THE SOIL 333 



Make a few fine tubes of different diameters by heating 

 a piece of soft glass-tubing in a flame and drawing out 

 portions to the thickness desired. Dip the ends of these 

 into a coloured liquid and note how high it rises in each case. 

 The liquid wets the inside of the tube and creeps up it, 

 forming a concave surface film. The pull which this film 

 exerts is proportional to the line of contact with the tube. 

 As we reduce the diameter of the tube, we not only reduce its 

 capacity, but at the same time reduce the pull, though we 

 see that in such a narrow tube it will support a much 

 higher column of water. In a wide tube the water rises 

 very little. This property of the ascent of water in narrow 

 tubes is called ' capillarity '. 



Apply this principle to the different soils. Sand with its 

 coarse grains and large spaces will correspond to the widest 

 tube, loam to the intermediate one, and clay to the finest. 

 The capillarity of humus is greater still. 



From these observations we see that water is present in 

 soils in different states : 



(i) That which fills the free spaces, and, when in excess, 

 displaces the air. 



(2) That which is still retained in air-dried soil, and may 

 be driven off at a high temperature. 



(3) What may be called capillary water, which forms films 

 over the particles in the soil and on the roots of plants, 

 behaving similarly to that noticed in the capillary tubes. 



The first readily drains away in a loose permeable soil ; 

 the second, which is called the hygroscopic water, is held 

 firmly by the particles and is of no use to plants ; the third 

 is the most important, because it is able to move in the soil 

 from wet to drier parts in any required direction, so that 

 the effect of absorption by root-hairs is to provide a space 

 into which more water is drawn by capillarity. By this 

 means, water may be drawn upwards in the soil eight to 

 ten feet. 



