Processes in Movement of Materials S;^ 



the room. Even if there were no air currents, the ether would 

 evaporate ; that is, particles of ether would rise from the surface 

 of the liquid, pass out of the dish, and move through the room in 

 every direction. This is an example of the diffusion of a vapor. 

 The vapor is concentrated in the dish and the particles move 

 outward into the room where there is none ; that is, the particles 

 move from the place where their concentration is greater to 

 where it is less. After the ether has evaporated, the vapor tends 

 to become evenly distributed throughout the room. SoUds like 

 camphor and naphthalene might be used in place of the ether. 



Similarly, if a few crystals of copper sulfate are placed in the 

 bottom of a vessel of water, particles of the copper sulfate diffuse 

 through the water. The crystals are blue in color, and as dif- 

 fusion proceeds, the water in the vessel gradually becomes blue. 

 The direction of the movement is again from the place where the 

 diffusing substance is most concentrated to where it is less con- 

 centrated. The particles pass from the place where they are 

 most abundant to where there are fewer of them, and this process 

 is continued until they are evenly distributed throughout the 

 water. 



Diffusion of a gas or vapor is very rapid. Diffusion of a dis-r 

 solved substance in a hquid is slow, but the distances that sub- 

 stances must travel in plant cells are very small. Oxygen and 

 carbon dioxide, when once dissolved in the water of cells, move 

 about partly by diffusion. The soluble foods in plants move 

 from one part to another by diffusion. Soil salts enter the root 

 by diffusion and are not carried into it by water. Diffusion may 

 occur under special conditions, and it is then conveniently spoken 

 of as imbibition and osmosis. 



Imbibition. The process of imbibition may be illustrated by 

 placing a sheet of gelatin in water. Dry gelatin is a hard, brittle, 

 partly transparent solid. After it has been in water for a few 

 minutes, it will be found to have increased in weight and in 

 length, breadth, and thickness. The gelatin, instead of being 



