540 



STATE BOARD OF AGRICULTURE. 



the air would be drawn only through the mass of the soil. The rate of 

 passage of air was measured in quartz sand, sandy loam, silt loam, and 

 clay of the Miami series, and Clyde silt loam and peat, at the tempera- 

 tures of 0°, 10°, 20°, 30°, 40°, and 50° C. The soil column was kept at 

 the same temperature from two to three hours before the transpiration 

 measurements were taken. 



The aspirator apparatus employed was similar to that of King's, with 

 few modifications. This apparatus is shown also in figure 10. It consists 

 of a sheet-galvanized iron can 15 inches high,' and 5 inches in diameter, 

 inverted over water into another can 14 inches high, and G^/o inches in 

 diameter. The suction tube is % inch in diameter and 14i/4 inches high 

 inside, and IG inches high outside of the fixed can. The aspirator bell is 

 attached to a fine brass wire which runs over a GVi inch grooved metallic 

 pulley, to a weight. The weight employed was sufficient to produce a 

 suction of 2 mm. pressure of mercury. The pressure was read on the 

 manometer. The volume of the aspirator bell was calibrated with air. 

 This was done by adding a definite quantity of water into the outer or 

 fixed can, and then allowing the aspirator bell to come to position, and 

 measuring the volume of air displaced by water in a gas burette. The 

 volume of air displaced is the reciprocal of the volume of air drawn in, 

 for the same water level in the tank. Many preliminary trials showed 

 that four liters of water in the tank gave an air displacement of 2G50 

 cc. on the aspirator bell. In all the experiments, therefore, this quantity 

 of water in the fixed can was used as a standard, and the relative rate 

 of flow of air through soils was measured by noting the time required for 

 2650 cc. of air to pass through the column of soil. The results obtained 

 are shown in table 7 and on the accompanying figure 11. 



TABLE 7.— EFFECT OF TEMPERATURE ON THE RATE OF FLOW OF AIR IN SOILS. 



Soil. 



Quartz sand . . . 

 Sandy loam . . . 

 Clyde silt loam 



Clay 



Peat 



Time required for 2650 cc. of air to pass through soil column. 



0° C. 



Min. 



1.45 



1.40 



11.00 



11.50 



13.30 



10° C. 



Min. 



1.60 



2.00 



15.20 



15.00 



16.40 



20° C. 



Min. 



2.00 



2.45 



18.30 



18.00 



20.15 



30° C. 



Min. 



40° C. 



Min. 



2.12 



3.37 



21.20 



26.00 



20.40 



2.28 



4.30 



29.30 



31.00 



27.30 



50° C. 



Min. 



2.50 

 10.35 

 41.20 

 33.00 

 38.40 



The preceding results show that the rate of passage of air through 

 soils decreases inversely with temperature. This is in conformity with 

 the law of viscosity of gases as far as the order is concerned, but not in 

 magnitude. The above velocity of transpiration is governed not only 

 by the internal friction of the gas, but also by the swelling of the col- 

 loidal material at the higher temperatures which closes up the pores, 

 and impedes the flow. This fact is very strikingly illustrated by the ab- 

 normal decrease in flow at the higher temperatures. The abnormal de- 

 crease at the higher temperatures, however, is also partly due to the 

 change in structure brought about by temperature and by the transpira- 



