ATMOSPHERIC INFLUENCE. Air con sists normally of oxygen, nitrogen, water-vapor, and carbonic acid; but owing to the continual de composition of organic bodies that is taking place in nature, its constituents usually include many other gases, as hydrogen disulphide, sulphur tri oxide, etc., which may act chemically on various bodies, and thus gradually cause their destruc tion. The destructive effects of atmospheric elec tricity and changes of temperature may also be included among the influences that tend to de stroy various materials.
Atmospheric influence is conspicuously shown on buildings and other structures that are ex posed to the air. The atmosphere of large towns usually contains an excess of carbon dioxide gas, and, where coal is burned, of sulphur. The re sistance of many building materials to atmos pheric influence is, however, very great. Granite is regarded as the most stable of building-stones. Egyptian porphyry is also remarkably enduring. Basalt disintegrates unequally according to the amount of feldspar that it contains. The dur ability of slate is in proportion to its density. Sandstone, millstone grit, and conglomerates are affected through the decomposition of the mate rial cementing their particles, or the mechanical effect of moisture, as by freezing. Limestone de cays with varying degrees of rapidity. The ap plication of melted paraffin to structures that tend to disintegrate has been found of value in protecting them from decay. A notable example of atmospheric influence is presented by the Egyptian obelisk in Central Park, New York, which had withstood the ravages of centuries in a dry atmosphere, but is rapidly disintegrating in the moist climate of New York. The power of brick, tile, and the like to resist the influence of the atmosphere is dependent on the chemical com position of the material and the amount of burn ing in their manufacture. If they contain lime
they tend to crack and crumble under moisture. Much care is usually exercised in the proper se lection of materials in making cements, and those best adapted to resist atmospheric influence are naturally selected. Wood and timbers are easily influenced and, when moist and exposed to cur rents, resulting in rapid evaporation, cracks en sue from shrinkage. Dry-rot is the result of ex posure to high temperature with consequent im prisonment of natural moisture; while common wet rot is the result of air and water eombined. For the protection of wood, ordinary oil paint is the usual preservative. Treatment with special preparations, such as kyanizing, creosoting, and pickling in mineral salts, is also not infre quently resorted to. (See NVOOD PRESERVING.) In the case of metals the influence is somewhat complicated by chemical and physical changes. • Iron, when exposed to the influence of the atmos phere, absorbs oxygen from the air and becomes rusty or coated with iron oxide. For its protec tion, as in the case of bridges, a suitable paint is the most satisfactory preservative. Zinc, when exposed, similarly becomes coated with oxide of zinc, which serves to prevent further oxidation. Copper likewise is soon coated with an oxide that serves as a protecting agent. Glass, which is deficient in silica, deteriorates in consequence of the decomposition of its potash and soda. Paint ings, statuary, and other works of art, as well as books and manuscripts, readily decay under the influence of the atmosphere. The bindings of books in libraries disintegrate in consequence of the sulphur that is often contained in illumi nating gas.