96 SEWAGE AND ITS PURIFICATION 



between them and the combustion which follows so closely. 

 *' Bacteriolysis " refers to the principal living agents of the 

 changes. " Hydrolysis " signifies a chemical breaking down by 

 combination with the hydrogen and oxygen of molecules of 

 water. Other cases, in which the decomposition takes place 

 without any absorption of water, are grouped under the general 

 term '' fermentation." Sometimes oxidation is simulated, since 

 the organic matter is partly converted into oxidized compounds; 

 but the oxygen is not derived from the air, being that which was 

 originally present in the organic matter or in the water taking 

 part in the reaction. As an example, albumin contains the 

 elements C, H, N, and O in the ratio represented by the 

 empirical formula C8H13N2O3. An anaerobic change, due to 

 hydrolysis, could be expressed thus : 



4C8H13N2O3 + 14H2O = 4N2 + 19CH4 + 1 3CO2 + 2H2. 



Such an ideal change would result in the production of all the 

 gases which are commonly met with in these decompositions, 

 and leave no soluble organic matter for oxidation. Non-nitro- 

 genous substances like cellulose and woody fibre can similarly 

 break down into starch, sugar, etc., and then, in presence of 

 yeast, into carbonic acid and alcohol. In most natural anaerobic 

 changes of this character it is found, however, that there are 

 residual compounds containing nitrogen, of a humus-like char- 

 acter, which are very stable, and resist chemical action.^ In 

 peaty soils they exist in appreciable quantity. Adeney has 

 noticed their formation in his experiments, and in the Exeter 

 septic tank the dark suspended matter is of allied nature. That 

 it does slowly disintegrate is shown from the experiments at 

 Harpenden, where crops have grown on unmanured land for 

 long periods under such conditions that it is difficult to ascribe 

 any other source for their nitrogen. Humus slowly undergoes 

 oxidation to CO2 and nitrate. 



I was one of the first to point out that when changes are 

 brought about by organisms which are facultative anaerobes, 

 the breaking down of gelatine to albumoses, ammonia, pep- 

 tones, etc.,^ is not accompanied by any absorption of oxygen 

 or the formation of any oxidized products, and it is, moreover, 

 obvious that in the natural process of digestion solid foods, 



^ See Chap. XI. ; also the author's paper on " Humus and the So-called Irre- 

 ducible Residue in Bacterial Treatment of Sewage " (British Association, 1901 ; 

 Chemical News, vol. Ixxxiv., p. 149). 



2 See also Selitrenny, Monatsch., x., 908. 



