EARTH-WORMS VEGETATION. 47 



colour. The change in volume, accompanied very 

 often by falling to powder, .aids in breaking up the 

 rock. Air in the presence of water also oxidises 

 metallic sulphides, c.(j., iron sulphide, and so pro- 

 duces disintegration in rocks containing such com- 

 pounds. The carbon dioxide of the air, acting with 

 water, is necessary for many of the chemical changes 

 described as being due to water. 



3. Earth-worms play an important part in the formation 



and modification of soil. Darwin" has shown that they 

 bring portions of the subsoil to the surface, render the 

 soil more porous and pulverulent, and aid greatly in the 

 conversion of vegetable refuse, leaves, &c., into humus. 

 This they do partly by drawing dead leaves, &c., into 

 their holes, and partly by actually passing the vege- 

 table matter and the soil containing it through their 

 bodies. This matter is ejected and deposited on the 

 surface of the ground at the rate of about 10 tons to the 

 acre per annum, burying all small objects, like stones, 

 fragments of bones, &c., to a depth which increases at 

 an average rate of about T ^ of an inch per year. The 

 number of earth-worms to the acre is estimated to 

 average over 25,000, so that the part played by them 

 must be most important. 



4. Vegetation. This acts in several ways : - 



(i.) Mechanically. The roots of a plant penetrate the rocks 

 or soil, rendering them porous, and so admitting air 

 and watsi\ Plants also tend to prevent the access 

 of sunlight and air to the surface of rocks, and so 

 favour their denudation by moisture. 



(ii.) Chemically. 



(a) During life, by the solvent action of the roots, which 



secrete an acid liquor capable of dissolving many 

 constituents of rocks. 



(b) After death, by decaying and producing both acids of 



the type of humic acid and also carbon dioxide, 



* " Vegetable Mould and Earth-worms," 1881. 



