EXPERIMENT STATION BULLETINS. . 



541 



FIG. 11. CURVE SHOWING EFFECT OF TEMPERATURE ON THE RATE OF FLOW 



OF AIR IN SOILS. 



tion of air. As the latter is sucked through the column of soil, some re- 

 arrangement of particles undoubtedly occurs. This change in structure 

 sometimes causes either an increase or decrease in the flow of air very 

 suddenly. 



The foregoing transpiration data may be expressed also as transpira- 

 tion constant for any soil and for any particular temperature. By a 

 transpiration constant is meant the number of cubic centimeters of air 

 which would in one second, flow through 1 cm- of cross section of a layer 

 of soil 1 cm. thick, if the pressure were 1 mm. of mercury greater on one 

 side of the layer than on the other. The following example will make 

 clear how this transpiration constant can be computed. Suppose that 

 2650 cc. of air are drawn by the aspirator through a layer of soil 326.8 

 cm.- in area and 14.7 cm. thick in 105 seconds by a suction of 2 mm. of 

 mercury. The total flow per second is 2650 -:- 105 or 25.2 cc. per second. 

 The flow through each square centimeter is 25.2 -f- 326.8 or 0.077 cc. per 

 second. The pressure gradient is 2 mm. in 14.7 cm. or 0.13 mm. p^r 

 centimeter. Therefore the flow per second through each square centi- 



