622 THE MECHANISM OF ABSORPTION 



whether the cell may be considered normal until that point is 

 reached is unknown. 



In considering intestinal absorption, therefore, it is obviously 

 necessary to remember the elementary facts connected with 

 the osmotic laws and their general relation to bioplasm ; and 

 a short account of them will help materially the discussion of 

 the experiments to be considered. 



Osmosis. By osmotic pressure is meant the pressure which 

 a diffusible substance in solution exerts on a semi-permeable 

 membrane, i.e. one which is permeable to the solvent, but 

 absolutely impermeable to the substance in solution. One 

 physical method of measuring osmotic pressure is to separate 

 in a copper ferrocyanide membrane osmometer a solution from 

 its solvent, and to measure the pressure exerted directly in 

 mm. Hg. This method, however, gives only approximate 

 values, for, the membrane is not strictly impermeable to most 

 substances, although nearly so to cane-sugar. The law of 

 Avogadro-van 't Hoff states that at the same osmotic pressure 

 and the same temperature equal volumes of all dilute solutions 

 contain the same number of molecules ; that is to say, at the 

 same temperature equimolecular solutions of different sub- 

 stances in the same solvent must be isotonic with each other. 

 This law opens up another and more accurate physical method 

 of measuring osmotic pressure, namely, by the determination of 

 the depression of the freezing-point. For, the freezing-point 

 of a solution is lower than that of the pure solvent, and the 

 depression of the freezing-point, A, is found to be proportional 

 to the concentration of the dissolved substance. It follows, 

 therefore, that if a gram molecule of any substance is dissolved 

 in 100 grm. of the same solvent it ought to have the same 

 A. And this is found to be the case. When the solvent is 

 water A = 18*6 C. Solutions of various substances dissolved 

 in water having the same A will consequently have the same 

 osmotic pressure ; and this osmotic pressure will correspond 

 to the pressure which the solution would exert on a true semi- 

 permeable membrane. But it was soon found that while equi- 

 molecular solutions of some substances, such as sugars, urea, &c., 

 gave their theoretical A, this was not so in the case of solutions 

 of the strong inorganic acids, bases, and salts, all of which gave 

 larger actual values than they theoretically should. Thus, a 



