74 GENERAL BIOLOGY 



their action, carbohydrates and fats are reduced to 

 CO 2 and H 2 O, and proteins are dissolved, by a long 

 chain of reactions, among other things into NH 3 and 

 CO 2 . The urea excreted by animals goes the same 

 way. By this means the rotation of -the elements 

 through organic and inorganic nature is hastened and 

 facilitated . The ammonia in the soil is taken in hand 

 by another group of bacteria, the nitrite bacteria, 





FIG. 25. Putrefactive bacteria of various sorts: A, Bacillus urea, 

 the agent that ferments urea into ammonium carbonate and water and 

 eventually into carbon dioxide and ammonia ; B, Bacillus subtilis, a putre- 

 factive organism commonly found in hay infusions ; C, the same in the 

 inactive zooglcea condition ; D, Spirilla, sp., from hay infusion ; E, Bac- 

 terium " termo," from fermenting infusion of peas. 



which oxidize it to nitrous acid (HNO 2 ). This 

 combines with potassium or ammonia in the soil to 

 form potassium or ammonium nitrite. Another 

 group then further oxidizes the nitrite into nitrate, 1 

 and makes it available for plant food. There is 

 a similar cycle for the sulphur and phosphorus, 

 but these elements, although absolutely essential 

 for organic life, are but a small fraction of the bulk 



1 I. 2 NH 3 + 3 O 2 = 2 HNO 2 + 2 H 2 O. 

 II. 2 HN0 2 + 2 = 2 HN0 3 



