ABSORPTION 



399 



motic pressure/ and ' gas ' for ' solution/ we have a statement of Boyle's 

 law, which asserts that the pressure of a gas is proportional to its den- 

 sity. Indeed, it has been shown that the osmotic pressure of the 

 dissolved substance is the same as the pressure that would be exerted by 

 a gas, say hydrogen, if all the water were removed, and a molecule of 

 hydrogen substituted for each molecule of the substance, or as would be 

 exerted by the substance itself if, after removal of the solvent, it could 

 be left as a gas filling the same volume. And .the osmotic 

 pressure of a solution of one substance is the same as 

 that of a solution of any other substance which con- 

 tains in a given volume the same number of mole- 

 cules of the dissolved substance. In other words, the 

 osmotic pressure is not dependent on the nature, but 

 on the molecular concentration, of the substance. The 

 analogy of the laws of osmotic to those of gaseous pres- 

 sure becomes still more obvious when it is added that 

 the osmotic pressure of a substance with any given 

 molecular concentration is proportional to the absolute 

 temperature ; and that when a solution contains more 

 than one dissolved substance the total osmotic pressure 

 is the sum of the partial osmotic pressures 

 which each substance would exert if it were 

 present alone in the same volume of the solution. 



The osmotic pressure of a solution may reach 

 an enormous amount. Thus, a i per cent, solu- 

 tion of cane-sugar has a pressure at o C. of 

 493 mm. of mercury. A 10 per cent, solution 

 of cane-sugar would have an osmotic pressure of 

 more than six atmospheres, and a 17 per cent, 

 solution of ammonia a pressure of no less than 

 224 atmospheres. The manner in which the 

 phenomenon known as osmotic pressure is de- 

 veloped is not definitely known. One theory 

 attributes it to the attraction between the 

 dissolved molecules and the molecules of the 

 solvent on the other side of the membrane. 

 The most commonly accepted view is that the 

 osmotic pressure is due to the kinetic energy 

 of the moving molecules. Where the mole- 

 cules are hindered from passing a bounding 

 membrane, the pressure exerted by their im- 

 pacts on the boundary is greater than where 

 the membrane is easily permeable, because in 

 the latter case many of the molecules pass 

 through, carrying with them their kinetic 

 energy. The pressure must be still less when 

 a dissolved substance diffuses freely into 

 water ; but however small it may become, it is in the same force which 

 gives rise to the osmotic pressure of the molecules of the dissolved sub- 

 stance that the cause of diffusion must be sought. Recently interest 

 in the nature of the membrane itself as an important factor in osmosis 

 has been revived (Kahlenberg, Armstrong, etc.). There are many facts 

 which indicate that in physiological processes the affinity of the dissolved 

 substances for, or their solubility in, the cell envelopes or the cytoplasm 

 plays an important role. 



It is as yet impossible to directly measure the osmotic pressure with 

 accuracy by means of a semi-permeable membrane like ferrocyanide of 



FIG. 153. BECKMANN'S 

 APPARATUS. 



For description, see 

 p. 492. 



