THE COLLOIDAL MATTER OF SOILS 159 



from a sol to a gel. It is what might be called a reversible 

 colloid. Moreover, it is not coagulated by ordinary addi- 

 tions of salt or by heating. The ferric hydrate colloid, 

 on the other hand, when precipitated or agglutinated by 

 any means may not easily be brought back again to the 

 sol state. It is a so-called irreversible colloid. Moreover, 

 it is thrown down by the addition of electrolytes. There 

 exist, then, the viscous, gelatinizing, reversible colloids, and 

 the non-viscous, non-gelatinizing, easily coagulable, and 

 non-reversible colloids, besides all gradations and variations 

 between the two. In the ordinary clay soil, both types 

 of these materials probably exist and play important parts 

 in the physical and chemical characteristics exhibited. 



106. Flocculation. — While the gelatinous colloids of 

 the soil, such as some of the humic materials, are not 

 agglutinated by the addition of electrolytes, most of the 

 colloids of a nature similar to colloidal silicic acid and 

 ferric hydrate are thrown down by this treatment. This 

 phenomenon is often spoken of as flocculation. A very 

 good example is afforded by treating a clay suspension 

 with a little caustic lime. The tiny particles almost 

 immediately coalesce into floccules, and, because of their 

 combined weight, sink to the bottom of the con- 

 taining vessel, leaving the supernatant liquid clear. 

 The same action will take place in the soil itself, but of 

 course with less rapidity and under conditions less notice- 

 able to the eye. The colloids thus thrown down, being 

 largely irreversible, cannot again assume their former 

 attributes and thus lose their distinguishing character- 

 istics. In general, acids bring about flocculation while 

 alkalies do not, calcium oxide and calcium hydrate being 

 the best-known exceptions to the latter. Ammonia is 

 an intense deflocculator. 



