88 SOILS AND MANUBES 



pended in water, and are not floculated by reagents to the 

 same extent as colloidal clay, if at all. On drying they 

 shrink greatly in volume and lose their binding power. 

 They cannot take up water again or resume the plastic state. 



Contraction. The shrinkage of colloidal matters on 

 drying, already referred to, causes a considerable contrac- 

 tion in the volume of the soil as a whole, and produces the 

 cracked appearance commonly seen in periods of drought. 

 The phenomenon does not occur in sands at all but only in 

 soils which contain much clay or organic matter. Air 

 finds access to the mass of the soil through the cracks 

 which spread out in all directions, and provides a supply of 

 oxygen of which these soils generally stand much in need. 

 The soils swell up again when moistened and the cracks 

 are soon obliterated. 



Diffusion. By the decomposition of minerals and 

 organic matter in the soil, various salts and compounds 

 are constantly passing into solution and are again with- 

 drawn by the roots of plants. Small local differences of 

 concentration produced in these ways are equalised by the 

 movements of the water which holds the substances in 

 solution, and also by the movement of the dissolved salts 

 through the water known as diffusion. The rate of diffu- 

 sion depends partly on the nature of the substance and 

 partly on the difference of concentration. Crystallisable 

 substances, such as salts, diffuse at different rates, but 

 all of them much more rapidly than colloids. Crystalloids 

 diffuse through colloidal jellies or colloidal membranes 

 nearly as rapidly as through pure water, but one colloid 

 does not diffuse through another. 



MECHANICAL ANALYSIS. 



The object of physical or mechanical analysis of soils is 

 to ascertain the physical or mechanical properties. It has 



