To preserve wood it must be protected from those causes which induce warping, checking, and discoloration; be removed from those, conditions which favor the development of fungi and the boring of insects.

Attention must first be given to the seasoning of the wood. The logs should be sawed into lumber as soon as possible after cutting, or, if they have been immersed in water, immediately on removal from the water, then stacked in the open air and allowed to remain until thoroughly seasoned, or be subjected to some other drying process now in use. If the logs are to be shipped a long distance, or remain un-sawed for even a few weeks, it is necessary to remove the bark and coat the surface, particularly the ends of the log, with a thick coat of tar or paint, to retard evaporation. From most logs the sap-wood should be removed, to prevent the attacks of fungi and insects. The sap-wood of lignum-vitse is allowed to remain, to prevent checking in the heart-wood.

Exposure of the raw surface of wood to the alternate action of rain or moisture and sunlight causes a discoloration called weather-stain, which penetrates into the tissue and renders the surface unfit for finished work. If exposed for a long time, the softer portions are worn away, giving a weather-beaten effect. To protect smoothed boards from the action of the weather, they are oiled, painted, or varnished. Sawed and weather-beaten surfaces require a large quantity of paint to cover them, and may be whitewashed or coated with some other lime preparation.

Few woods can resist the constant alternation of dampness and dryness occurring in those portions of timber in contact with the soil. Here we have the most favorable condition for the attacks of fungi and eventually decay or rot. The ends of beams and joists resting on damp walls, posts set in stone foundations, fences and railroad-ties, are wellknown examples of wood exposed to this condition. Those woods which have the least tendency to decay in contact with the soil are the cypress, redwood, cedar, locust, and white oak. The others require some one of the various artificial means to preserve them.

Charring, in which the wood is held for a few minutes in a fire until the surface is evenly and completely converted into charcoal. This will be effectual only in well-seasoned woods, because, if the wood checks after the operation, fungus-spores may germinate in the check and cause rotting of the wood. A specimen observed by the author had a large, well-developed polypore in a stick that had been charred only one year.

Creosote. - The protective substance developed in charring seems to be creosote, which is one of the best preservatives we have. The ends of timbers are placed in the creosote until they have drawn up into their pores a sufficient quantity, and, as long as it gives a perceptible odor to the wood, fungi and insects, including even the white ants, leave it alone.

Wood-Tar and Coal-Tar are quite frequently used in America as preserving coats for wood. They are to be recommended as cheap and effective, and especially adapted to out-of-door structures.

Paint. - Although the so-called metallic paint, in which an oxide of iron is the basis, and common paint, with carbonate of lead as a basis, have been used to a great extent for preserving wood, they are desirable only for those portions of wooden structures not in contact with the soil. In any event, they need renewal every two or three years to continue their preservative action.

Many chemical solutions have been used to protect wood from fungi, insects, and even from fire. Of these a ten-percent solution of sulphate of copper, in which the wood is placed until its cells and vessels have absorbed a sufficient quantity, is the most prominent. A mixture, of one part of silicate of sodium and three of water, applied to the wood, renders it fire-proof and free from the attacks of parasites. Acid solutions of various alums, together with sulphates of zinc and potassium, have been strongly recommended. For railroad-ties a solution of rosin and paraffin in benzine has been used effectually. In most of these solutions the wood is simply immersed; but, to render the absorption very complete, the air is first removed by vacuum-pumps, and the wood then immersed in the preserving fluid.

Wood will not decay as long as it is kept well ventilated and dry. It may become brittle with age, but no sign of fungus growth will make its appearance. This is shown in the wood of old pieces of furniture and the interior woodwork of houses, which the coat of paint or varnish has kept perfectly sound.

The opposite condition, in which the wood is constantly covered by water, will also preserve it; as examples of this, we have the oak of vessels sunken for a hundred years or more, and the remains of ancient lake-dwellers in Switzerland and England. It is because of this peculiar preservative action of water that foundations of great structures of granite and marble are laid upon the tops of wooden piles, driven below the low-water mark.

In America, with its bountiful supply of wood, which is easily obtained and cheap, little attention has been paid to means of preserving it. But now we begin to note the result of extravagant and unchecked destruction of timber-lands by the increasing scarcity of some of the ordinary kinds, and in the attempts made to preserve railroad-ties.