EXPERIMENT STATION BULLETINS. 513 



not he attributed entirely to the forces exerted hy the curvature of the 

 ca/pillary films, hut also to the forces exerted by the unsatisfied attractive 

 power of the soil for icater. When a moist soil, tlierefore, begins to lose 

 water at the surface two effects are produced: (1) the attractive forces 

 of the soil for water are increased, and (2) the curvature of the capillary 

 films is increased, both of these effects exert a pull on the moist soil be- 

 low and tend to draw water to the surface. As to which one of these two 

 forces exerts the greatest pull, it is impossible to say, because there is 

 no way of measuring them. It is certain, however, that the force re- 

 sulting from the attractive power of the soil for water must be very 

 considerable, and probably it is the predominant of the two. 



It might be argued that the preceding phenomena of thermal translo- 

 cation of water could be explained entirely by the film theory, without 

 having to resort to the conception of the attractive forces of the soil. 

 Such a contention, however, cannot be maintained, first because it can- 

 not be conceived that the tension of the capillar}^ films is operative and 

 effective at such high moisture contents employed, and secondly, because 

 the fact remains nevertheless that the soil exerts a pull due to its attrac- 

 tive forces for water as has been abundantly proved. Furthermore, if it 

 is maintained that the attractive forces of the soil for water are satis- 

 fied as soon as the soil is merely dampened, then why should the soil 

 hold additional large amounts of water with such a great force 

 that it is impossible to extract it with mechanical means? It seems 

 reasonable, therefore, to believe that if the soil holds large amounts of 

 water with a great force, it should attract or absorb it with a force of 

 equal magnitude. 



MOVEMENT OF MOISTURE FROM MOIST AND WARM COLUMN TO DRY AND COLD 

 COLUMN OF SOIL WITH AN AIR SPACE BETWEEN THE TWO COLUMNS. 



In the preceding section the thermal translocation of water was con- 

 sidered as occuring as water film phenomena. There is still another way 

 in which this thermal mobility of moisture might take place, this is by 

 evaporization and condensation of soil water from a point of high tem- 

 perature to a point of low temperature. It is well known that water 

 undergoes a transformation into the vapor state by the application of 

 heat, and the quantity of liquid vaporized increases with rise in tempera- 

 ture. One of the remarkable characteristics of aqueous vapor is its 

 sensitiveness to heat, changing from gaseous to liquid state and vice 

 versa with very small variations in temperature. As an excellent para- 

 digm of this latter fact may be cited the relative humidity of the air at 

 different temperatures. 



Since the temperature gradient of the soil reverses itself during the 

 night, that is, it increases with depth, it is believed that there is a rising 

 of vapor or moist air from the warmer soil below to the colder soil above 

 where the moisture is condensed. As a manifest proof of this theory 

 the morning dew is cited. It is concluded, therefore, that a large part 

 of the water movement in soils is due to this process, 

 65 



