_Durability_--In Egypt where climatic conditions are highly favorable, Lebanon cedar, North African acacia, East African persimmon, and oriental sycamore have remained sound during three or four thousand years. In the moist forests of the northwestern Pacific coast, an alder log six or eight inches in diameter will decay through and through in a single year. No wood is immune to decay if exposed to influences which induce it, but some resist for long periods. Osage orange and locust fence posts may stand half a century. Timber from which air is excluded, as when deeply buried in wet earth or under water, will last indefinitely; but if it is exposed to alternate dampness and dryness, decay will destroy it in a few years. It is apparent that resistance to decay is not a property inherent in the wood, but depends on circumstances. However, the ability to resist decay varies greatly with different species, under similar circumstances. Buckeye and red cedar fence posts, situated alike, will not last alike. The buckeye may be expected to fall in two or three years, and the cedar will stand twenty. Timbers light in weight and light in color are, as a class, quick-decaying when exposed to the weather. The rule holds in most cases that sapwood decays more quickly than heart when both are subject to similar exposure. The matter of decay is not important when lumber and other products intended for use are in dry situations. Furniture and interior house finish do not decay under ordinary circumstances, no matter what the species of wood may be; but resistance to decay overshadows almost any other consideration in choosing mine timbers, crossties, fence posts, and tanks and silos. Decay in timber is not simply a chemical process, but is due primarily to the activities of a low order of plants known as fungi, sometimes bacteria. The fungi produce thread-like filaments which penetrate the body of the wood, ramifying in and passing from cell to cell, absorbing certain materials therein, and ultimately breaking down and destroying the structure of the wood. Both air and dampness are essential to the growth of fungus. That is the reason why timbers deep beneath ground or water do not decay. Air is absent, though moisture is abundant; while in the dry Egyptian tombs, air is abundant but moisture is wanting, fungus cannot exist, and consequently decay of the wood does not occur. Nothing is needed to render timber immune to decay except to keep fungus out of the cells. Some of the fungus concerned in wood rotting is microscopic, while other appears in forms and sizes easily seen and recognized.