RELATION OF THE SOIL TO WATER 



27 



20. Capillary Water. — The water held in the 

 capillary spaces above the bottom water is known as 

 the capillary water. The capillary spaces of the soil 

 are the small spaces between the soil pat tides in 

 which water is held by surface tension ; that is, the 

 force acting between the soil and the water is greater 

 than the force of gravity. If a series of glass tubes of 

 different diameters be placed in w r ater it will be ob- 

 served that in the smaller tubes water rises much 

 hiofher than in the larger. The water rises in all of 



LI 



v5=^3gr» 7 



Fig. 12. Comparative height to which water rises in glass tubes. 



the tubes until a point is reached where the force of 

 gravity is equal to the force of surface-tension. In 

 the smaller tubes surface-tension is greater than the 

 force of gravity, and the water is drawn up into the 

 tube. In the larger tubes the surface-tension is less 

 and water is raised only a short distance. There 

 are present in the soil many spaces which are capable 

 of taking up w T ater in the same way as the small glass 

 tubes. The height to which water can be raised by 



