THE FORMS OF SOIL WATER 



207 



film, where the water to a large extent loses its movement, 

 is considerably without this zone of influence. In order 

 to give some idea of the extreme minuteness of the hygro- 

 scopic film, it may be said that its thickness is less than 

 the diameter of the smallest known soil bacteria. In 

 moving from the surface of a particle outward through 

 an ordinary water film, passage is first made through the 

 zone of influence. When the edge of this is reached, an 

 area is passed through which continues with constantly 

 increasing capacity for molecular motion until the outer 

 edge of the hygroscopic film is crossed, where molecular 

 activity reaches its maximum. 



136. Effect of humidity and temperature on hygro- 

 scopic water. — Two external conditions seem to affect 

 the amounts of hygroscopic water that a soil may hold 

 under definite conditions — humidity and temperature. 

 As a general rule, the higher the humidity, the higher is 

 the hygroscopic moisture. The experiments of von Dobe- 

 neck ^ with quartz and humus illustrate this point : — 



Percentage of Hygroscopic Water held at Various Humid- 

 ities AFTER AN EXPOSURE OF. TWENTY-FOUR HoURS AT 



20" C. 



Quartz 

 Humus 



30 

 Per cent 



.045 

 4-055 



50 

 Per cent 



.053 

 7.765 



70 

 Per cent 



.076 

 10.589 



90 

 Per cent 



.119 

 15.676 



100 

 Per cent 



.175 

 18.014 



The results as to the effects of a rise in temperature 

 on the hygroscopic film are not so definite. Most in- 



1 Dobeneek, A. F. von. Uutersueliungen tiber das Absorp- 

 tionsvermogeu und die Hygroskopizitat der Bodenkonstitu- 

 enten. Forsch, a. d. Gebiete d. Agri.-Physik, Band XV, Seite 

 163-228. 1892. 



