THE CONSTITUTION OF THE SOIL 



59 



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 the ratios to A1 2 O 3 . 



TABLE XXVII. RATIO 



MOLECULES OF 



MOLECULES OF A1 2 O 3 



VAN BEMMELEN (22). 



ExTRACTED FROM VARIOUS SOILS. 



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 

 of the original silicates and also of easily soluble substances including 



a fusible group with ratio molecules of SiO, ing from to 6 



molecules of A1O 



and a silicate resembling kaolin with ratios varying between 2 and 3. 

 The easily soluble material represents the products of weathering since 

 it does not occur in rocks. If it were a definite chemical compound the 

 ratio of its constituents should be constant whole numbers, but this is 

 not the case. It behaves, however, precisely like a solid solution and 

 is therefore regarded by van Bemmelen as an " absorption compound," 

 SiO 2 , mAl 2 O 3 , nFe 2 O 3 . . . pH 2 O, in which the constituents are not 

 chemically united but are held by the feebler forces manifested by 

 colloids in their attractions one for the other. 



The Physical Properties of the Various Fractions. Serious 

 studies of the soil by competent physicists have scarcely been at- 



