HUMUS OR VEGETABLE MOLD. 19 



and nitrates. It also changes the disgustingly offensive 

 sulphuretted hydrogen of decaying manure and other or- 

 ganic matter, into sulphuric acid, and thus removes a 

 sometimes intolerable nuisance of barn yards and hog pens 

 to uninitiated passers on the road. 



Humus is another form of carbon, although an impure 

 one, which deserves notice. It is the decomposed remains 

 of vegetable matter which has undergone the slow process 

 of decay a kind of combustion and oxidation in the 

 open air. It exists in swamps in the form of peat and black 

 porous soil ; in woods as a dark spongy mass on the sur- 

 face, covering the lower soil, and wherever a mass of veg- 

 etation has. slowly decayed. The leaf mold so much prized 

 by gardners is chiefly humus. 



When the woody matter of plants, large and small alike, 

 is exposed to moisture and air, it undergoes a slow decom- 

 position, in which oxygen is absorbed. It is in fact pre- 

 cisely similiar in its operation and effects to a slow com- 

 bustion or charring, although accompanied by so small a 

 quantity of heat as to be almost imperceptible. With the 

 absorption of oxygen and its combination with a portion 

 of the carbon, carbonic acid is formed. Some of the oxy- 

 gen also combines with hydrogen and forms Avater ; but as 

 the hydrogen is taken first, a large portion of carbon re- 

 mains and the mass gradually assumes a dark brown or 

 black color and becomes what is termed : "vegetable mold.'* 

 To this crumbled porous substance the term humus is ap- 

 plied. It contains various acids, as geic, ulmic and humic 

 acids. This class of substances is of great importance in 

 agriculture, as by their decomposition they yield up car- 

 bonic acid to plants, and have the power of absorbing and 

 retaining ammonia to be yielded up for the same purpose. 



Carbonaceous matter gradually accumulates in soils that 

 are always covered with vegetation, as in forests, pastures 

 and prairies. This is a conclusive proof that the carbon 

 of it is derived from the atmosphere, and that growth is 

 more rapid than decay. When land is brought under cul- 

 tivation this carbonaceous matter is consumed by the crops 



