EXPERIMENT STATION BULLETINS. 515 



The above results 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 indeed. It will be 

 seen that at the temperature amplitude of 40° C. the quantity of moisture 

 moved is only about 0.25%, and at the amplitude of 20° C. the value is 

 only about 0.035%. In comparison with the results of table 2 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% in some cases, then the above values, due only to vapor move- 

 ment and condensation, are extremely insignificant. 



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 of 20° and 40° 0. loJiich never exist dur- 

 ing the night at the different adjacent depths in the soil, nor during such 

 a long continuons period as 8 hours. This conclusion is indirectly sub- 

 stantiated by the studies of Buckingham" 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, Bucking- 

 ham 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 to be noticed 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 

 principally upon the temperature and is not governed by the amount 

 of water present. Oti 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 be within the experimental error, and the method of moisture de- 

 termination may not be sufficiently sensitive and accurate to show any 

 decreased evaporation by the soils with 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 

 is attributed by many writers almost entirely to this thermal movement 



of vapor. Thus, in discussing the subject Hilgard^^ states "dew 



is formed from vapor rising from the warmer soil into a colder at- 

 mosphere, and condensed on the most strongly heat-radiating surfaces 

 near the ground, such as grass, leaves both green and dry, wood and 

 other objects first encountering the rising vapor." Farther on he says: 

 "The fact that dew is most couiiuonly derived from the soil could have 

 been foreseen from the other fact, long ascertained and known, that dur- 



"n. R. Dcpt. of Act.. Bureau of Soils Bulletin No. 38 (1907). 

 "Soils, p. 307 (1912). 



