WOOD PRESERVATION. The tendency of wood to rot when exposed to the weather, and especially when placed in contact with the soil, and when partially submerged in water, fresh or salt, is something every body knows. Season a stick of timber, then build it into your house where it never gets damp and it is practically imperishable. But lay it in the sill, and unless the foundation is exceptionally high, dampness may creep up and fungous disease attack and ultimately destroy the timbers upon which your house rests. No matter how long the albuminous substance in the cells of sap wood has been dry and inert, moisture softens it and it becomes a favourable soil for wood-destroying fungi. Rot is the result. Every decayed twig in the woods is a menace to the healthy trees. In time it scatters disease far and wide.
A telegraph pole a few years old breaks off at the ground in a stiff wind storm. It is rotten to the heart. But the wood above ground is sound; so it is below the surface. This is not a marvel. Rot is an organism that breathes while it grows. Oxygen avail able for growth is not found far below the entrance. Rot is dependent on moisture; the wind keeps the exposed parts of the pole dry. Hence the rot is restricted to a narrow zone—the surface of the soil—and the broken ends show how far its growth has proceeded. Posts break off at the ground for like cause. Piles in wharves rot off at some point between high and low water mark. Railroad ties and paving blocks suffer a more general decadence. Mine timbers fall a prey to their own particular subterranean fungi.
Weathering boards turn grey in the alternation of sun and rain, heat and cold. The outer fibres weaken and disintegrate. Oak can be scraped off with the finger nail. Hornets chew it into pulp to build their paper castles.
The protection and preservation of wood has been one of the problems of civilisation. A vast body of experience has accumulated, and we are nearer to-day to a satisfactory, a trium phant, solution of the problem than ever before. Decay is some
thing that enters wood from the outside, at some time. To prevent the entrance of the spores of the disease into sound timber is to save it. A protective covering that will effectively do this is the quest of science and of all the wood-consuming industries. One of the earliest hints came to men before the days of Plato and Aristotle. The lasting qualities of charred wood were observed. So they learned to char the lower parts of all stakes, posts and poles before setting them in the ground. The ancient practice is still held to in many regions. The timbers in salt mines last indefinitely. So the suggestion to soak posts in brine has been eagerly followed. But the salt soon leaches out in contact with soil water. Impregnation of timbers with chemi cals has been practised commercially for about one hundred years. Numerous preparations and processes have been tried with varying success. Chloride of zinc and of mercury, sulphate of iron and of copper, and other things have had their advocates. Most of them fail because the preservatives are lost to the sur rounding soil or water, in a short time. Some are too expensive to be practical. Impregnation by soaking, steeping, boiling and pressure has been tried. High temperature, while it produces thorough impregnation, has a disintegrating effect upon the wood fibres as a whole. Soaking takes too much time. Pressure requires elaborate and expensive machinery. Each seems to have its drawbacks.
Creosote oil, a by-product of illuminating gas, is believed now to be the best substance available for impregnation, and the following the best method of treating the timbers. Seasoned railroad ties are placed in a tank in the hot oil until a high temper ature is reached. The oil is drawn off, and a cold supply pumped in. The sudden cooling and condensing of gases and vapour in the wood cells produce a vacuum suction, to which is added the force of capillarity. Thus oil is forced into the wood.