30 PHYSICAL CONSTITUTION OF SOIL 



not without influence on the cohesion of the soil particles, but 

 nothing very definite can be said on the subject. 



Coagulation of Clay. The tenacity of a soil containing clay 

 is greatly influenced by the condition of the colloid clay which 

 it contains. If this jelly-like matter is in its fully swelled 

 condition, the soil exhibits its maximum stickiness and is 

 perhaps quite impervious to water ; while if this jelly is in 

 a shrunk, coagulated state, the same soil may be pervious to 

 water, and capable of successful tillage. 



In the case of a natural soil the facts are complicated by 

 the circumstance that the permeability of a clay soil to water, 

 and the production of a good tilth, largely depend on the 

 formation and maintenance of compound particles, and the 

 conditions under which compound particles are produced are 

 sometimes also favourable to the coagulation of clay. The 

 conditions which destroy compound particles are also some- 

 times those which bring about the expansion or diffusion 

 of the colloid clay. We may at present state, that a change 

 in the clay from the coagulated to the diffused condition is 

 necessarily destructive to compound particles in all cases in 

 which the colloid clay is an essential constituent of such 

 particles; but the converse is not necessarily true, for it is 

 quite possible to destroy compound particles without affecting 

 the coagulated condition of the clay. The phenomena presented 

 by soils have thus in some cases a mixed origin ; in our 

 attempt to understand them we must consider the causes 

 separately. 



The behaviour of colloid clay is best studied when we have 

 it diffused in water. Pure clay, when mixed with distilled 

 water, remains permanently suspended in the same, however 

 long the mixture may remain at rest. The addition of 



