CHEMICAL CHANGES 97 



both nitrogenous and non-nitrogenous, are digested in the 

 stomach and intestines before the products are absorbed by the 

 blood, and so rendered useful by oxidation. 



The amount of oxygen required to render inoffensive the 

 substances occurring in sewage depends on the species of the 

 bacteria which are acting, as they determine whether the result 

 should be a complete burning to CO2, H2O, and N, or a partial 

 decomposition to equally harmless compounds like NH3 and 

 CH^. There are a number of intermediate products ; thus, 

 Streptococcus longus liquefies fibrin to ammonia, methylamine, 

 trimethylamine, tyrosine, leucine, fatty acids, succinic acid, 

 collidine, and peptones.^ 



Elastin, with anaerobic organisms, evolves CO2, H, CH^, 

 and N, whilst the sulphur remains in solution as mercaptan, 

 and is not evolved as HgS.^ Grass similarly evolves CO2 and 

 N under the action of B. subtilis and other organisms. B, my- 

 coides also acts upon the carbohydrates in grass, ferments 

 glucose to inactive lactic acid, and hydrolyses cane-sugar, 

 maltose, and glycogen.^ Pakes and Jollyman^ describe an 

 apparatus for examining the gas generated by bacteria, and 

 have specially investigated B. pyocyaneus and B, coli communis^ 

 which produce CO^ and H, or N if nitrate be present. 



The gases found in the septic tank at Exeter are as follows : 



k Per Cent. By Volume. \ 



\ CO2 0-3 ... 0-6 \ 



' CH4 203 ... 24-4 ' 



H 



N ...' 



lOO'O lOQ-Q 



According to Wood and Wilcox, similar gases are produced 

 in the manufacture of leather by Bacterhim furfuris, which does 

 not attack cellulose, but only starch and nitrogenous matter. 

 A sample of the gas contained in per cents. : CO^ and traces of 

 HjjS, 25*2; O, 2*1; H, 467; N, 26*0; while formic, acetic, 

 butyric, and lactic acids were produced : these in sewage would 

 combine with ammonia. Much of the CO2 dissolves in the 

 water, as does also the ammonia formed, whilst the hydrogen, 

 .J from its easy diffusibility, escapes from the tank more rapidly 

 than the heavier gases. As to H2S, see p. 114. 



^ Emmerlich, Ber., 1897, xxx., p. 1863. 



2 Zoja, Zeit. Pliysiol. CJiem., 1897, xxiii., 236. 



3 Emmerlich, Ber., 1897, xxx., p. 1896. 



^ Journal of the Chemical Society, 1901, pp. 322, 459. 



