204 SOILS: PROPERTIES AND MANAGEMENT 



expanses, but is distributed over the great internal sur- 

 face exposure. Such water cannot be expelled by ordi- 

 nary drying, but the material must be subjected to a high 

 heat in order to drive off even a part of the water so held. 

 The question is greatly complicated also by the fact that 

 some bodies have a chemical affinity for water. This 

 results in the formation of hydrates and other salts. Such 

 water cannot be expelled without the breaking-up of the 

 compounds. 



Ordinary soil possesses to an extraordinary degree 

 the three characteristics already cited : that is, it exposes 

 a very large amount of free surface ; it tends to generate 

 continuously large amounts of colloidal material such as 

 ferric hydrate, aluminium hydrate, silicic acid, and espe- 

 cially humic materials in a colloidal state ; and it always 

 has present compounds having an affinity for water. 

 However, since these compounds are easily satisfied, and 

 also since the adsorptive power of colloids is due to the 

 surface exposed, it may be considered that, other condi- 

 tions being equal, the hygroscopicity of the soil is essen- 

 tially a surface phenomenon. Although for all practical 

 purposes hygroscopicity may be considered as having 

 special relation to surface, exact correlation is not easy 

 partly because of the difficulty of accurately determining 

 the surface exposed by a normal soil. 



134. Effect of texture and humus on hygroscopicity. - 

 The question being thus reduced to a surface consideration, 

 it is evident that the texture of the soil, external factors 

 being under control, is the determining factor. The fol- 

 lowing figures from Loughridge, 1 by whom the hygroscopic 



1 Loughridge, R. H. Investigations in Soil Physics. 

 California Agri. Exp. Sta., Kept, of Work of the Agri. Exp. 

 Stations of California for 1892-3-4, pp. 76-77. 



