AIR IN SOILS 



67 



the air within the soil. They come together at the sur- 

 face of the ground, and this exchange is brought about by 

 diffusion, which is dependent upon the pore space within 

 the soil. 



Diffusion is the mingling of gases into each other. It 

 may be measured by the passage of a gas through a porous 

 partition. It has been demonstrated that the rate of 



Porous Cup 



- Hydrogen. 



FIG. 22. Diffusion of hydrogen through a porous cup. 



diffusion of a gas is inversely proportional to the square 

 root of the density of a gas. The rapidity of diffusion 

 may be shown by the apparatus in Fig. 22. Hydrogen 

 surrounding the unglazed porcelain cup and air within 

 it each diffuse through the pores of the cup, but the air, 

 being nearly sixteen times as dense as hydrogen, will dif- 

 fuse one-fourth as rapidly. The hydrogen will penetrate 

 to the interior of the cup more rapidly than the air can 

 diffuse outward, consequently there will be increased pres- 



