74 GENERAL BIOLOGY 



their action, carbohydrates and fats are reduced to 

 CO2 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, 



O ? 



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 zoogloea 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 HNO 2 + O 2 = 2 HNO 3 



