98 THE TEXTURE OF THE SOIL [chap. 



night cannot be passed on to the subsoil and given up 

 to the roots, yet by its evaporation the next day it 

 may help to keep the temperature of the soil down, 

 and so indirectly diminish the loss of water to the 

 soil. Of course in certain conditions of air and soil 

 temperature there is condensation upon the soil of visible 

 water which can be available to the crops ; for example, 

 in some months drain-gauges yield more water than the 

 rainfall, though a certain amount of loss by evaporation 

 must also have taken place. This usually happens in 

 the early spring, and can be set down to the conden- 

 sation of dews by the thoroughly chilled ground from 

 a warm and moist atmosphere. Warington has sug- 

 gested that the persistent wetness of the soil in February 

 must be attributed to this cause. In a coarse-grained 

 soil mostly filled with air, the cooling of the surface that 

 comes by radiation at night may result in an upward 

 distillation of water from the wetter and warmer subsoil. 

 Hilgard has suggested this explanation to account for 

 the capacity of some Californian soils to maintain a 

 crop during a rainless winter, when the soil itself shows 

 only 3 per cent, or so of water. 



A. Mitscherlich has made a number of determina- 

 tions of the heat that is evolved on moistening dry soil 

 (benetzungs-warme), due to the condensation of the 

 hygroscopic water on the surface of the soil particles, 

 and to the imbibition of water by the soil colloids, 

 which is accompanied by a contraction of the total 

 volume and a consequent evolution of heat. The 

 figure thus obtained is of some significance in judging 

 of the physical properties of a soil, since it provides a 

 measure of the effective surface of all the particles 

 composing the soil if we regard a colloid as essentially 

 a matter of surface. Mitscherlich obtained results of 

 the following order, in calories evolved per gram of 



