^2 THE CHEMISTEY OF THE fAEM 



to diminish the proportion of interspace, the latter to 

 increase it. Some of the particles of a soil are them- 

 selves porous, as particles of humus and limestone, 

 and aggregates of smaller masses ; this condition 

 may considerably increase the volume of interstices. 



The total surface presented by a mass of spherical 

 particles doubles w^hen their diameter is halved. The 

 internal surface of a soil is thus much greater when 

 it is made up of fine constituents ; it is also in- 

 creased when the particles are themselves porous. 

 Upon the proportion of the interstices, and upon the 

 amount of surface presented by the particles, the 

 physical properties of a soil and its fertility largely 

 depend. 



By mechanical analysis the constituents of soil may 

 be separated into groups of definite sizes ; the coarser 

 particles are separated by means of sieves, the finer by 

 means of currents of water of different velocity. The 

 finest particles in soil are those of pure clay, which 

 remain permanently suspended in distilled water. 

 Next to these come silt and sand of very varying 

 degrees of fineness, and often very different chemical 

 composition. The still coarser particles are grit, 

 gravel and stones. The physical character of a soil 

 depends in great measure on the prevailing size of its 

 particles. 



2. Tenacity of Soil. — The coarser elements of soil, 

 including the fine sand, exhibit httle cohesion ; the 

 tenacity of a heavy soil is due to the fine silt and clay. 

 Clay owes its cementing power to the presence of a 

 small quantity of a hydrated colloid (jelly-like) body, 

 rarely, according to Schloesing, exceeding 1-5 per cent. 



