume and pressure of gases. Also, that equimolecular solutions 

 have the same osmotic pressure, corresponds to AVOGADRO'S law, that 

 equal volumes of different gases under the same pressure contain the 

 same number of molecules. GAY-LUSSAC'S law that the pressure of 

 a gas changes in proportion to the absolute temperature cannot be 

 absolutely proven for the osmotic pressure because of the inaccuracy 

 of the methods. 



1 Osmotische Untersuchungen, Leipzig, 1877. 



2 Zeitschr. f. physik. Chem., 1, 481, 1887. 



