t 43 ] 



When organic matter decays in very high temperature, the 

 Carbon and Hydrogen may get entirely oxidized into Co 2 and 

 H 2 O, but with limited access of air the oxidation is slow 

 and the formation of bodies which resist decay for a long 

 time is the result. These are found in the lower layers of 

 turfs and in meadows and forests. The humus so formed is 

 of a complex composition. The acids and other organic 

 substances formed are not clearly understood. The common- 

 est are humic acid ( C 20 H 12 O 6 ) Ulmic acid (C 20 H 14 O 6 ), 

 Geic acid (C 20 H 12 O 7 ), Crenic acid and Apocrenic acid. 

 The composition of Crenic and Apocrenic acids, discovered 

 by Berzilius, is uncertain. All these compounds retain am- 

 monia with great tenacity. Humus is also a highly hygros- 

 copic substance tending to keep rocks moist and thus helping 

 their further disintegration by hydration. The generation of 

 CO 2 in humus is profuse and constant. The air of all soils 

 contains a much higher proportion of CO 2 than ordinary 

 atmospheric air, which contains only 4 or 5 parts of CO 2 in 

 10,000 parts, while the air in soils contains from 10 to 250 

 parts of CO a in 10,000 parts. The organic acids and CO 2 of 

 humus assist in the decomposition of minerals. The nitrogen- 

 ous matters of humus are gradually converted into ammonia- 

 salts and nitrates, especially inthe presence of lime and 

 nitrifying bacteria. These salts in their turn assist in disin- 

 tegration. Plants thus have the effect of disintegrating rocks 

 in various ways, both in their live and dead state. Not only 

 limestones but even quartz and other hard silicates are found 

 eaten into by roots of plants. Clubmosses which contain a 

 good deal of A1 2 O 3 ( which is not an essential constituent of 

 every plant), are of great help in disintegrating rocks con- 

 taining alumina. Of agricultural plants, mangold wurzcl and 

 Chukd-Pdlam, containing a good deal of oxalic acid, have a 

 considerable power of assimilating phosphates from the soil, 

 and they have also considerable power of disintegrating 

 rocks. 



