THE COLLOIDAL PROPERTIES OF SOIL 149 



turn that the colloidal nature of the soil modifies or even deter- 

 mines its behaviour and leads to all sorts of unexpected 

 results. 



So far as the soil is concerned the most striking colloidal 

 properties are : — 



(i) The power of absorbing substances from their solu- 

 tions. 



The absorption is in effect a precipitation : it may be re- 

 garded as a concentration of the absorbed body on the surface 

 of the colloid and in this sense it is called an " adsorption ". 

 It differs from a simple chemical precipitation in that it does 

 not follow the ordinary laws of chemical reaction but special 

 laws of its own. In general, but not invariably, the adsorption 



by colloids can be expressed by the equation — = K^p where 



m 



c = the concentration of the dissolved substance when 

 equilibrium is attained ; 



y = the amount absorbed by a quantity m of the ad- 

 sorbent ; 



(this can readily be expressed as {a - c) where a = the 

 initial concentration) ; 



K and / = constants depending on the nature of the solu- 

 tion and adsorbent. 



As we shall see, this equation fits a large number of soil 

 reactions. 



(2) The power of absorbing water in considerable amount 

 and holding it rather loosely: parting with it again by 

 evaporation in a continuous manner without any critical 

 points or regions. 



(3) The change that can be brought about in some of the 

 finer mineral particles from the flocculated to the defloc- 

 culated state, and vice versa. This is particularly shown by 

 clay and has been discussed on p. 105. 



(4) The property whereby some of the soil components 

 can enter into solution in pure water (not, however, a true 

 solution) and be thrown out again on addition of small 



