(3) Short, direct, clean, well-flushed, gas-tight branch pipes to connect receptacles with vertical pipes. (4) _Disconnection_ of the house sewer from the house pipes by the main trap on house drain, and disconnection of house from the house pipes by traps on all fixtures. (5) _Ventilation_ of the whole system by the fresh-air inlet, vent pipes, and the extension of all vertical pipes. =Definitions.=--The _House Drain_ is the horizontal main pipe receiving all waste water and sewage from the vertical pipes, and conducting them outside of the foundation walls, where it joins the house sewer. The _Soil Pipe_ is the vertical pipe or pipes receiving sewage matter from the water-closets in the house. The _Main Waste Pipe_ is the pipe receiving waste water from any fixtures except the water-closets. _Branch Soil and Waste Pipes_ are the short pipes between the fixtures in the house and the main soil and waste pipes. _Traps_ are bends in pipes, so constructed as to hold a certain volume of water, called the water seal; this water seal serves as a barrier to prevent air and gases from the sewer from entering the house. _Vent Pipes_ are the special pipes to which the traps or fixtures are connected by short-branch vent pipes, and serve to ventilate the air in the pipes, and prevent siphonage. The _Rain Leader_ is the pipe receiving rain and storm water from the roof of the house. =Materials Used for Plumbing Pipes.=--The materials from which the different pipes used in house plumbing are made differ according to the use of each pipe, its position, size, etc. The following materials are used: cement, vitrified pipe, lead; cast, wrought, and galvanized iron; brass, steel, nickel, sheet metal, etc. _Cement and Vitrified Pipes_ are used for the manufacture of street and house sewers. In some places vitrified pipe is used for house drains, but in most cities this is strongly objected to; and in New York City no earthenware pipes are permitted within the house. The objection to earthenware pipes is that they are not strong enough for the purpose, break easily, and cannot be made gas-tight. _Lead Pipe_ is used for all branch waste pipes and short lengths of water pipes. The advantage of lead pipes is that they can be easily bent and shaped, hence their use for traps and connections. The disadvantage of lead for pipes is the softness of the material, which is easily broken into by nails, gnawed through by rats, etc. _Brass, Nickel, Steel_, and other such materials are used in the manufacture of expensive plumbing, but are not commonly employed. _Sheet Metal_ and _Galvanized Iron_ are used for rain leaders, refrigerator pipes, etc.