THE MATERIALS USED IN THE CONSTRUCTION OF A HOUSE. In this country walls of houses are usually built of brick, stone, timber, or concrete, of which the first two are the most important. Timber is, owing to its inflammability, only allowed to be used in towns under special restrictions. Bricks and stones are bonded together and imbedded in mortar or cement. There are several kinds of =bonds= in brickwork, of which the strongest is the _English_. This consists of alternate courses of “headers” and “stretchers,” the former being bricks carried through the wall from face to back, the short end showing on the face, and the latter bricks laid lengthwise along the face of the wall. Hence the wall is held together in every direction. A _Flemish bond_ consists of alternate headers and stretchers in the same course. It is used where a specially smooth wall is desired, but is not so strong as the English bond. =Bricks= are generally of a uniform size, of 9 inches in length by 4½ in width and 2-3∕4 inches in thickness. Those bricks which are heaviest and hardest are generally the most durable; bricks of good quality when knocked together give a clear ringing sound. The relative conductivity for heat of brick as compared with other materials, is shown in the following table, from Galton, which gives the units of heat transmitted per square foot per hour by a plate 1 inch thick, the two surfaces differing in temperature 1° Fahr.:— _Stone—ordinary free stone_ 13·68 _Glass_ 6·6 _Brickwork_ 4·83 _Plaster_ 3·86 _Fir planks_ 1·37 _Brick dust_ 1·33 It is evident that in this respect, brick walls compare very favourably with stone walls, and are much more economical of heat. Increased conductivity of a material may be counteracted by increased thickness. Brick is very porous, as shewn by its power to absorb moisture. A good brick can absorb from 10 to 20 per cent. of its weight of water; while good granite only takes up 1∕2 per cent., sandstone usually from 8 to 10 per cent., marble only a trace, and Portland limestone 13½ per cent. Being porous, brick allows the passage of a considerable amount of air, unless its pores are occupied by moisture. The following table, from Galton, shews the number of cubic feet of air which every hour pass through a square yard of wall-surface of equal thicknesses, built of the following materials, there being a temperature of 72° Fahr. on one side the wall, and of 40° on the other:— _Wall built of brick_ 7·9 _cubic feet_. „ _quarried limestone_ 6·5 “ „ _sandstone_ 4·7 “ „ _limestone_ 10·1 “ „ _mud_ 14·4 “