40 THE MECHANICAL ANAL YS1S OF SOILS [chap. 



upon the nature of the material ; in the soil they depend 

 upon the presence of the zeolitic double silicates derived 

 from the weathering of the felspars in the fundamental 

 rocks. 



The power of flocculation plays a very important 

 part in the cultivation of clay soils. When such a soil 

 possesses a good texture its finest particles are in a 

 state of temporary aggregation or flocculation, so that 

 they behave as if the soil, as a whole, were built up of 

 much coarser particles. Just as a potter or a brick- 

 maker brings his material into its highest condition of 

 plasticity by repeatedly kneading and working it, by 

 which process the naturally formed aggregates are 

 resolved into their ultimate particles and the material is 

 made as fine-grained as possible, so if a clay soil be in 

 any way worked or disturbed when in a wet condition, it 

 becomes apparently more clayey than before. It remains 

 persistently wet and impervious to the percolation of 

 water, and shrinks when dry into hard tenacious clods. 

 But if the clay be exposed to the weather for some time, 

 so that it undergoes alternations of temperature, freez- 

 ing and thawing, wetting and drying, it will experience 

 a certain amount of spontaneous flocculation and behave 

 as though it were coarser grained, so that if caught 

 in the right state of partial dryness it may easily be 

 crumbled. 



Flocculation may also be aided or otherwise by the 

 use of certain artificial manures, as will be explained 

 later ; the incorporation again of humus much improves 

 the texture, while the action of lime is particularly 

 effective and is much employed in practice to ameliorate 

 the working. of clay soils. 



Lime itself can be shown in the laboratory to possess 

 little flocculating power, for though its base is calcium, a 

 highly effective metal, it is combined as a hydrate, which 



