AGRICULTURE AND RURAL ECONOMY. 541 



alumina with other bases, as lime, magnesia, potash, soda, etc. 

 These " double silicates" are somewhat soluble in acids. 



Absorption by Exchange of Bases. 



Van Bemmelen has published a long series of investiga- 

 tions on soil-absorption, which throw a great deal of light on 

 the ways in which bases replace each other and are other- 

 wise absorbed in the soil. His work adds much strength to 

 the opinion which is becoming prevalent, that absorption is 

 due to chemical rather than physical processes. 



The agency of the soluble (zeolitic) silicates was tested by 

 first determining the absorptive power of soils which con- 

 tained them; then extracting them by hot hydrochloric acid, 

 and noting the loss of absorptive power ; and, finally, by treat- 

 ing the extracted soil with alkalies and alkaline earths, by 

 which bases were restored, and with them the lost absorp- 

 tive power. 



His experiments show very strikingly how absorption with 

 change of bases is effected. When solutions of salts of al- 

 kalies or alkaline earths come in contact with the zeolitic 

 silicates, the silica of the latter gives up some of its bases to 

 the solution, taking bases from the solution in return. The 

 silicates part most readily with lime, then with soda, and 

 then magnesia, retaining potash most firmly. Conversely 

 they absorb potash most readily, and then the others in the 

 inverse order, showing the least affinity for lime. In general, 

 strong acids e. g., sulphuric acid and chlorine seem loath 

 to give up their bases to the soil unless they get other bases 

 in return. The zeolitic silicates are rich in bases which are 

 easily exchanged. If a solution of chloride of potassium, for 

 instance, be brought in contact with such silicates, the latter 

 may give up some of their lime to the solution, and take pot- 

 ash from it in return ; but the soil cannot get much of the 

 potash from the salt unless it has the double silicates with 

 their bases for the barter. This much has been understood, 

 though not in so complete detail before. Van Bemmelen 

 shows, in a very striking way, how one base may be driven 

 out by a second, this latter replaced in its turn by another, 

 or by the first, and so on. 



