MATERIALS FOR BUILDING. In some of the previous articles reference has been made to the conditions which should regulate the application of particular building materials [Aestosrnemio INFLUENCE; FOUNDATIONS; MASONRY]; but the leading principles connected with their selection, and the mode of their applications, are of such paramount importance to all parties connected with this branch of the constructive arts, that it appears desirable to allude, in general terms at least, to their nature. The manner in which this will be done will be by noticing, in the usual order, the materials used in the various details of a building.
External walls (which are mainly required to resist the transmission of inoisturo from the ground upwards, or from the exterior to the interior of a building, and to maintain an equable temperature in the interior if possible,) may be constructed of either stone, brick, or wood, according to local circumstances ; provided the requisite precautions be taken to guard against the destructive action of humidity, or against the various mechanical efforts exerted, and that them be no danger from the propagation of fire, in cases where many buildings are in proximity to one another. The stone may either be used in large or small rough blocks, devoid of bedding or of regular vertical joints ; or it may be used in the form of coursed masonry in the manners described under that head; and the only general principles it may be desirable hero to allude to with respect to the use of stones of various descrip tions, are, that such of them as are dense and of small absorptive powers should be used near the ground ; whilst the use of the lighter and the more hygrometric ones must be confined to the positions where they will not be exposed to great efforts of compression, or to external sources of moisture; and care should be taken that the stones are in no case laid with their planes of bedding otherwise than in the direction which they may have occupied in the quarries. The various descriptions of stones (such as the granites, whin stenos, slates, marbles, carbonates, argillo-carbonates, and sulphates of lime, and tho magnesian limestones) have already been alluded to in the articles quoted, or they will be finally discussed under that of STONE; so that, for the present, it may suffice to say that any material of this descrip tion may be advantageously used for building purposes when it is uniform in its grain, or in its molecular structure; when it has com paratively small powers of absorbing water; and when it is not a very ready conductor of heat. The latter condition is too often neglected,
and it is on this account that many of the buildings in cold, moist, countries, present hygienic properties of a very dangerous character; for the condensation and the evaporation which are thus continually going on, must render latent a great amount of heat furnished by surrounding objects. The absorptive powers of stones have an im portant bearing, not only upon the comfort of the interior of the buildings in which they are employed, but also upon their durability, on account of the disintegration and decay which attends the evapora tion of the salts they are able to liberate from their original combina tions in the stones. It appears, however, that the mere fact of the existence of great powers of absorption does not constitute an absolute reason for the rejection of a building stone; because there aro some materials of this class which are of great constructivo value, but which are highly absorbent; such as the Yorkshire sandstones, amid sonic of the silicious conglomerates. The structure of a stone has a marked influence upon its powers of resistance to natural external causes of decay, and it is found that the more highly crystalline materials aro the most durable ; whilst those in which the molecules of the stone are both of an earthy and of a decidedly marked sub-crystalline character (as is often the case with the inferior varieties of the oolites), two sure to decay in on irregular manner. Some descriptions of granite even are exposed to decay in the open air, on account of the differences in the molecular structure of their constituent elements.