ABSORPTION OF MATERIAL IN GENERAL 



lOS 



Walden* obtained semi-permeable precipitation membranes in the following 

 manner. The upper end of a glass tube 5 cm. long and i cm. wide is closed by 

 the finger and the lower end is dipped into a solution containing 50 g. of water, 

 10 g. of gelatine, and i g. of ammonium chromate. When the tube is lifted 

 from the solution, the lower end remains closed by a thin membrane, which is 

 rendered insoluble in water by the action of light. A precipitation membrane 

 of copper ferrocyanide is then deposited in the hardened gelatine film, according 

 to the method employed by Pfeffer. 



Experiments with precipitation membranes have given the general results 

 summarized below. Other conditions remaining the same : — 



1. Osmotic pressure is proportional to the concentration of the solution. 

 Thus I-, 2- and 4-per cent, solutions of cane sugar developed osmotic pressures 

 equivalent to 53.2 cm., 101.6 cm. and 208.2 cm. of a mercury column, respectively. 



2. Osmotic pressure increases with rise in temperature. A i-per cent, 

 saccharose solution at temperatures 6.8°, 13.7" and 22°C. gave osmotic pressures 

 of 50.5 cm., 52.5 cm. and 56.7 cm. of a mercury column, respectively. 



3. Osmotic pressure depends upon the nature of the dissolved substance. 

 Six-per cent, solutions of (i) gum arabic, (2) gelatine, (3) saccharose and (4) 

 potassium nitrate gave osmotic pressures of (i) 25.9 cm., (2) 23.8 cm., (3) 287.7 

 cm. and (4) 700 cm. of a mercury column, respectively; Colloids (such as 

 gum arabic and gelatine) thus produce much lower osmotic pressures than do 

 crystalloids.' 



4. Osmotic pressure depends upon the nature of the membrane. Six-per 

 cent, solutions of the four substances named above gave the following osmotic 

 pressures (in centimeters of a mercury column) with membranes of copper 

 ferrocyanide, parchment paper and animal bladder, respeqtively.' 



1 Walden, Paul, Ueber Diffusionserscheinungen an Niederschlagsmembranen. Zeitsch. physik* 

 Chem. 10: 699-732. 1892. 



' As is brought out a little farther on, the concentration of the solutions should not be 

 stated in terms of percentage for such comparisons; they should be given in terms of a volume- 

 molecular, or still better, of a weight-molecular solution. The former gives the number of 

 gram-molecules of solute dissolved in a liter of solution (at a stated temperature) and the latter 



1000 

 gives the number of gram-molecules of solute dissolved in rooo g. ( — 3- = 55.56 g.-mol.) of 



water taken as H2O. For a valuable discussion of the relation of volume-molecular and 

 weight-molecular solutions to physiological considerations, see: Renner, O., Ueber die Berech- 

 nung des osmotischen Druckes. Biol. Centralbl. 32 : 486-504. 1912. The general principle 

 holds, as stated in the text, however. See also note n, below (p. 114.) — Ed. 



