20 GENERAL PROPERTIES OF LIVING TISSUES 



Ten years after these observations were pub- 

 lished by Pfeffer, Van't Hoff pointed out the 

 analogy between osmotic pressure and gas pres- 

 sure. It has just been shown that, if the tem- 

 perature be constant, the osmotic pressure is 

 proportional to the amount of dissolved substance 

 in a given volume ; thus the osmotic pressure 

 of a solution, like that of a gas, varies inversely 

 with the pressure. Moreover, the osmotic pres- 

 sure, like gas pressure, increases with the tempera- 

 ture. Pfeffer observed that the osmotic pressure 

 of a cane sugar solution at 142 C. was 51 cm. 

 Hg. Assuming that the osmotic pressure, like 

 gas pressure, is proportional to the absolute 

 temperature, Pfeffer then determined by calcula- 

 tion that the osmotic pressure of this same 

 solution at 32.0 C. should be 54.^ cm. Actual 

 observation gave 54.4 cm. 



In short, the osmotic pressure of a dissolved 

 substance is numerically equal to the pressure 

 which the substance would exert were it present 

 as a gas. 



Plasmolysis. Isotony. The cells of Trades- 

 cantia discolor possess a strong outer envelope 

 .permeable by both water and salts, and a thin 

 inner envelope permeable by water, but not by 

 salts. The cell is filled with an aqueous solution 

 of glucose, salts of malic acid, etc. The osmotic 



