PRELIMINARY PHYSICO-CHEMICAL DATA 



421 



or, in other words, to the number of molecules of the dissolved substance 

 in a given volume of the solution. If 'in this sentence we substitute 

 ' gaseous pressure ' for ' osmotic 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 density. 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 contains in a given volume the 

 same number of molecules 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 pressure 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 pres- 

 sure 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 substance 

 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 sub- 

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

 plays an important role. 



Fig. 171. Beckmann^ 

 Apparatus. For df- 

 scription, see p. 521. 



