384 SOILS. 



of the large-scale use of the salts of manganese; which are 

 obtained in large quantities as a comparatively valueless by- 

 product of the bleaching industries. 



The " Insoluble Residue." 



Remembering, in discussing the facts shown by the table, 

 that the fundamental difference between the regime of the 

 humid and arid regions is the presence in the latter of an al- 

 most continuous leaching process, in which the carbonated 

 water of the soil is the solvent; remembering, also, that the 

 least soluble portion of rocks and soils is quartz or silica ( sand, 

 as usually understood), it would be predicable that this ingredi- 

 ent should in the humid region be found to be more abundant 

 in soils than in the arid. This portion is represented by the 

 " insoluble residue " of the table. 



Inspection shows that both in the averages of the single 

 states, and in both of the general averages, this difference be- 

 tween the soils of the humid and the arid regions of the United 

 States is strongly pronounced ; the ratio being substantially as 

 69% in the arid region to 84% in the humid. 



We must then conclude that the leaching process must have 

 influenced materially other soil ingredients than lime, which 

 have remained behind in such amounts as to depress the per- 

 centage of insoluble residue in the soils. It remains to be 

 shown what are the substances so retained. 



Insoluble and Soluble Silica and Alumina. 



The ingredient most nearly correlated with the insoluble 

 residue is the free silica which remains behind with it when the 

 acid with which the soil has been treated is evaporated to dry- 

 ness. The silica is separated from the practically insoluble, 

 undecomposed minerals by boiling with a strong solution of 

 sodic carbonate. The amount of this " soluble silica " is obvi- 

 ously the measure of the extent to which the soil-silicates have 

 been decomposed in the treatment with acid. 



The most prominent of these is usually supposed to be clay 

 the hydrous silicate of alumina that in its purest condition 

 forms kaolinite or porcelain earth. Any alumina found in the 



