24. =Pipes= are =tarred= by being heated to 500° Fahr. and then dipped perpendicularly into a hot bath of coal-tar pitch mixed with a small proportion of heavy coal-oil. This coating prevents corrosion, or at least greatly retards it. 25. =Wrought-iron pipes=, with screw joints, are used by the Durham House Drainage Company, the joints being gas-tight. They have the advantage of being very durable and impervious to gases, but the disadvantage of being too permanent. If it is desired to change the position of a fixture, or to connect a new one, it is easy to break a hole in a cast-iron pipe, or to remove a portion of it, but where wrought-iron pipe is used this is almost impossible. The Durham system also costs about twenty-five per cent more than the common one, and it is not strictly correct to say that its use renders a house absolutely secure against leakage of sewer-air, because it does not do away with lead branch-pipes, in which frequent leakages occur, and all irregular portions of pipe, with branches, etc., in this system are made of cast-iron with calked joints, as in the ordinary system. The =water-test= is applied by plugging the pipe carefully, and closing all openings below a certain point, and then filling it with water to that level. If there is any leakage, the water-level will gradually sink. If the pipes are tight, it will remain the same. =Peppermint-test.= (See hereafter.) 26. Oakum is packed into the space between the spigot-end of one pipe and the hub of the other, and driven against the bead above mentioned. This packing is called a =gasket=. Melted lead is poured in on top of the oakum, and, when it has cooled, is driven in tight with a calking iron and mallet. When ordinary pipe is used, the calking will sometimes split the hub, and this furnishes an argument for the general use of extra-heavy pipe. =Iron-filings=, etc., making what is called a =rust-joint=. 27. =Ferrules=, or =sleeves=, are used because lead pipes are too soft and yielding to withstand the calking. Lead can not be soldered to iron, and therefore brass or copper has to be used. If iron ferrules are used, the lead pipe must be turned up on the outside far enough so that its edge shall be covered by the melted lead used in calking, so as to prevent leakage of gas, as hereafter described. (See “Bad Odors,” 1, _b_.) 28. =Wiped-joints= are those always used for lead pipe, and are indicated by a raised ring of solder surrounding the pipe. 29. A =trap=, with a good water-seal, is an effective barrier against the passage of sewer-air or particles of organic matter. The experiments of Carmichael, confirmed by Wernich and Pumpelly, have amply demonstrated that sewer-air passes through the water-seal in such minute quantities that it can not possibly harm any one, and that solid particles of matter can not pass at all. =Separate traps=, except, etc. (See “Bad Odors,” 1, _d_.) 33. =Siphonage=. (See “Bad Odors,” 1, _d_ 37.)