THE WATER IN THE SOIL 247 



has run away, but moisture is still present. You can 

 demonstrate this even more satisfactorily by heating 

 a little of the soil and seeing the steam escape. 



The water that is thus present in soils from which the 

 free water has run away is called capillary moisture. 

 Around each tiny particle of the soil is a very thin film 

 of water that is held there by capillary attraction. If 

 you place the lower end of a vertical string or a lamp 

 wick in water, you will find that this string or wick soon 

 becomes wet for some distance above the surface. The 

 water is drawn upward along the thread by capillary 

 attraction. This is the same force that serves to keep 

 the film of moisture around the soil particles. 



Capillary attraction also helps to keep soils moist by 

 causing the water from below to rise constantly toward 

 the top, very much as the oil in a kerosene lamp rises 

 constantly to the flame. The particles at the surface 

 lose a great deal of moisture through evaporation, espe- 

 cially when the ground is baked. This loss is replaced 

 by the water that rises from below in very much the same 

 way that the oil burned in a lamp flame is replaced by 

 the oil contained in the lamp. 



The amount of capillary moisture held by a soil 

 depends very largely upon the fineness of the particles. 

 The smaller and more numerous these particles are, the 

 larger is the surface to be covered by the film of water. 

 You can easily understand this if you make the following 

 experiment : 



Cut from a potato a cube an inch square. Estimate 

 the amount of surface it presents. Then cut this cube 

 into eight smaller cubes, all of equal size. Estimate the 

 surface area which the original cube now has. By cut- 



