290 IMPROVEMENT OF THE SOIL 



the air, and in putrefaction the air is not needed at all, but 

 the process is often promoted by excluding oxygen altogether. 



The carbon, oxygen, hydrogen, nitrogen and other substan- 

 ces of which manures are composed, form themselves into 

 several new compounds which, without attem])ting to point 

 out all the changes which take place, finally result in the for- 

 mation of several bodies already considered. 



These substances depend upon the conditions under which 

 the changes take place. If the changes occur in the earth, 

 they give rise to fossil coal. If they take place near the sur- 

 face of the ground, or in the open air, which is the case un- 

 der consideration, they give rise to the substances found in 

 vegetable mould, p. 215, and in the atmosphere. 



Theories of the changes which take place in fermenting 

 dung-heaps^ in the process of decomposition. 



(1) Carbonic acid in large quantities is formed. This re- 

 sults either from the direct combination of the oxygen of the air 

 and of water with the carbon of the plant, or from the union 

 of the oxygen of the air with the hydrogen of the plant to 

 form water, while the carbon and the oxygen of the vegeta- 

 ble is evolved in the form of carbonic acid. By this process, 

 a large portion of the carbon is abstracted in a gaseous form, 

 and unless alkalies or earths are present to absorb it, passes 

 off into the atmosphere. 



(2) Water is formed at the same time with carbonic acid. 

 The hydrogen is furnished from the vegetable matter, and the 

 oxygen from the air. The quantity of water thus annually 

 formed in the soil, is probably greater than that which falls 

 in rain on the same surface. In dung-heaps, the quantity of 

 water formed is far greater than in the soil. In these pro- 

 ducts there is a genuine process of decay. 



(3) As all parts of the heaps are not exposed alike to the 

 action of the air, the hydrogen and the carbon combine and 

 form carbureted hydrogen. The hydrogen is fiirnished either 

 from the vegetable itself, or from the water which is known 



