ATMOSPHERIC INFLUENCE, the power of the air over inorganic bodies to affect them chemically, varying in degree with the constituents and condition of the air itself. .Normally, 1000 parts of the air contain 203 parts of oxygen and 792 of nitrogen; but many other gases are taken up, so that the air varies widely at places not far apart. Electricity, humidity, and temperature are other disturbing agents. The effect of A. I. is shown on buildings, many fine structures having been speedily ruined by it. Granite, in its order of purity, best withstands it among building stones. Egyptian porphyry is also remarkably enduring. Basalt is disintegrated unequally, according to the amount of feldspar therein. The atmosphere of large towns usually contains an excess of carbonic acid gas, and is hurtful to turpentine in timber or other material. Slate is durable in proportion to its density. Sandstone, millstone grit, or conglomerates are affected through the decomposition of the material cementing their particles, or by the mechanical effect of moisture, as by freezing. Limestone decays with varying degrees of rapidity. A. I. on bricks, tiles, etc., depends on the chemical composition of their material and the amount of baking or burning in their manufacture. If bricks contain lime, they will crack and crumble under moisture. In making cements, the A. I. is carefully studied and guarded against by a proper selection of materials. All timbers are affected easily.
If moist and exposed to currents of air, evaporation is rapid and cracks are produced by shrinkage. Dry-rot comes from exposure to high temperature, and consequent impris onment of natural moisture. Common wet rot is well known to come from air and water combined. The influences of both are Featly lessened by covering the wood with oil paint. Kyanizing, creosoting, and pickling in mineral salts are methods of protecting wood. On metals A. I. is complicated by electrochemical changes. Iron becomes rusty, that is, the surface is converted into hydrous oxide and will scale off. The more iron is used the less the rusting, as may be seen on comparing a well-worn railway track with a little used siding. Zinc when exposed to air and moisture is rapidly covered with white oxide of zinc, a coating which arrests further oxidation. For this reason. also, galvanizing or plating with zinc is a means of protecting iron. Copper strongly resists A. I.; like zinc, it is soon covered with an oxide that serves as a protec tion. Lead changes but little in air or water. Glass which is deficient in silica is exposed to decay by the decomposition of its potash and soda. The A. I. causes decay of paintings, statuary, and other works of art, and the destruction of books and manu scripts.