no GEORGE F. KAY AND J. NEWTON PEARCE 



impart to these strata many of their most important properties, 

 it will be necessary to mention in detail some of the more important 

 colloid properties and phenomena. 



Colloids are divided into two general classes, namely, suspen- 

 :soids and emulsoids. Briefly stated, suspensoids are suspensions 

 of solid particles, chiefly inorganic, in a fluid medium; emulsoids 

 are suspensions of fluid or semi-fluid particles. The emulsoids 

 found in the soil are chiefly of organic origin, resulting from the 

 i^secretions of animals, the exudations of plant roots, the humus, 

 and other products of decaying organic matter brought about 

 through the assistance of bacteria and fungi. 



Colloids are also classified as reversible and irreversible. Most 

 of the suspension colloids when desiccated, sometimes when frozen, 

 or when in the presence of electrolytes, coagulate into a sohd or 

 semi-solid water-insoluble precipitate. When this sohd is placed 

 in water it does not again pass into suspension. It is therefore 

 ^aid to be irreversible. To this class belong the hydrated ferric 

 ^xide, the gelatinous siHcic acid, the gelatinous, hydrated aluminum 

 silicates, the clays, kaolin, etc. In rare cases one may find alumi- 

 num hydroxide.^ 



Suspended colloid particles are either positively or negatively 

 charged. Thus siHcic acid, kaoHn, and clay particles are nega- 

 tive; the basic hydrated ferric oxide is positive. These charged 

 particles are precipitated by electrolytes, and it has been found 

 that the precipitating power of the electrolyte is specifically a 

 property of the ion bearing a charge opposite in sign to that of the 

 particle. Further, the precipitating effect is greatest for those 

 ions carrying the greatest number of charges. 



Factors determining the stability of colloidal clay suspensions. — 

 The effective properties of any colloid suspension depend upon its 

 stabihty — its power to exist in the colloidally suspended state. 

 The stabihty is also one factor in the slow transportation of the 

 colloid particles through the soil capillaries. This stabihty depends 

 not only upon the absence of precipitating ions but also upon the 

 potential difference between the charged particles and the oppo- 



^ Cameron and Bell, "The Mineral Constituents of Soil Solution," U.S. Dept. 

 .vf Agric, Bull. 30 (1905), p. 22. 



