69 



of the soil. During the following dry period plants use 

 this capillary water or it is lost "by surface evaporation, 

 the capillary water system is thinned and air is drawn 

 into the soil. The quantitative importance of this form 

 of soil ventilation depends upon so many factors and such 

 complex ones that it is impossible to estimate it accurate- 

 ly. It is certain, however, that it is anong the most 

 important of the factors acting and it is quite probable 

 that it is the most important one. 



The circulation of gases, as a result either 

 of water changes or of differences in temperature, is 

 a circulation by virtue of differences of gas pressure. 

 Physically, tiiis is the case of movement of gases through 

 a porous medium under a small pressure or "head", and 

 it is important to see how largely this depends on the 

 size of the passages in the porous medium. It is usually 

 assumed that the flow of gas through soils is represented 

 by the formula of Poiseuille 1 for the flow of fluids 

 through capillary tubes. This formula is: 



Q - K Ml 



1 



in which "Q" is the rate of movement of the fluid, "h" is 

 the "head" or driving pressure, "d" is the diameter of the 

 passage, "1" is its length and K is a constant which depends 



1. Ann. Ohim. Ihys. (3) 7: 50 (1843). 



