258 PHYSIOLOGY OF ALIMENTATION. 



stance has distributed itself uniformly throughout the solu- 

 tion. We say that the soluble substance has diffused through 

 the solvent. This process of diffusion is analogous to the 

 diffusion of gases described in the preceding paragraph. 

 When a crystal of copper sulphate is covered with pure water, 

 the diffusion of the copper salt shows itself as a blue zone 

 about the crystal, which gradually spreads until all the water 

 is tinged uniformly blue and the crystal (provided it has not 

 been too large) has entirely disappeared. It is not necessary 

 to start with a crystal of the copper sulphate, a solution of 

 .this salt may be used instead and it be carefully covered 

 with distilled water. Diffusion occurs in the same way and 

 continues until the entire volume of water is tinged uni- 

 formly blue. What has been said holds for any solvent and 

 any soluble substance, be the latter a solid, a liquid, or a gas. 



What is the cause of this movement of particles of dis- 

 solved substance from places of higher concentration to places 

 of lower concentration, in other words, this diffusion? 



In the case of the odorous gas liberated in a room, we 

 attribute the spread of the odorous substance throughout the 

 room to the fact that its partial pressure is greater at the 

 point of liberation than anywhere else*in the room. In other 

 words, the gas pressure is highest where the odorous sub- 

 stance is liberated, and in consequence the odorous particles 

 of gas are driven through the room until the pressure is every- 

 where the same. Entirely analogous to the movement of 

 the particles of a gas through a vacuurn or another gas is the 

 movement of the particles of a dissolved substance through 

 a solvent, only, while we call the force which causes the move- 

 ments of a gas, gas pressure, we call the force which causes 

 the movement of a dissolved substance osmotic pressure. 

 Just as a gas exerts a certain (gas) pressure upon the walls 

 of its container, so a dissolved substance exerts a certain 

 (osmotic) pressure upon the walls of its container. This 

 pressure, indicative of the movement of the dissolved sub- 

 stance, we can render apparent by separating the region of 



