THERMAL MOVEMENT OF WATER 189 



eight and ten feet, or even more, may be had. This 

 is an important body of information yet to be gained. 



90. Thermal movement. Water moves through the 

 soil in the form of vapor. If a glass vessel or tube filled 

 with moist soil be set on a hot surface, the bottom of the 

 column will be seen to become lighter in color, indicating 

 a loss of moisture. If the whole column is not heated 

 and the moisture is determined in successive sections, 

 beginning at the top, or coldest portion, the moisture 

 content will be found greatest a short distance above 

 the heated layer at the bottom. 



When the moist soil is heated, steam is formed, which 

 develops a pressure that forces the vapor rapidly through 

 the soil. But, at ordinary temperatures, this vapor 

 movement is the result of simple diffusion, and it obeys 

 the same laws. Buckingham has shown that the diffu- 

 sion of air through 'the pores of the soil is exceedingly 

 slow, and therefore that this phase of soil aeration is 

 of small effect. (See page 439.) He has also shown 

 that the diffusion of water vapor through the fine pores 

 of the soil is very slow. (See table below.) 



It is well known that water does not necessarily 

 evaporate at the surface of the soil. It may evaporate 

 in the deep pores in the soil if the air at that point 

 is sufficiently dry. Atmosphere in a moist soil is very 

 near saturation. In a mulched soil (see page 199) evapo- 

 ration may take place at the top of the moist layer. 

 The loss of water will therefore depend very largely 

 upon the loss of moisture by diffusion through the 

 mulch. Buckingham obtained the interesting results 

 given in Table XXXV bearing on this point: 



