4 PHYSICAL CONSTITUTION OF SOIL 



soil, rich in humus, and possessing a good tilth, is thus one 

 attended with a large proportion of interspaces. We shall 

 go into further details on these points when we treat of the 

 capacity of soils for water. 



Another important fact is plainly taught by the system 

 of solid spheres which we have taken as the simplest example 

 of a soil. Although the size of the spheres is without 

 influence on the proportion of the interspaces, it has an 

 enormous influence on the extent of surface which the spheres 

 present. In the case of the spherical particles contained in 

 one cubic inch, we have the following variations in surface 

 brought about by alterations in the size of the spheres. 



Number of spheres in Diameter of each Total surface of spheres 



i cubic inch. sphere. contained in i cubic inch. 



One ...... ... 1 inch ... 3-1416 square inches 



A thousand ...... T V inch ... 31416 



A million ......... T ^ inch ... 314-16 



A thousand millions ... iifov inch ... 3141-6 , t 



A million millions ... -- inch ... 31416 



A solid mass of matter, reduced to particles of T tfi<T^h its 

 former diameter, has thus its surface increased ten thousand 

 times. 



The extent of internal surface possessed by a soil thus 

 depends on the fineness of the particles of which it is 

 composed, and is immensely greater in the case of a clay 

 soil than in the case of a coarse sand. King has calculated, 

 that in the case of a soil composed of particles TTHJ^^ i nc h * n 

 diameter, a cubic foot of soil will possess an internal surface 

 of about one acre. Soyka states, that in one litre of spherical 

 particles o-oi mm. in diameter, the total surface will be 

 444-0 square metres with the tightest packing, and 314-4 

 square metres with the loosest packing. The internal surface 



