CHAPTER II 



THE COMPOSITION OF THE SOIL 



The reader must often have noticed in walking along 

 a lane after a heavy rainfall, that the water streaming 

 down a bank has washed away the soil in a somewhat 

 uneven manner, leaving behind the grit and small stones 

 but carrying away the rest. In following the course of 

 such a streamlet one observes that at certain points a 

 smooth cake is formed which cracks as soon as it begins 

 to dry, and is much more sticky and clay-like than the 

 original soil. Closer observation shows that the original 

 soil has been separated into various constituents by the 

 running water, the heavier coarser particles being left 

 behind while the finer lighter particles are carried on. 



This effect of a flowing stream has suggested a inethod 

 for analysing soil that has proved extremely valuable 

 and is largely adopted by soil investigators. It consists 

 in allowing a stream of water to flow over the soil and to 

 sort out the particles according to their degree of fine- 

 ness. One form of the apparatus for doing this, designed 

 by Nobel, is illustrated in Fig. 7. 25 grams of the soil 

 are put into the smallest of the pear-shaped vessels A, 

 and water is run in. As the vessels are of different 

 diameters the water flows through them at different 

 rates, going most rapidly through the narrowest and 

 most slowly through the widest, D. When it runs rapidly 

 it carries away the fine particles leaving only the coarse : 

 when it goes more slowly it deposits some of the fine 

 particles. Hence after a time the soil put into the 



