“The fæcal matters and other impurities attached themselves to the surfaces of the particles of earth by a kind of cohesive attraction, and in this state were readily attacked by the oxygen of the air. Their organic carbon became carbonic acid, their nitrogen was converted into nitrous or nitric acid, which united with the lime, magnesia, and other basic matters present. Mechanically suspended impurities were arrested as by a sieve, and the water issued from beneath--not indeed fit for dietetic or domestic purposes, but at any rate in a fair state of purity and quite inoffensive to the senses.” (_Vide_ W. Crookes in the discussion on the Sewage Question by Norman Bazalgette, ‘Min. of Proceedings of the Institution of Civil Engineers,’ vol. xlviii. p. 164.) The land thus used as a sewage filter requires constant aëration by being dug over or ploughed, and if this precaution is taken, it is surprising to what a wonderful extent the land will take sewage without becoming what is called “sewage sick.” Clay soils are, however, stated to be ill-adapted for this purpose. The next method, that of mechanical subsidence of the sewage in large tanks, has been attempted in conjunction with irrigation and filtration without much benefit, nor has the fifth method I have mentioned, viz., that of mechanical filtration of the sewage, met with any better result. Artificial filters have been constructed of burnt clay, cinders, coke, charcoal, peat, chalk, gravel, broken stone, sand, spongy iron (this is now being applied very successfully for the purification of water), straw, cocoa-nut matting, wicker-work, and wire gauze of different degrees of fineness of mesh. The late Mr. Odams spent a considerable sum in endeavouring to strain sewage through revolving screens of wire gauze with but little success, and Mr. Bannehr has striven to achieve the same object by passing sewage over oscillating screens of the like material. In all these cases of mechanical filtration, however, the effluent has either not been sufficiently pure or the screens and filters have become clogged and refused to act.[191] The last method that I have mentioned for the disposal of sewage is that of precipitation, or what may be more properly called the chemical treatment of sewage. Precipitation means the production, by the introduction of chemical substances within the body of the sewage, of certain solid compounds, which, in settling, drag down with them the suspended matters in the sewage, together with a small proportion of the polluting matters which are in solution in the sewage, this proportion varying with the quantity of solid matters deposited. The effluent from the tanks in which this precipitation takes place is then allowed to flow direct into a river or stream, or is still further purified by being passed over land or filtered through deep-drained soils.