CHAPTER V 



CHEMICAL CHANGES PRODUCED BY BACTERIA 



Hydrolysis and oxidation — Nature and order of the reactions- 

 Symbiosis and antagonism — Enzymes — Classes of transforma- 

 tions — Utilization of gases produced — Sources of energy — 

 Nitrosification, nitrification, and denitrification. 



Some thirty-five years ago trials of upward filtration (Chap. XL) 

 in the place of chemical precipitation gave such satisfactory 

 results that it is difficult to understand w^hy chemical treatment 

 was almost universally adopted. In slow upward filtration of 

 sewage the arrested suspended matter slowly disappears just in 

 the same way as when the solids, after being removed by strain- 

 ing or by chemical precipitation, subsequently disappear when 

 buried in the soil. Similar changes take place in mud-banks in 

 estuaries, below the surface of the water, and the conversion of 

 organic matter of vegetable or animal origin at the bottom of a 

 stagnant pool into harmless gases is of the same nature. Such 

 transformations formerly almost escaped attention, yet are as 

 important as those which take place in the presence of light 

 and air. In nearly all cases of destruction of organic matter 

 preliminary disintegration takes place before the final oxidation 

 of the elements. Solid organic matter capable of undergoing 

 change can only be oxidized by air directly on its surface, 

 whereas in a rotten apple or cheese changes take place beneath 

 the surface, which pave the way for oxidation. Similarly, 

 organic matter in solution seldom oxidizes directly to its final 

 oxidation products, but passes through intermediate conditions 

 until the complex organic forms are resolved into others of 

 more simple structure, and these are subsequently burnt up to 

 the stable oxidized compounds — water and carbonic acid. 



The older terms for these phenomena — "decay," "putre- 

 faction," "eremacausis"^ — did not sufficiently differentiate 



^ From vpi/xa, quietly ; /caOcrts, burning. 

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