210 SOILS: FBOPJEBTIES AND MANAGEMENT 



Heat Evolved by Wetting Soils Dkied at 110° C. 



Soil 



Calories per 

 Kilo or Dry Soil 



Coarse quartz . 



150 



Podunk fine sandy loam 



Norfolk sand 



200 

 347 



Hagerstown loam 



1108 



Galveston elay 



Mnek soil (25 per cent organic matter) . . . 



3785 

 6413 



139. Capillary water. — It has been shown in the pre- 

 vious discussion that a large proportion of the hygroscopic 

 fihn is beyond the radius of influence of the particle and 

 is not held so rigidly as is the inner portion. In other 

 words, in this film a certain amount of molecular move- 

 ment is possible, this movement depending on the dis- 

 tance from the particle. As soon, how- 

 ever, as the boundary of the hygroscopic 

 film is crossed, a comparatively thick 

 film of moisture is reached in which 

 molecular movement, except for the 

 influence of viscosity, is perfectly free 

 and unimpeded. These two zones (see 

 Fig. 31) — one in which capillary move- 

 ment is more or less free, and a com- 

 paratively thin film in which molecular 

 movement becomes increasingly slug- 

 gish as the radius of influence of the 

 soil grain is approached — are there- 

 fore clearly differentiated. The capillary water differs 

 from the hygroscopic moisture (l) in that it is largely in 

 a liquid state and consequently is governed by the ordi- 

 nary laws of liquids ; (2) in that it evaporates at ordinary 



Fig. 31, — Diagram 

 showing the rela- 

 tionship of the 

 hygroscopic and 

 capillary water 

 films surrounding 

 a soil particle, 

 (s), particle; 

 (i), zone of influ- 

 ence of particle; 

 (w), outer edge of 

 hygroscopic zone ; 

 (c), capillary filna. 



