THE FORMS OF SOIL-WATER 153 



as the air with which they are in contact, provided, of course, 

 that the air contains water-vapor. Such condensation is 

 largely a function of the surface exposed. 



One of the characteristics peculiar to colloidal materials is 

 a high absorptive power for water, whether it is presented in 

 the form of a liquid or vapor. This capacity is due to the 

 tremendous surface exposed by matter in a colloidal state, 

 which not only may hold the moisture physically but may 

 even force it into loose chemical combination. 1 The hygro- 

 scopic water is probably not in the form of a film around 

 the particles but in a much more intimate relationship. That 

 which is held physically is probably, in part at least, in a con- 

 dition of solid solution. If any of the hygroscopic water is 

 held chemically, the bond is probably a rather loose one. 



A large proportion of the hygroscopic moisture is obviously 

 not in a liquid state and consequently is immovable as such. 

 When a hygroscopically saturated soil is exposed to a partially 

 saturated air, a portion of the hygroscopic moisture will be 

 lost through vaporization. In order to expel the remainder 

 of the hygroscopic water, the soil must be heated. For con- 

 venience of determination, it is generally assumed that all of 

 the hygroscopic moisture will be driven from an air-dry soil 

 by heating it for four or five hours at a temperature of 100° 

 or 110° C. This is only an assumption, however, as some of 

 the moisture in intimate relationship with the colloidal com- 

 plexes probably still remains. 



The amount of energy necessary to expel the hygroscopic 

 moisture from the soil is very great, since its only movement 

 is thermal and because it is held so closely. As so much 

 energy is expended in removing this water, it is reasonable to 



1 See, Bouyoucos, G. J., Classification and Measurement of the Dif- 

 ferent Forms of Water in the Soil by Means of the Dilatometer Method; 

 Mich. Agr. Exp. Sta., Tech. Bui. 36, Sept., 1917. Belationship between 

 the TJnfree Water and the Beat of Wetting of Soils and its Significance; 

 Mich. Agr. Exp. Sta., Tech. Bui. 42, Mar., 1918. A New Classification 

 of the Soil Moisture; Soil Sci., Vol. XI, No. 1, pp. 33-47, Jan., 1921. 



