HYGROSCOPIC MOISTURE 



195 



tin's form of moisture. Sandy soils with a relatively small surface 

 area contain only small amounts of hygroscopic water. 



The preceding tahle shows the amount of hygroscopic moisture 

 as determined hy Hilgard. The soils were exposed in saturated 

 atmosphere at 15 degrees C. and dried at 200 degrees (>. 



This shows the effect of texture and consequently the internal 

 surface upon the amount of hygroscopic moisture. In another case 

 the hygroscopic capacity was determined for the whole soil and then 

 for the separates as follows : 



Per cent 



Whole soil 5.24 



Clay 1 7.1)0 



Hydraulic Value less than 0.25 nun. per second. . . . 7.00 



Hydraulic Value 0.25 mm 2.01 



Hydraulic Value 0.50 mm 1.7.3 



(h) Colloids. The colloids have a very high adsorptive power 

 for water, and their presence in soils increases very strikingly the 

 hygroscopicity. Nearly all soils, and more particularly the heavy 

 ones, contain considerable amounts of colloids. They may occur 

 as humus, ferric oxide, silicic acid, or hydrous aluminum silicates. 

 Ilydrated ferric oxide and some other minerals may unite with 

 water chemically, but the common hygroscopic phenomenon is one 

 of adsorption. This table shows the effect of colloids on the. hygro- 

 scopic moisture. 



Influence of Silt, Sand, Clay, Ferric Hydrate and Humus on Hygroscopic 



Capacity 3 



It will be seen from the above table that clay, ferric hydrate and 

 humus have the greatest effect upon hygroscopic capacitv. 



(c) Temperature. The temperature affects the amount of 

 hygroscopic moisture, since under temperatures higher than normal 



