65 



a free space equal in volume to thB pore space of the 

 soil. It was found that the rate of escape of carbon 

 dioxide from the soil by diffusion is equal to its 

 rate of free diffusion in air multiplied by the square • 

 of the porosity, the porosity being expressed as the 

 decimal fraction of space occupied by air; thus, a 

 porosity of 50 percent is expressed as .5. The method 

 of calculating the free diffusion of gases has been 

 explained above (pages £b ). Investigations of the 



penetration into the soil of pressure waves resulting 

 from changes of atmospheric pressure showed that these 

 were relatively unimportant in altering the composition 

 of the soil air, having, at most, an effect one-twentieth 

 as great as the effect of simple diffusion. Buckingham 

 gives a table^ of the rates of escape of carbon dioxide 

 and entry of oxygen, by diffusion, for soil porosities 

 of .2 to .7 (SO to 70 percent) and concentration grad- 

 ients of the diffusing gases between .1 percent per 

 inch and 1.0 percent per inch. All percentages are by 

 volume. The values, expressed as cubic feet per day 

 per square foot of soil surface vary from .0082 cubic 

 foot for a concentration gradient of .1 percent per 

 inch *t a porosity of ,2,^and .10 cubic foot for the 



1. On penetration of pressure-waves into the 

 soil see also Bornstein,- Physik, Zeits. 12: 771-776 (1911) 



2. loc . cit . . page 38. 



