OXIDS OF CARBON, CARBONATES, ETC. 



119 





with fermentation, are uninterrupted, the organic mate- 

 rials are decomposed until carbon dioxid, water, ammonia 

 gas, and hydrogen sulfid are ob- 

 tained as the final products. 

 When such changes take place, 

 the mineral matter combined and 

 associated with the organic mat- 

 ter is left as non-volatile products. 

 In economic agriculture, it is the 

 aim to conserve and return to the 

 soil these essential elements, as 

 nitrogen, potassium, phosphorus, 

 and calcium, which are fre- 

 quently unavailable or present in 

 scant amounts in soils, so that 

 the fertility will not be impaired. Fi s- 5.-Decay of wood. 

 The elements present in plant and animal bodies pass 

 through a cycle of chemical changes ; they are never lost 

 to nature, but appear in different chemical compounds, as 

 exemplified by the law of indestructibility of matter. 

 A. Elements of plant growth in soil and air. 



B. Elements from soil and air elab- 

 orated into plant tissue. 



C. Elements in plant tissue elab- 

 orated into animal tissue. 



The elements in either plant or ani- 

 mal bodies may pass back to A, and 

 then pass again through the same 

 cycle of chemical changes. 



