As the durability of *life in years' of a given variety of building stone, therefore, varies greatly under different climatic con ditions, it is obvious that the engineer and architect cannot be too careful in selecting the stones best suited to the climatic and other conditions of the localities in which his struc ture is to be built. In order to make the selection intelligently, it is necessary for them to know the special qualities of the various stones, according to their structural constitu ents, and the natural causes by which those constituents were formed into rocks. The porosity, or the capacity to absorb moisture, of the various classes of stone available for building purposes, varies from 1/2 to 1 per cent in the granites, 7 to 8 per cent in the limestones and to 8 per cent in the sand stones, and their hardness or resistance de pends on the firmness with which the particles of which they consist are bound together. In the igneous or metamorphic rocks the bond is the result.of crystallization, and in the sedi mentary rocks it is due to cementation by depositions of silica, etc., between the individ ual grains as already described. The resist ance of a stone to frost action may be con veniently and accurately tested by soaking it in water and then freezing it; the process be ing repeated a number of times and the amount of disintegration noted.
Artificial
These are represented by a great variety of artificial compositions • of which the basis is hydraulic cement. The best-known varieties are the "Beton-Coignet," a French production composed of Portland cement, silicious hydraulic cement and clean sharp sand. These constituents when mixed together with a small amount of cold water make a plastic compound which is hardened in molds to the consistency of a stone, very suitable for various building purposes; the I
Sand lime brick is increasing rapidly iu favor. In its early use in the United States it was often so imperfectly made that readily it absorbed moisture and rapidly deteriorated through weathering. But it was learned that the'secret of good sand lime brick lay in form ing the brick under tremendous pressure, about 100 tons to a brick being often employed. This produces very solid as well as handsome brick, accurate in form and with good moisture resistance.
Ordinary brick is now artificially colored to many pleasing hues which greatly aid the archi tect in securing harmonious effects. In large buildings the using of different tints of bricks in groups adds vastly to the appearance. Pressed brick, once favored for fronts, owing to its smooth, handsome surface, is largely in disuse, owing to its tendency to crack.
Terra cotta, which is really nothing but brick of superior quality, tinted and molded to attractive designs, is in greatly increased use, especially for covering steel work, and for ornamental decoration of buildings, inside and out. It is commonly made in blocks, 24 to 30 inches long, and 6 to 12 deep, the height varying with the ornamental design on the face. The interior is usually hollowed, with partitions about 1(4 inches thick.
The use of steel is no longer confined to the skeleton frame of buildings. There are more than 30 factories in the United States making steel doors and shutters, and probably even more making metal lath and steel sashes. Steel and concrete construction is generally replacing wood, brick and stone, in the large cities, and in all important buildings.
The use of various kinds of brick and tile, and of iron, steel and other metals for build ing purposes, is too familiar to require a de tailed account in this connection, especially as they are exhaustively treated under their spe cial titles. It is important to note, however, that during the last decade the tensile strengths of structural iron and steel have been more than doubled by improved processes of manu facture, and they have almost entirely sup planted wood and stone in those portions of large structures which are subjected to the greatest strains, and also where economy of space is of vital importance.
For further detailed information relative to the production and strength of various kinds of building materials, and the specific purposes for which they are most suitable, see ARCHITECTURE; BRICK; CEMENT; CONCRETE; CONCRETE CON STRUCTION; IRON; LUMBER INDUSTRY IN THE UNITED STATES; MASONRY; STEEL; TIMBER