THE FORMS OF SOIL WATER 



203 



far more hygroscopic when pulverized. Porous bodies 

 are especially high in hygroscopic water, sometimes 

 holding as much as 20 to 30 per cent of moisture. The 

 following data, drawn from Ammon l and von Dobeneck, 2 

 although no doubt faulty, illustrate the differences in 

 hygroscopicity of materials commonly found in soils 

 and make plain the complexity of the question when 

 applied to soil phases : - 



PERCENTAGE OF HYGROSCOPICITY OF DIFFERENT SUBSTANCES 

 AT 20 C. WHEN EXPOSED FOR ONE DAY TO SATURATED 

 AIR 



One of the characteristics peculiar to colloids in partic- 

 ular is a high adsorptive power for moisture, this giving 

 them properties not usually possessed by crystalloids. 

 Gelatinous precipitates of silica, ferric oxide, and alumin- 

 ium oxide are good examples. Colloidal humus, gela- 

 tin, and agar are noted for their adsorptive powers. The 

 water in such cases is not simply adsorbed on the external 



1 Ammon, Georg. Untersuchungen iiber das Condensa- 

 tionsvermogen der Bodenconstituenten fur Gase. Forsch. 

 a. d. Gebiete d. Agri.-Physik, Band II, Seite 1-46. 1879. 



2 Dobeneck, A. F. von. Untersuchungen iiber das Absorp- 

 tdonsvermogen und die Hygroskopizitat der Bodenkonstitu- 

 fiiton. Forsch. a. d. Gebiete d. Agri.-Physik, Band XV, 

 Seite 163-228. 1892. 



