158 Jotimal of Agricultural Research voi. v, N0.4 



The results in Table III show the most surprising fact that the 

 amount of moisture moved from the moist and warm column of soil to 

 the dry and cold column of soil by vapor is very insignificant. It will 

 be seen that at the temperature amplitude of 40° the quantity of mois- 

 ture moved is only about 0.25 per cent, and at the amplitude of 20° the 

 value is only about 0.035 P^r cent. In comparison with the results of 

 Table II, where it is shown that the maximum thermal movement of 

 water at the thermal critical moisture content, when the soil mass is 

 continuous, runs as high as 3.68 per cent in some cases, the above values, 

 due only to vapor movement and condensation, are extremely insig- 

 nificant. 



From these results then it is safe to conclude that the thermal move- 

 ment of moisture due to distillation is practically negligible, even at 

 such high amplitudes of temperature as 20° and 40° C, which never 

 exist during the night at the dift'erent depths on the soil, nor during 

 such a long, continuous period as eight hours. This conclusion is indi- 

 rectly substantiated by the studies of Buckingham (5) on the loss of 

 soil moisture by direct evaporation from points below the surface. By 

 exposing a surface of water or moist soil to evaporation into a confined 

 space which was in communication with the outside air through a column 

 of soil, Buckingham found that the actual mean rate of loss of water 

 through diffusion of water vapor through soils in still air was very small. 



Another noteworthy fact in the foregoing experimental data is that 

 the amount of distillation from moist and warm to the dry and cold 

 column of soil is the same for all moisture contents. This might have 

 been anticipated, since the amount of water vaporized depends prin- 

 cipally upon the temperature and is not governed by the amount of 

 water present. On the other hand, if the amount of water present in 

 the soil is extremely small, the water is held by the soil grains with an 

 attraction of great magnitude, causing a lowering of the vapor pressure 

 of the absorbed water film and thereby producing a diminution in the 

 rate of evaporation. Perhaps the water contained in the soil with the 

 lowest moisture content was above the point where this lowering of 

 vapor pressure occurs ; and consequently the partial pressure of the vapor 

 in the air space in this soil was the same as in the air space of the soil 

 with the greater moisture contents. Furthermore, the values are so 

 small as to lie within the experimental error, and the method of mois- 

 ture determination may not be sufficiently sensitive and accurate to 

 show any decreased evaporation by the soils vdth the lowest moisture 

 content. 



In undertaking and performing the foregoing series of experiments 

 it was taken for granted that there really is an upward movement of 

 moist air during the night from the warmer soil below to the colder soil 

 at the surface, where its vapor is condensed. This theory seems to be 

 now very widely accepted, as already stated. The formation of the dew 



