The interior of all stone walls, and in fact all masonry walls, will show condensation of moisture over the interior surface, and if they are plastered directly on the interior the decorations will be ruined by the collection of so much water. The cause of this condensation is the same as that which forms sweat on the exterior surface of a glass of cold water. In order to eliminate this disagreeable feature, all masonry walls are furred on the interior before the lath and plaster is applied. The furring makes an air space between the wall and the plaster, and all dampness is prevented from penetrating to the interior surface of the plaster. To further increase the damp-proof qualities of a masonry wall they are sometimes built hollow, as, for example, the hollow brick wall, or the hollow terra-cotta tile wall. This air space also serves as an insulator for heat, preventing the escape of heat from the interior of the building in winter and the penetration of it into the structure in the summer. [Illustration: 10. Furring Strip] The commonest type of furring is the 1-inch by 2-inch wooden strip, nailed to the joints of the masonry or to wall plugs inserted in the joints. Metal furring strips are also extensively used, and occasionally hollow terra-cotta furring blocks. _Brick House_ Like the stone house, the brick dwelling is one of the oldest types in this country. Examples of early brick houses show a taste for good brick, which later died out on account of the introduction of the first American machine-made bricks. These early machine-made bricks were extremely ugly, due to their perfection of geometric shape, smoothness of surface, and monotony of red color. Later improvements in the manufacture of brick have released this material for extensive artistic use. The surface was given a varied color and texture, and the form was not made so machine-like. To-day we have a variety of bricks which range in colors through reds, yellows, buffs, greens, blues, and even dark violets. Textures of wire-cut bricks are rich and varied, and, if properly handled, can produce the very finest architecture. [Illustration: 11. Running Bond and method of Bonding 14. Flemish Bond] [Illustration: 12. English Bond] [Illustration: 13. Dutch Bond or English Cross Bond] BONDING AND CONSTRUCTION