ABSOIIPTIVK POWEU OF THE SOIL. . 335 



the soil just as a mass of clean wool separates indigo from 

 the liquor of a dye-vat, or as bone-charcoal removes the 

 brown color from syrup. 



Chemical absorptions depend upon tlie formation of 

 new compounds, and in many cases occasion chemical 

 decompositions and displacements in such a manner that 

 while one ingredient is absorbed, and becomes in a sense 

 fixed, another is released from combination and becomes 

 soluble. Brief notice has already been made of the 

 chemical absorption of ammonia by the soil (p. 243). 

 We shall now enter upon a fuller discussion of this and 

 allied phenomena. 



When solutions of the various soluble acids and bases 

 existing in the soil, or of their salts, are put in contact 

 with any ordinary earth for a short time, suitable exami- 

 nation proves that in most cases a chemical change takes 

 place, — a reaction occurs between the soil and the 

 substance. 



If we provide a number of tall, narrow lamp-chimneys 

 or similar tubes of glass, place on the flanged end of each 

 a disk of cotton-batting, tying over it a piece of muslin, 

 then support them vertically in clamps or by strings, and 

 fill each of them compactly, two-thirds full of ordinary 

 loamy soil, which should be free from lumps, we have an 

 arrangement suitable for the study of* the absorptive 

 power in question. 



Let now solutions, containing various soluble salts 

 of the acids and bases existing in the soil, be pre- 

 pared. These sohations should be quite dilute, but 

 still admit of ready identification by their taste or by 

 simple tests. We may emploj^ for example, any or all of 

 the following compounds, viz., saltpeter, common salt, sul- 

 phate of magnesia, phosphate of soda, and silicate of soda. 



If we pour solution of saltpeter on the soil, which 

 should admit of its ready but not too rapid percolation, 

 we shall find that the first portions of li(piid which pasa 



