AGRICULTURE AND RURAL ECONOMY. 357 



bases of hydrous silicates in the soil for the base absorbed 

 from the solution, though several have considered that the 

 oxides of iron and alumina, and some organic compounds, 

 play a very important role. Armsby concludes from two 

 series of experiments, one with soils and the other with hy- 

 drous silicates, that " the absorption of combined bases by 

 the soil consists in an exchange of bases between the salt 

 and the hydrous silicates of the soil; and that this exchange, 

 which is primarily chemical, is only partial, its extent vary- 

 ing 1st, with the concentration of the solution; 2d, with 

 the ratio between the volume of the solution and the quan- 

 titv of the soil used." 



" The cause of these variations is probably the ' action of 

 mass,' or the tendency of the resulting compounds to re-form 

 the original bodies, the absorption actually found in any 

 case marking the point where the two forces are in equilib- 

 rium " (Am. Jour. Sei., XIV, 1877, p. 25). 



On the whole, the results of later investigation seem to 

 lead towards the conclusion that the cause of the absorption 

 of acids is mainly, if not entirely, chemical ; that of bases, 

 partly physical but chiefly chemical. 



Oxidation of Nitrogen Compounds in the Soil. 



To test the action of organized ferments upon the oxida- 

 tion of nitrogenous compounds, Schloessing and Miintz tilled 

 a wide glass tube, a meter in length, with ignited sand mix- 

 ed with 100 grams of powdered chalk. The mixture in the 

 tube was watered every day with a fixed quantity of sewage. 

 After twenty clays nitric acid appeared in the water flowing 

 from the bottom of the tube, and increased rapidly until the 

 last trace of ammonia disappeared. After four months chlo- 

 roform was passed through the tube to kill the ferments. 

 The formation of nitric acid ceased at once., and did not be- 

 gin again until fresh germs, obtained from land possessed of 

 marked nitrifying properties, were sown in the sand. It is 

 thus shown that nitrification may be caused by organized 

 ferments ; and that a sandy and barren soil, if it contain 

 enough lime to neutralize the nitric acid, may serve admira- 

 bly for the purification of sewage. 



