Of the various phenomena of oxidation due to bacteria, the formation of nitrites and nitrates has the greatest practical importance, since it is by this means that the ammonia which results from the decomposition of animal and vegetable tissue and waste products is again rendered available to green plants as food in the form of nitrates. Practically all the nitrates found in nature, sometimes in large quantities, are formed in this way. There are two distinct kinds of bacteria involved. One, the nitrous bacteria, oxidizes the ammonia to nitrous acid which forms nitrites with bases, and the other, the nitric bacteria, oxidizes the nitrous to nitric acid, giving nitrates with bases. A striking peculiarity of these two classes of organisms is that they may live entirely on inorganic food materials, are proto-autotrophic, prototrophic for oxygen (aërobic) 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. [Illustration: FIG. 73.--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 putrefaction 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 “phosphate rock” owes its origin to similar bacterial action in ages past. The oxidation of H₂S to free S may be an explanation of the origin of the great deposits of sulphur which are found in Louisiana and along the Gulf coast. These deposits occur in the same general regions as natural gas and oil. The sulphur might have been derived from the same organic material carried down by the Mississippi which yielded the oil and gas.[15] A purposeful utilization of the oxidizing power of bacteria is in “contact beds,” “sprinkling filters” and “aërated sludge tanks” in sewage disposal works. In these instances the sewage is thoroughly mixed with air and brought in contact with large amounts of porous material so as to expose an extensive surface for oxidation (Fig. 73). [Illustration: FIG. 74.--One of the University hot beds.] PRODUCTION OF HEAT.