196 



SOILS. 



3. Liquid water seeking its level ; bottom, ground or hydro- 

 static water. 



HYGROSCOPIC WATER. 



Soils artificially dried so as to deprive them of all their mois- 

 ture, when exposed to moist air absorb water vapor with great 

 energy at first ; both the rapidity of absorption and the amounts 

 absorbed, when full time is given, varying greatly with their 

 nature. Sandy soils, broadly speaking, absorb the smallest 

 amounts ; while clayey soils, and those containing much humus, 

 or finely divided ferric hydrate, take up the largest proportion. 



The figure expressing the amount of aqueous vapor absorbed 

 at the standard temperature of 15 Cent., is called the coef- 

 ficient of moisture absorption. For one and the same sub- 

 stance, this coefficient rises as the grain becomes finer, the 

 surface being correspondingly increased (see chapt. 6). 



The table below indicates the effect of the three substances 

 mentioned in increasing moisture absorption as compared with 

 a very sandy soil from the pine woods of Mississippi, and a 

 gray silt or " dust " soil from Washington, very fine-grained 

 but poor both in humus and ferric hydrate. (For details of 

 the physical composition of the Mississippi soils see table in 

 chapt. 6, p. 93). A highly ferruginous soil from Oahu shows 

 plainly the effect of that substance. 



TABLE SHOWING INFLUENCE OF SILT, SAND, CLAY, FERRIC HYDRATE, AND 

 HUMUS ON MOISTURE ABSORPTION. 



