IMPORTANCE OF WATER 



111 



providing a sufficiently strong support for the membranes to sus- 

 tain a pressure of several atmospheres (Fig. 44). 



Pfeffer's osmometer shows a certain similarity to the plant 

 cell. In the latter, we also find a readily permeable, yet solid 

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

 permeable protoplasm. Therefore, this osmometer has fre- 

 quently been called an artificial cell. In comparison with a real 



Fig. 43. — Osmometer {after 

 Dutrochet) . 



Fig. 44. — Pfeffer's osmometer 

 (after Molisch). 



cell capable of being distended, Pfeffer's osmometer has rather 

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



Pfeffer established that the osmotic pressure increases in direct 

 proportion to the concentration of the solution and to the absolute 

 temperature. It is governed by the laws of Boyle and Gay-Lussac, 

 established for gaseous pressures. Even the absolute magnitude of 

 gaseous and osmotic pressures, at a concentration of one gram 

 molecule of substance per liter, has proved to be the same, namely, 

 22.4 atmospheres. In spite of the unity of the laws governing 



