After a variable interval urine and fæces begin to decompose, ammonia and fœtid gases being disengaged in large quantities. Urea the chief constituent of urine is decomposed into carbonic acid and carbonate of ammonia. Thus CH₄N₂O + 2H₂O = (NH₄)₂CO₃ In addition to the excreta, house-slops have to be got rid of, and “dust.” =House-slops= vary greatly in quantity, but probably amount to as much as sixteen gallons per head daily. They consist chiefly of the water used in cooking and washing and for baths. It would be a mistake to suppose that only urine and fæces need careful disposal. There are masses of decaying epithelium from lavatories and baths, organic matters from soiled apparel, and various organic matters from culinary operations, all of which may cause serious nuisance unless promptly disposed of. The =Dust= consists chiefly of the ashes from fires; but the dust-bin also forms a favourite refuge for kitchen refuse, composed of various animal and vegetable matters, as well as for broken pots and tins. It is dealt with apart from the house-slops and excreta, except in certain dry methods of disposal of sewage. Two chief plans of getting rid of the sewage have been proposed, though there are many varieties of these. They are— 1. The Water Method, and 2. The Dry Methods. For towns the water carriage of sewage is indispensable, and in this chapter we shall confine ourselves to the part of this system which relates to the =Drainage of the House=. The chief sanitary appliances of a house, which empty their contents into the drain and thence into the street sewer, are—(1) Rain-water Pipes; (2) Bath-room and Sink-pipes; (3) Water-closets; (4) Soil-pipes; and (5) The House Drain. We will consider these in detail. =Rain-water Pipes= collect the water from the roof by means of gutters, and carry it down to the house drain, except in the few cases in which the rain-water is collected for use. The rain-water or stack pipe was formerly joined directly at its base with the underground drain. This was evidently bad, because the upper end of the pipe was frequently near windows, and foul gases from the drain might be conducted by it into the house. It is equally objectionable to connect the rain-water pipe with the soil pipe and for the same reasons as above. _The general rule with regard to all pipes carrying away water from the house, with the sole exception of the soil-pipe, is that they must be disconnected from the underground drain and discharge into the open air over a gulley-trap._ This rule applies to rain-water pipes, and to waste pipes from baths, lavatories, sinks. It does not apply to the soil-pipes leading from water-closets and slop-sinks, which must discharge directly into the drain.