1867.] and the Cholera. 319 



remaining three schemes have not yet been sufficiently matured to 

 permit of their treatment from a chemical point of view. The first 

 two colossal schemes are truly worthy of this age of engineering 

 triumphs, and of the great city on behalf of which they are pro- 

 jected. Before L the advantages of such magnificent undertakings 

 can be appreciated, however, it is necessary first to consider the 

 chemical quality of our present supply, and then to compare it with 

 that of the water which the new sources are capable of furnishing. 



The impurities present, or liable to be present, in the waters 

 supplied to the metropolis may be thus classified : — 



Previous sewage Present sewage 



Innocuous. Soap destroying. contamination. contamination. 



Common salt. Salts of lime and Nitrates, nitrites, Nitrogenous organic 

 magnesia. and ammonia. matter. 



The most important things to be ascertained respecting the im- 

 purities in water used for domestic purposes are — first, the amount of 

 sewage contamination previous and present ; and secondly, the quan- 

 tity of soap-destroying materials. The organic matters, containing 

 nitrogen which occur dissolved in water, are chiefly, if not entirely, 

 of animal origin, being derived either from sewage or manured 

 land ; be their origin, however, animal or vegetable, no distinction 

 founded upon their source can be drawn between their respective 

 noxious qualities. After admixture with spring or river water, 

 these noxious organic matters undergo slow oxidation, by which 

 they are finally resolved into comparatively innocuous mineral com- 

 pounds : their carbon is converted into carbonic^ acid gas, and their 

 hydrogen into water, and these products can no longer be identified 

 in the aerated waters of the river or spring ; but the nitrogen is 

 converted into nitrons and nitric acids, which, combining with the 

 bases contained in most waters, remain dissolved, and constitute a 

 record of the sewage or other analogous contamination to which 

 the water has been subject. With certain corrections presently to 

 be mentioned, the analytical determination of the nitrogen contained 

 in these salts and in the form of ammonia writes, as it were, the 

 history of the water, as regards its contact with decomposing animal 

 matter. Such previous organic contamination may be conveniently 

 expressed in parts of average filtered London sewage, which, if thus 

 completely oxidized in a river, would yield a like amount of nitrogen, 

 in the form of nitrites, nitrates, and ammonia. For this purpose, 

 average filtered London sewage may be taken as containing ten 

 parts of combined nitrogen in 100,000 parts, as deduced from the 

 numerous analyses of Way, Hofmann, and Witt. The number so 

 obtained as the previous sewage contamination of a water requires, 

 however, a correction, since rain-water itself contains combined 

 nitrogen in the forms of ammonia, nitrite of ammonia, and nitrate 



