Chap. I] EXPLANATIONS AND DEFINITIONS 11 



if two solutions of different concentrations are used with the 

 separating membrane, the volume on the side of the solution of 

 greater concentration will be increased. 



For example, if a carrot with the inside hollowed out is filled 

 with a sugar solution, a long glass tube secured in the opening 

 and the carrot then placed in a vessel of water or a solution of less 

 concentration, the volume of the solution inside the carrot will 

 increase as indicated by the rise in the glass tube. If conditions 

 are reversed and the less concentrated solution is placed in 

 the membrane, the increase in volume will be in the outer vessel. 

 This phenomenon of osmosis, therefore, accounts for the swelling 

 of dried fruits in water and the rising of water into the stems and 

 leaves of plants. (See page 154.) 



No completely satisfactory explanation is given for this 

 process of osmosis. A most acceptable one, however, is found 

 in the kinetic theory. According to this theory, the molecules 

 of liquid and solute are both bombarding the membrane on each 

 side. The molecules of the liquid can pass through, but the mole- 

 cules of the solute cannot, the membrane, therefore, being called 

 semi-permeable. The number of liquid molecules bombarding the 

 membrane, per unit area, on the side of the clear solvent, or less 

 concentrated solution, will be greater than the number of liquid 

 molecules bombarding the membrane on the side of the more con- 

 centrated solution, hence, more liquid molecules will pass from 

 the less concentrated to the more concentrated solution and the 

 volume of the latter will be thereby increased. This increase will 

 continue until the pressure in both directions is the same. 



Osmotic pressure. — This is a term that is used indefinitely, and 

 to express a fact rather than an understood force. A possible 

 explanation is as follows : the molecules in their vibratory motion 

 in the process of diffusion exert a certain force which is evident 

 as a pressure when striking against anything in their path. This 

 force or pressure is spoken of as osmotic pressure. When there- 

 fore a solution is held confined the vibratory force or osmotic 

 pressure of the molecules within the mass produces an evident 

 pressure against the sides of the containing vessel. It is to this 

 pressure that the process of osmosis is due, for if the side of a 

 vessel is a permeable membrane, the molecules in pressing against 

 it will pass through. 



