CHAPTER V 



DIFFUSION AND OSMOSIS 



Diffusion and Osmosis. — If we place a lump of sugar in the 

 bottom of a glass of water, it will dissolve, and spread by slow- 

 degrees equally throughout the whole volume of the liquid. 

 If we pour a concentrated solution of sulphate of copper into 

 the bottom of a glass vessel, and carefully pour over it a layer 

 of clear water, the liquids, at first sharply separated by their 

 difference of density, will gradually mix. so as to form a 

 solution having exactly the same composition in all parts 

 of the jar. The process whereby the sugar and the copper 

 .sulphate spread uniformly through the whole mass of the 

 liquid in opposition to gravity is called Diffusion. This 

 diffusion of the solute is a phenomenon exactly analogous to 

 the expansion of a gas. It is the expression of osmotic 

 pressure, or rather of the difference of the osmotic pressure of 

 the solute in different parts of the vessel. The molecules of the 

 solute move from a place where the osmotic pressure is greater 

 towards a position where the osmotic pressure is less. The 

 water molecules on the other hand pass from positions where 

 Hie osmotic pressure of the solute is less towards positions 

 where it is greater. As a consequence of this double circula- 

 tion the osmotic pressure tends to become equalized in all 

 parts of the vessel. 



Diffusion appears to be the fundamental physical pheno- 

 menon of life. It is going on continually in the tissues of all 

 living beings, and a study of the laws of diffusion and osmosis 

 is therefore absolutely necessary for a just conception of vital 

 phenomena. 



Coefficient of Diffusion. — The coefficient of diffusion has 



