_a_—Cavity. _b_—Tie. _c_—Floor-joist. _d_—Wall-plate. _e_—Concrete foundation of wall.] The _thickness_ of the walls of a house requires to be sufficient to ensure stability, to keep out the damp, and to prevent a too rapid loss of heat from the walls. The relative merits of the different materials employed for these purposes have been already considered. A thin-walled house is hot in summer, and cold in winter. The upper stories of houses are often built with too thin walls, the result being chilly bedrooms. A single-brick wall (9 inches thick) will rarely keep out the weather effectually, and frequently a brick-and-a-half wall (14 inches thick) is insufficient for this purpose. The bricks should be so interlaced as to “bond” or tie the wall together in all directions. The strength of walls may be increased by the introduction of hoop-iron between the courses of brickwork. In the construction of fire-places and chimneys, it is important to avoid the proximity of timber and wood-work to the inside of flues, as this is a common cause of fires. Inside Coverings of Walls. =Plaster= is made of lime mortar, or cement mortar; the former is generally preferred for domestic dwellings because it remains porous and moisture does not condense on it. In houses built by speculative builders, the plaster commonly used consists of a mixture of lime with road scrapings. The result is a composition which unless supported by the wall-papering, is soon damaged. Ordinary plaster consists usually of three layers. The first is laid on with a mixture of about equal parts of lime and sand with long ox-hairs if required for ceilings. The second coat consists of slaked lime, mixed to the consistency of cream. The last or setting coat consists of a thin layer of slaked lime called plasterers’ putty. Some plaster of Paris (gypsum) may be added, to ensure rapid setting, but it should only be used in small quantities. For the internal plastering of rooms _serapite_ (a form of cement) is now commonly employed. This is not so absorbent as mortar, but is sufficiently so to prevent condensation of moisture on the walls. Its chief advantage over plaster is that it hardens quicker and is smoother, and can be used in a single thin layer. This, however, diminishes the impermeability of ceilings for sound. Keene’s cement and Parian cement are mixtures of calcined gypsum and other substances; Keene’s cement being the hardest, and capable of receiving a high polish. Selenitic cement contains a small proportion of plaster of Paris ground along with lime. Lime may also be selenised by the addition of any other sulphate, or of sulphuric acid. The presence of the sulphate causes the lime to set rapidly. Selenitic cement is useful in plastering, as a backing of cements, such as Parian.