Page:Popular Science Monthly Volume 15.djvu/542

526 current, containing the products of leaf-action, passes back from the leaves, and is distributed for the uses of the tree. As wood grows older it grows darker, particularly in the center of the stem or heart. This darkening is due to the deposit within the fibers; and when a tree reaches maturity the fibers are so filled as no longer to join in the general circulation. Now, this inner or heart wood is less liable to decay than the outer or sap wood, and sap, as is well known, is the agent of destruction. Sap is water with sugary, saline, albuminous, mucilaginous, and gummy matters dissolved in it, and such solutions ferment easily and rapidly. Fermentation is a state of vegetable matter in which the various molecules, sugary, oily, albuminous, etc., exert their peculiar attractive and repulsive powers, forming new combinations, which at first change and at length destroy the texture of the substance of which they were formerly a part. Every one knows the smell of pure, fresh wood. If you bore into wood in which the sap has just begun to ferment, you get a vinous smell, which is soon followed by the smell of putrefactive decay, unless means are taken to arrest the chemical changes that are in progress. This decomposition of wood containing sap is ordinary rot or wet-rot. It is the most general and the most fatal cause of decay in wood; but it has attracted less attention than the more startling but less common evils of dry-rot, and the destruction of timber by insects.

The seasoning of wood, whether naturally or artificially, is simply the evaporation of its sap. Decay can not occur in well-seasoned wood if it is kept dry. It matters little whether wet is applied to timber before or after the erection of a building: it can not resist the effect of what must arise in either case; for heat and moisture will produce putrid fermentation. In basement stories with damp under them, dry timber is but little better than wet, for if it is dry it will soon be wet, and decay will only be delayed while the timber is absorbing moisture; and the amount it receives will depend upon the closeness of the deposit within its fibers. This moisture dissolves the substances held in solution by the sap, and fermentation begins, with its usual train of consequences.

Dry-rot is one of these consequences. Ordinary decay must have begun before dry-rot sets in. When the moisture in wood begins to ferment, whether it be the natural sap, or the water absorbed by seasoned timber, the conditions are ripe for the inroads of dry-rot, which can no more occur without moisture than wet-rot. The immediate agent of destruction, in this case, is of vegetable origin. It takes its name from the dust to which it reduces timber. That degree of moisture which is favorable both to natural decay and to the growth of plants is essential to the process of dry-rot. The vegetation that produces it belongs to the natural group of fungi. This group is made up of plants having distinct vegetative and reproductive systems, and their best known representative is the common mushroom. If you