Oct. 25. 1915 Temperature and Capillary Moisture in Soils i6i 



in the fall, when the trend of the air temperature is downward and the 

 surface soil temperature continually falls. At this time the variation in 

 temperature between the surface and 6 inches of the mineral soils may 

 be as high as 8° C. 



The truth of the matter, however, seems to be that instead of vapor 

 rising from the warmer soil below to the colder soil at the surface, vapor 

 enters the soil from the atmosphere. This is a natural conclusion from 

 the law enunciated that during the day air is exhaled from the soil and 

 during the night air is inhaled from the atmosphere. The amount of 

 moisture that will thus enter the soil will depend upon the quantity of 

 air inhaled and upon its absolute humidity, but, as calculations show, 

 it is extremely small. The water may be abstracted by the dry 

 soil at the surface as the air is drawn in or it may enter unaffected. 

 Thus, it is possible that the moisture lost by the soil during the day by 

 the expulsion of its moist air is partly, if not wholly, regained at night. 



What is, then, the source of the water of the dew ? The greatest part 

 of it comes from the lower layer of the atmosphere itself by condensation. 

 Some of it comes from the leaves of trees and plants; and a certain 

 amount comes from the soil by capillary and thermal capillary action, 

 as set forth previously. 



According to the foregoing consideration, therefore, the notion that 

 " dew is formed from the vapor rising from the warmer soil into a colder 

 atmosphere" is wrong, and those who proposed and adhere to this theory 

 seem to be laboring under a misapprehension of facts. 



MOVEMENT OF MOISTURE FROM A MOIST AND WARM COLUMN TO A 

 DRY AND COLD COLUxMN OF SOIL AND FROM A MOIST AND COLD 

 COLUMN TO A DRY AND WARM COLUMN OF SOIL 



The soil moisture under field conditions exists during the warm period 

 of the year nearly always in a gradient form. During a long drought even 

 the upper surface dries out, either of its own accord or induced by 

 artificial means. This layer of dry soil formed at the surface is known 

 as mulch. To this mulch is ascribed the important function of conserv- 

 ing the moisture in the soil by its ability to reduce evaporation of water 

 at the surface. It accomplishes this conservation of moisture, it is 

 claimed, by producing a change or break in the capillary connections 

 between itself and the moist soil below. 



Since, on account of the kinetic energy, the absorptive and adhesive 

 forces of the solid substance decrease with a rise in temperature, the 

 interesting question arose whether the dry mulch with an excessively 

 high temperature would absorb moisture from a moist soil with low tem- 

 perature, even when the capillary connections were ideal. The desire 

 to secure information upon this important and exceedingly interesting 

 point led to the execution of the following experiments: Brass tubes, as 

 described in the preceding sections, were filled with soil, one half with 



