SOIL MOISTURE 9 



of glass are separated by the thinnest piece of paper, 

 they do not adhere; if the iron surfaces are a trifle dusty 

 they do not adhere. The distance apart determines 

 the adhesion. 



It is generally held that the ultimate particles of all 

 substances are held together by molecular attractions. 

 Iron is a solid mass and not a pile of loose particles, 

 because of the mutual attraction of like particles, known as 

 cohesion. These molecular forces may be overcome by 

 other forces, notably by heat. When the solid iron is 

 heated, the molecular attraction is weakened until the 

 iron is melted. If the heating is further continued, the 

 molecular attraction is finally overcome, and the iron 

 becomes a gas, in which state the ultimate particles, or 

 molecules, actually repel each other. 



This theory of attraction is of great help in under- 

 standing the phenomena observed in soils, especially in 

 relation to water. When a pebble is dipped in water, a 

 thin water-film clings around its whole extent. The water 

 has come into very close contact with the surface of the 

 pebble, within the reach of molecular forces, and a cer- 

 tain quantity of water adheres. The quantity of water, 

 thus adhering, is just in proportion to the force of adhesion 

 existing between the water and the rock surface. On the 

 basis of this fact the adhesion between rock surfaces 

 and water rest the tillage methods of moisture-conser- 

 vation. 



12. Soil particles. Soil is composed of broken-down 

 rock mixed with decaying animal and plant remains. The 

 most notable properties of soil result from the minute 

 size of the constituent particles. The coarsest particles 

 useful to plants are from 1 to 3 millimeters in diameter, 

 which means that about twenty-three placed side by side 



