[19] The bacilli of typhoid can live in crude sewage (Klein), but only for a very short period. When sewage is diluted with large quantities of water the case is very different. _Bacillus coli_ flourishes in sewage. [20] Annual Report of the Medical Officer of the Local Government Board, 1897-98, p. 210. [21] John Tyndall, F.R.S., _Floating Matter of the Air_. [22] Flügge has lately attempted to demonstrate that an air current having a velocity of four metres per second can remove bacteria from surfaces of liquids by detaching drops of the liquid itself. [23] Hewlett and Thomson graphically demonstrated the bactericidal power of the nasal mucous membrane by noting the early removal of _Bacillus prodigiosus_, which had been purposely placed on the healthy Schneiderian membrane of the nose. [24] _Pathological Society of London, Transactions_, 1897. [25] _Annali d'Igiene Sperimentale_, vol. v. (1895), fasc. 4. [26] _Public Health_, vol. x., No. 4, p. 130 (1898). [27] Flügge, _Grundriss der Hygiene_, 1897. [28] _Zeitschrift für Hygiene_, vols. xxiv.-xxvi. [29] _Annales de Micrographie._ [30] E. A. Schäfer, F.R.S., _Text-book on Physiology_, vol. i., p. 312. [31] The unorganised ferments are frequently otherwise classified than as above, not according to the locality, but according to the function. The chief are these:--_amylolytic_, those which change starch and glycogen (amyloses) into sugars, _e. g._, ptyalin, diastase, amylopsin; _proteolytic_, those which change proteids into proteosis and peptones, _e. g._, trypsin, pepsin; _inversive_, those which change maltose, sucrose, and lactose into glucose, _e. g._, invertin; _coagulative_, those which change soluble proteids into insoluble, _e. g._, rennet; _steatolytic_, those which split up fats into fatty acids and glycerine, _e. g._, steapsin. [32] A chemical change obtained by the action of sulphuric or some other acid, or by the influence of _diastase_. [33] _Bacteriology and Infective Diseases_, Appendix. [34] E. C. Hansen, _Studies in Fermentation_ (Copenhagen), p. 98. [35] _Proc. Royal Soc. of Edin._, xxxvii., pt. iv., p. 759. [36] E. A. Schäfer, _Text-book of Physiology_, vol. i., p. 25 (W. D. Halliburton). [37] "Denitrifying" means reducing _nitrates_. [38] R. Warington, M.A., F.R.S., _Journ. Roy. Agricultural Soc. Eng._, series iii., vol. viii., pt. iv., pp. 577 _et seq._ [39] The saltpetre beds of Chili and Peru are an excellent example of the industrial application of these facts. Nitrates are there produced from the fæcal evacuations of sea-fowl in such quantities as to form an article of commerce. A like form of utilisation of the action of these bacteria was once practiced on the continent of Europe. Economic application is also seen in the treatment of sewage referred to elsewhere.