Conditions and Principles of Absorption 67 



of the 20 per cent sugar solution. If there were a less 

 concentrated sugar solution in the outer vessel, or any 

 solution of salts containing fewer solvent particles per 

 volume, the major flow of water would still be inward, and 

 the extent of this pressure would be in direct proportion to 

 the difference in number of solvent particles. 



Special diffusion shells (Fig. 18) are also prepared for 

 demonstration experiments, and these likewise give quick 

 results. In this connection it might be mentioned that 

 accurate measurement of osmotic pressures involves 

 special apparatus in which the natural membrane is re- 

 placed by an artificial precipitation membrane deposited 

 in the interstices of a porous cup, the pressure being indi- 

 cated by a mercuric manometer. The original of this 

 apparatus, described by Pfeffer in 1877, has been greatly 

 improved in recent years by Morse. 



42. An explanation of osmotic pressure. In the 

 thistle-tube demonstration experiment it was suggested 

 that the molecules of sugar or other solute tend to diffuse, 

 to distribute themselves equally, but the semipermeable 

 membrane retards and almost prevents this outward 

 diffusion. The inflow of water is conceived to be in direct 

 response to this force, analogous to a pressure, and the 

 water would continue to flow inward until equilibrium 

 between the pressures were established. This commonly 

 accepted view of osmosis regards the solute as obeying 

 the laws of gases. At a constant temperature, therefore, 

 the osmotic pressure varies with the density or concentra- 

 tion; that is, with the number of particles of the solute. 

 The molecular weight of cane-sugar (a nonelectrolyte) 

 in grams, 342, dissolved in water to 1000 cc. (called a gram- 



