102 PHYSIOLOGICAL ACTIVITIES 



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. 



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.^ 



A purposeful utilization of the oxidizing power of bacteria 

 is in "contact beds," "sprinkling filters" and "aerated 

 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. 67). 



Fig. 68. — One of the University hot-beds. 



PRODUCTION OF HEAT. 



A direct result of the oxidizing action of bacteria is the 

 production of heat. Under most conditions of bacterial 

 growth this heat is not appreciable. It may become well 

 marked. The "heating" of manure is one of the commonest 



^ Dr. Green (1. c.) has also isolated an organism which causes some deterio- 

 ration of cattle dips by oxidizing arsenites to arsenates. 



