in.] HYGROSCOPIC MOISTURE 93 



films only. The clay is neither so wet that the particles 

 just slip over one another when pressure is applied the 

 pasty condition ; nor have they been drawn so closely 

 together as to cohere without the aid of any water. 

 A piece of clay swells again when wetted because the 

 "gels" can imbibe water and expand. 



Hygroscopic Moisture. 



If the withdrawal of water from a soil by evapora- 

 tion be continued, a point is at last reached when the 

 soil becomes air-dry : it still retains some water, which 

 will vary in amount with the degree of humidity of the 

 atmosphere and the temperature. This film of hygro- 

 scopic moisture can be produced by condensation alone, 

 when perfectly dry soil is placed in an atmosphere 

 containing water vapour : the surface of materials like 

 glass, sand, etc., has sufficient attraction for water to 

 condense it from a state of vapour. The amount of 

 water retained or absorbed will in fact vary with the 

 tension of water vapour in the atmosphere with which 

 the soil is in contact until a state of equilibrium is 

 attained. 



The hygroscopic moisture, however, retained by soil 

 is not simply determined by the surface, as it would be 

 if the material were sand or powdered glass. The soil 

 colloids exert a considerable effect in retaining these 

 last traces of water and in absorbing them from the 

 moist atmosphere. 



If a layer of powdered glass or sand in a moist 

 condition is set to evaporate in a dry atmosphere and 

 the rate at which the water is lost is determined, it will 

 be found that while the rate falls off as the material 

 dries, the results are such as might be expected from 

 the extent of the evaporating surface of the particles 

 and the rate of diffusion of water vapour out of the 



