THE FORMS OF SOIL WATER 213 



the water is drawn into the space between the particles, 

 where it becomes thicker than the capillary film about 

 the particles. This readjustment continues until the 

 forces developed by the two films become equal. An 

 equilibrium is now established. It is evident, then, that 

 as the capillary water becomes less in a soil from any 

 cause, the moisture collected in the spaces between the 

 particles becomes less and less, but still remains thicker 

 than the films about the particles themselves. What 

 percentage of the capillary water is held in the thickened 

 waists of the soil grains cannot be calculated, but it is 

 probable that this moisture makes up the major part 

 of the capillary water of any soil. One of the errors 

 in the determination of the hygroscopic coefficient of a 

 soil, as already pointed out, arises from the tendency 

 toward the formation of capillary water in these angles 

 between the soil particles before the hygroscopic film on 

 the grains themselves becomes satisfied. 



142. Factors affecting amount of capillary water. — 

 As might naturally be expected, the factors that tend to 

 vary the amount of capillary water in a soil are several, 

 and their study is more or less complex, due to the second- 

 ary influences that they may generate. These factors 

 may be discussed under four heads : (1) surface tension, 

 (2) texture, (3) structure, and (4) organic matter. 



143. Surface tension and the amount of capillary 

 water. — Any condition that will influence surface ten- 

 sion will obviously influence the thickness of the capillary 

 film, because of a variation in the forces thereby de- 

 veloped. A rise in temperature, by lowering the surface 

 tension, would consequently lower the capillary capacity 

 of the soil, and if the soil were capillarily saturated would 

 allow some of the water to become gravitational in its 



