40 



Introduction. 



55. Diffusion When water has evaporated into the air; 

 when a salt has dissolved in water, there is a tendency for 

 these separated molecules to travel in any and all direc- 

 tions until the whole body of the liquid in which the solu- 

 tion is taking place contains the same number of the dis- 

 solved molocules per cubic inch. A lump of sugar placed 

 in the bottom of a cup of tea dissolves in time and be- 

 comes scattered uniformly through the whole mass, making 

 all parts equally sweet. This scattering of molecules is 

 called diffusion and the rate varies with the temperature 

 and the individual velocities of the molecules dissolved. 



B A 



FIG. 5. Illustrating the difference in Ihe rate of diffusion in soil and in liquids 



The rate of diffusion of salts in a vessel of water 

 is much more rapid at the same temperature than it could 

 be if the water were filled with sand. This will be under- 

 stood from a study of Fig. 5, where A is supposed to be 

 a place from which salts are diffusing through the water 

 surrounding a set of soil grains, while B is a correspond- 

 ing point from which diffusion is taking place in direc- 

 tions indicated by the arrows of that figure. Where the 

 diffusion must take place through the films surrounding 

 the soil moisture not only is there less water to travel in 

 but the course of the molecules must be many times ar- 

 rested by the soil grains themselves'. 



56. Gaseous Pressure, The pressure which is exerted 



