Flue lining must withstand rapid fluctuations in temperature and be resistant to the action of ordinary flue gases. The shapes used as flue lining should be of fire-clay, with shells not less than five-eighths of an inch thick, and should be vitrified. As a safeguard against over-burning and brittleness, the lining should be tested by submersion in water at room temperature for 24 hours, during which a quantity of water weighing more than 3 percent of the dry weight of the lining should not be absorbed. Place each length of flue lining in position, setting it in cement mortar with the joint struck smooth on the inside, and then lay the brick around it. If the lining is slipped down after several courses of brick have been laid, the joints cannot be filled and leakage is almost sure to result. Fill any spaces between the lining and the brickwork completely with mortar, especially if the round type of flue is used. The lower section of flue lining, unless resting on solid masonry at the bottom of the flue, should be supported on at least three sides by brick courses projecting to the inside surface of the lining. When laying brick and lining, it is advisable to draw up a tight-fitting bag of straw as the work progresses so as to catch material that might fall and block the flue. Where offsets or bends are necessary in lined flues, tight joints can be made by mitering or cutting equally the ends of abutting sections (fig. 8, _B_). This can be done if a cement sack of damp sand is stuffed firmly into the lining and a sharp chisel is tapped with a light hammer along the line where the cut is desired. If the cutting is done after the lining is built into the chimney, the lining may be broken and fall out of place. The hole for the thimble can be cut the same way when a special thimble section is not used. The linings commonly used are rectangular or round. Rectangular linings are better adapted to brick construction than round linings, but the latter are considered more efficient. The sizes commonly used are indicated in table 2. =Wall Thickness= Walls of chimneys not more than 30 feet high when lined should be 4 inches thick if of brick and reinforced concrete, 8 inches if of hollow building units, and 12 inches if of stone. Linings may be omitted in chimneys having walls of reinforced concrete at least 6 inches thick or of unreinforced concrete or brick at least 8 inches thick, although lining is desirable in the case of brick construction. Also the outside wall of a chimney exposed to the weather is best made at least 8 inches thick.