*202 SOILS: PROPERTIES AND MANAGEMENT 



or of free water to capillary with a change of structure, 

 temperature, or pressure, as was seen to be the case be- 

 tween the hygroscopic and capillary moisture. The forms 

 of water present in a saturated soil may be conveniently 

 represented by the following diagram : — 



HYGROSCOPIC! CAPILLARY | FREE 



Fig. 30. — Diagram representing the three forms of water that may 

 be present in a soil. 



£1.33. Hygroscopic water. — The hygroscopic water 

 a soil has been spoken of as the water of condensation, 

 adsorption. It is, however, quite distinct from water 

 condensed on a surface colder than the atmosphere in 

 which it is placed. All bodies possess the power, to 

 a greater or less degree, of adsorbing water even when 

 at the same temperature as the air with which they are 

 in contact, provided, of course, that the air contains water 

 vapor. The hygroscopic film may be continuous or only 

 partly continuous, depending on the condition of the 

 surface. In fact, the movement of water over surfaces 

 is often greatly facilitated by an already existing hygro- 

 scopic film. External conditions being constant, the 

 amount of hygroscopic water of various materials is 

 determined by two factors: (1) the characteristics of the 

 material itself, and (2) the amount of surface it exposes. 



It is a well-known fact that various materials differ 

 in the amount of hygroscopic water they will hold, due 

 to the attraction of the substances themselves for water. 

 The differences in the thickness of the film is so slightly 

 altered, however, by differences in materials, that, other 

 factors being constant, the hygroscopic water becomes 

 a function almost entirely of surface. Glass becomes 



