OXIDATION 



103 



and the other, the nitric bacteria, oxidizes the nitrous to 

 nitric acid, giving nitrates with bases. A striking pecuHarity 

 of these two classes of organisms is that they may hve entirely 

 on inorganic food materials, are proto-autotrophic, proto- 

 trophic for oxygen (aerobic) and autotrophic for the other 

 elements. Their carbon is derived from CO2 or carbonates. 

 The importance of such organisms in keeping up the supply 

 of nitrates in the soil can scarcely be overestimated. 



Fig. 72. — Sprinkling filters of the Columbus sewage-disposal plant — 

 devices which provide a good supply of oxygen for the bacteria that 

 oxidize the organic matter in the sewage. 



The oxidation of the H2S, which is formed in the putre- 

 faction of proteins, to free S by the sulphur bacteria and the 

 further oxidation of this free S to sulphuric acid, and of the 

 phosphorus, so characteristic of the nucleins, to phosphoric 

 acid have been referred to. These activities of bacteria are 

 of great value in the soil. Doubtless the commercial "phos- 

 phate rock" owes its origin to similar bacterial action in 

 ages past. 



