382 The Flocculation of Soils. Ill 



minerals of the cores of the particles causes no increase in the base 

 absorbing power of the remaining soil. The absorbed bases are easily 

 soluble in dilute acid and their extraction brings the soil at once, and 

 decidedly, to its maximum base absorbing power. From this it seems 

 justifiable to conclude that the absorption of hme which is measurable 

 by titration methods is solely confined to the surface colloids. The 

 insignificant amount of lime absorbed by powdered granite, after ex- 

 traction with acid, also goes to show that the unweathered and crystalhne 

 minerals have little to do with the absorption of lime under such con- 

 ditions as those of the Hutchinson-McLennan "lime requirement" 

 determination. 



It is not suggested that these minerals are not reactive but that any 

 part they play in base absorption is small. Powdered and extracted 

 crystalline minerals and rocks do show a shght absorptive power. But 

 even this may be due to the colloidal surface, for the grinding of rocks — 

 particularly the wet grinding of rocks — has a decomposing effect similar 

 to weathering. This is apparent from the classical work of Daubree^ and 

 the more recent work of Cushman*. 



The whole of the absorption of lime — reversible and irreversible — is 

 therefore a phenomenon associated with the colloidal surface. Until 

 sufficient lime has been apphed to react with that surface, the lime may 

 exercise a deflocculating action because of the combined effects of the 

 decreasing hydrogen ion concentration and the uncoagulated colloid 

 stabihzing the particles. 



After a soil has been treated with excess of calcium hydroxide, washing 

 with water readily removes lime at first, but as the amount of lime gets 

 less it becomes more difficult to remove it. The actual process of washing 

 a soil at the pump after treatment with calcium hydroxide until the 

 filtrate no longer colours phenolphthalein, is a long one. The colour pro- 

 duced by phenolphthalein becomes very gradually fainter in successive 

 washings and in such a way as to make it clear that the operation is not 

 one of merely washing away an excess of the hydroxide in solution, like 

 the washing away of hydrochloric acid after treatment of a soil therewith. 

 During the washing, calcium hydroxide is being removed less and less 

 effectively from the colloidal surface in which it is in some way held. 



The retention of hme by this colloidal surface seems to fall on the 

 borderhne between the physical phenomenon of a reversible absorption 

 and the chemical formation of compounds such as Way's double silicates, 



' ComiA. Html. 1857, 44. 



« U.S. Dept. Agric. Bur. Chem. Bulletin, 1905, 92. 



