I 



CHEMICAL CHANGES 113 



due to the enzyme, " cellulase " or cytase. One of the causes 

 of the formation of humus colouring matters is the production 

 of pigments from cellulose by the action of bacteria and fungi, 

 such as the " Bismarck-brown " cladothrix found in dust-heaps. 

 Anaerobically, in absence of nitrates, CO 2 and hydrogen or 

 methane, together with acetic and butyric acids, are formed ; 

 whilst in presence of a nitrate the cellulose is decomposed by 

 denitrifying bacteria, with formation of nitrogen, CO2, and 

 water. 



6. Fermentation of Other Carbohydrates. — Starch, different 

 sugars, and gummy substances undoubtedly enter into sewage, 

 but their hydrolysis is so rapid that very little trace of them 

 is found after a short period. The ferments in human faeces 

 allied to diastase and invertase were investigated in 1887-1888 

 by O. Loew,^ Pavy," and R. von Jaksch,^ and later by 

 MacConkey* and others. Those fermentations, such as the 

 alcoholic, which are usually occasioned by higher fungi like 

 yeasts and moulds, do not present themselves distinctly, j 

 although the B. coli communis is capable of fermenting sugars | 

 and producing lactic acid, alcohol, and a volatile acid. The ^ 

 changes are either lactic, from B, acidi lactici, or butyric, from 

 Clostridium butyricum or B. butyricus (both anaerobic), and 

 give, besides the respective acids, carbonic acid, hydrogen, and 

 water. 



7. Decomposition of Fats, — Soapsuds and greasy matters 

 occasion considerable trouble in the mechanical treatment of 

 sewage. At Bradford ftie refuse of wool-scouring has been the 

 chief difficulty for years, and has led to several special pro- 

 cesses (see later, Chap. XIV.). 



In a bacterial tank the grease is first emulsified by the 

 ammonia. There are several bacteria that attack fats in 

 presence of nitrogenous substances,^ breaking them up into the 

 simpler acids of the fatty series, like acetic and butyric, which 

 in their turn are finally resolved, as on p. 109. Mucor mucedo, a 

 common mould, resolves glycerides into their constituent parts, 

 and at the same time produces a secondary change, giving 

 bodies of an aldehydic character,*^ which would be subsequently 

 oxidized. Many other moulds also act on fats, notably the 



^ Pfiiiger's ArcJiiv., xxvii., 203. 



• Maly'sjahresb., xiv., 294. -^ Zeit. phys. Chem., xii., 116. 



* Journal of Hygiene, 1905, v., 333. 

 \ Sommaruga, ZfizYs. Hyg., xvii. 441. 

 '^ "Fat-consuming Organisms," Konig, etc. {Analyst, January, 1902, p. 10); 



also Hanns and Stocky {ibid., 1901, p. 15). 



8 



