526 THE POPULAR SCIENCE MONTHLY. 



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 com- 

 binations, 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 at- 

 tracted 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 ab- 

 sorbing 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 sea- 

 soned 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 moist- 

 ure which is favorable both to natural decay and to the growth of 

 plants is essential to the process of dry-rot. The vegetation that pro- 

 duces 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 ex- 



