THE CONSTITUTION OF THE SOIL 



57 



those shown by colloids ; the only hypothesis capable of explaining 

 the observed facts is that some of the constituents of the soil, and espe- 

 cially of the clay, are colloidal. 



This view is fully borne out by van Bemmelen's fractionations of 

 soil and of clay. By successive extraction with acids of increasing 

 concentration van Bemmelen found (22) two distinct groups of silicates 

 in the Dutch alluvial soils, one soluble in dilute hydrochloric acid in 



, . , , . molecules of SiOo , , , , ,, , . 



which the ratio — -, -, , . , ^ = ? to 5,^ the other soluble only m 



molecules of AljOg ^ ^> ^ 



hot, strong sulphuric acid in which the ratio is approximately equal 



to 2. Other soils of volcanic origin from Java gave up larger amounts 



of base relative to the silica, but in no case were the ratios constant 



whole numbers ; the alkaline bases showed the same lack of constancy 



in their ratios to AljOg. 



Table XXVII.— Ratio Molecules of SiO, extracted from Various Soils. 

 Molecules of AljO, 



Van Bemmelen (22). 



Alkaline bases extracted from a heavy clay, Surinam. 



Different soils gave up different proportions of alkaline bases, but 

 again without showing definite simple ratios one to another. Detailed 

 studies of clay revealed the presence of chemically unchanged crystals 



^ The higher numbers were obtained from sandy clays and the lovsfer from heavy clays. 

 As the silica was insoluble in the acid it was extracted by digesting the residue for a few 

 minutes at 55° with dilute alkali of sp. gr. i-04. 



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