Thermal Movements of Soil Moisture. 177 



of sensible cohesive attraction. It appears clear also that 

 no hard and fast line can be drawn between capillary and 

 hygroscopic moisture, nor indeed between either of these 

 and the gravitational water; each must shade by insensi- 

 ble degrees into the other. 



206. The Amount of Moisture a Soil May Absorb from the 

 Air. The amount of so-called hygroscopic moisture a given 

 soil may absorb from the air depends primarily upon the 

 relative temperature of the soil and of the air and its de j 

 gree of saturation. If the temperature of a soil could be 

 maintained continually below that of a saturated atmos- 

 phere above, it would in time become so fully charged with 

 water as to result not only in capillary saturation but in 

 percolation as well ; and it frequently occurs on clear 

 nights in summer, when dews are heavy, that a thick, loose, 

 dry dust blanket will cool down so much that moisture 

 condenses upon it in sufficient quantity to make it appear 

 damp. Indeed dew, wherever it forms, is a demonstra- 

 tion of the truth of the statement made; when it evapo- 

 rates with the rising of the sun the loss of moisture from 

 the blades of grass may carry the amount all the way from 

 the drops, too heavy to be retained upon the blades, through 

 the thick adhering films, to those which become invisible 

 and are called hygroscopic. 



207. Observed Absorption of Moisture from the Air. The 

 rate and amount of moisture which may be absorbed from 

 the air is influenced by many factors. Hilgard has studied 

 the rate and amount of absorption of moisture by soils when 

 spread out in layers about 1 m. m. thick in a fully saturated 

 and a half saturated atmosphere, maintained at a uniform 

 temperature. He finds that fully 7 hours are required 

 for an equilibrium to be reached in so thin a layer. In 

 the table which follows are given some of his observations. 



