225. The action of high winds, or of severe frosts, injures the tree while standing: the former separating the layers from each other, forming what is denominated rolled timber; the latter cracking the timber in several places, from the surface to the centre. These defects, as well as those arising from worms, or age, are easily seen by examining a cross section of the trunk.

226. Wet and Dry Rot. The wet and dry rot are the most serious causes of the decay of timber; as all the remedies thus far proposed to prevent them are too expensive to admit of a very general application. Both of these causes have the same origin: fermentation, and consequent putrefaction. The wet rot takes place in wood exposed, alternately, to moisture and dryness; and the dry rot is occasioned by want of a free circulation of air, as in confined warm localities, like cellars and the more confined parts of vessels.

Trees of rapid growth, which contain a large portion of sap-wood, and timber of every description, when used green, where there is a want of a free circulation of air, decay very rapidly with the rot.

227. Preservation of Timber. Numberless experiments have been made on the preservation of timber, and many processes for this purpose have been patented, both in Europe and this country. Several of these processes have yielded the most satisfactory results; and nearly all have proved more or less efficacious. The means mostly resorted to have been the saturation of the timber in the solution of some salt with a metallic or earthy base, thus forming an insoluble compound with the soluble matter of the timber. The salts which have been most generally tried are, the sulphate of iron or copper, and the chloride of mercury, zinc, or calcium. The results obtained from the chlorides have been more satisfactory than those from the sulphates; the latter class of salts with metallic bases possess undoubted antiseptic properties; but it is stated that the freed sulphuric acid, arising from the chemical action of the salt on the wood, impairs the woody fibre, and changes it into a substance resembling carbon.

228. The processes which have come into most general use are those of Mr. Kyan and of Sir W. Burnett, called after the patentees kyanizing and burnetizing. Kyan's process is to saturate the timber with a solution of chloride of mercury; using for the solution one pound of the salt to five gallons of water. Burnett uses a solution of chloride of zinc, in the proportion of one pound of the salt to ten gallons of water, for

common purposes; and a more highly concentrated solution when the object is also to render the wood incombustible.

229. As timber under the ordinary circumstances of immersion imbibes the solutions very slowly, a more expeditious, as well as more perfect means of saturation has been used of late, which consists in placing the wood to be prepared in strong wrought-iron cylinders, lined with felt and boards, to protect the iron from the action of the solution, where, first by exhausting the cylinders of air, and then applying a strong pressure by means of a force-pump, the liquid is forced into the sap and air vessels, and penetrates to the very centre of the timber.

230. Among the patented processes in our country, that of Mr. Earle has received most notice. This consists in boiling the timber in a solution of the sulphates of copper and iron. Opinion seems to be divided as to the efficacy of this method. It has been tried for the preservation of timber for artillery carriages, but not with satisfactory results.

231. M. Boucherie, to whose able researches on this subject reference has been made, noticing the slowness with which aqueous solutions were imbibed by wood, when simply immersed in them, conceived the ingenious idea of rendering the vital action of the sap-vessels subservient to a thorough impregnation of every part of the trunk where there was this vitality. To effect this, he first immersed the butt-end of a freshly-felled tree in a liquid, and found that it was diffused throughout all parts of the tree in a few days, by the action in question. But, finding it difficult to manage trees of some size when felled, M. Boucherie next attempted to saturate them before felling; for which purpose he bored an augerhole through the trunk, and made a saw-cut from the augerhole outwards, on each side, to within a few inches of the exterior, leaving enough of the fibres untouched to support the tree. One end of the anger-hole was then stopped, as well as all of the saw-cut on the exterior, and the liquid was introduced by a tube inserted into the open end of the augerhole. This method was found equally efficacious with the first, and more convenient.

232. After examining the action of the various neutral salts on the soluble matter contained in wood, M. Boucherie was led to try the impure pyrolignite of iron, both from its chemical composition and its cheapness. The results of this experiment were perfectly satisfactory. The pyrolignite of iron, in the proportion of one-fiftieth in weight of the green

wood, was found not only to preserve the wood from decay, but to harden it to a very high degree.

233. Observing that the pliability and elasticity of wood depended, in a great measure, on the moisture contained in it, M. Boucherie next directed his attention to the means of improving these properties. For this purpose he tried solutions of various deliquescent salts, which were found to answer the end proposed. Among these solutions he gives the preference to that of chloride of calcium, which also, when concentrated, renders the wood incombustible. He also recommends for like purposes the mother-water of salt-marshes, as cheaper than the solution of the chloride of calcium. Timber prepared in this way is not only improved in elasticity and pliability, but is prevented from warping and cracking; the timber, however, is subject to greater variations in weight than when seasoned naturally.

234. M. Boucherie is of opinion that the earthy chlorides will also act as preservatives, but to insure this he recom mends that they be mixed with one-fifth of pyrolignite of iron.

235. From other experiments of M. Boucherie, it appears that the sap may be expelled from any freshly-felled timber by the pressure of a liquid, and the timber be impregnated as thoroughly as by the preceding processes. To effect this, the piece to be saturated is placed in an upright position, so that the sap may flow readily from the lower end; a watertight bag, containing the liquid, is affixed to the upper extremity, which is surmounted by the liquid, the pressure from which expels the sap, and fills the sap-vessels with the liquid. The process is complete when the liquid is found to issue in a pure state from the lower end of the stick.

237. Either of the above processes may be applied in impregnating timber with coloring matter for ornamental purposes. The plan recommended by M. Boucherie consists in introducing separately the solutions by the chemical union of which the color is to be formed.

238. The rapid decay of railroad sleepers has led to more recent experiments in Europe, where timber is scarce and dear. Opinion now is in favor of impregnating them with creosote, as the best preservative from wet rot.

239. The effect of time on the durability of timber, prepared by any of the various chemical processes which have just been detailed, remains to be seen; although results of the most satisfactory nature may be looked for, considering the severe tests to which most of them have been submitted,

by exposure in situations peculiarly favorable to the destruction of ligneous substances.

240. Durability of Timber. The durability of timber, when not prepared by any of the above-mentioned processes, varies greatly under different circumstances of exposure. If placed in a sheltered position, and exposed to a free circulation of air, timber will last for centuries, without showing any sensible changes in its physical properties. An equal, if not superior, durability is observed when it is immersed in fresh water, or embedded in thick walls, or underground, so as to be beyond the influence of atmospheric changes.

241. In salt water, however, particularly in warm climates, timber is rapidly destroyed by the two animals already noticed: the one, the limnoria terebrans, attacking, it is said, only stationary wood, while the attacks of the other, the teredo navalis, are general. Various means have been tried to guard against the ravages of these destructive agents; that of sheathing exposed timber with copper, or with a coating of hydraulic cement, affixed to the wood by studding it thickly over with broad-headed nails to give a hold to the cement, has met with full success; but the oxidation of the metal, and the liability to accident of the cement, limit their efficacy to cases where they can be renewed. The chemical processes for preserving timber from decay do not appear to guard them in salt water. A process, however, of preserving timber by impregnating it with coal tar, patented in this country by Professor Renwick, appears, from careful experiments, also to be efficacious against the attack of the shipworm. A coating of Jeffery's marine glue, when impregnated with some of the insoluble mineral poisons destructive to animal life, is said to subserve the same end.

242. The best seasoned timber will not withstand the effects of exposure to the weather for a much greater period than twenty-five years, unless it is protected by a coating of paint or pitch, or of oil laid on hot, when the timber is partly charred over a light blaze. These substances themselves, being of a perishable nature, require to be renewed from time to time, and will, therefore, be serviceable only in situations. which admit of their renewal. They are, moreover, more hurtful than serviceable to unseasoned timber, as by closing the pores of the exterior surface they prevent the moisture from escaping from within, and therefore promote one of the chief causes of decay.

243. Forest Trees of the United States. The forests of our own country produce a great variety of the best timber for

every purpose, and supply abundantly both our own and foreign markets. The following genera are in most common

use.

244. Oak. About forty-four species of this tree are enumerated by botanists, as found in our forests and those of Mexico. The most of them afford a good building material, except the varieties of red oak, the timber of which is weak and decays rapidly.

The White Oak (Quercus Alba), so named from the color of its bark, is among the most valuable of the species, and is in very general use, but is mostly reserved for naval constructions; its trunk, which is large, serving for heavy frame-work, and the roots and larger branches affording the best compass timber. The wood is strong and durable, and of a slightly reddish tinge; it is not suitable for boards, as it shrinks about in seasoning, and is very subject to warp and crack.

This tree is found most abundantly in the Middle States. It is seldom seen, in comparison with other forest trees, in the Eastern and Southern States, or in the rich valleys of the Western States.

Post Oak (Quercus Obtusiloba). This tree seldom attains a greater diameter than about fifteen inches, and on this account is mostly used for posts, from which use it takes its name. The wood has a yellowish hue, and close grain; is said to exceed white oak in strength and durability; and is therefore an excellent building material for the lighter kinds of frame-work. This tree is found most abundantly in the forests of Maryland and Virginia, and is there frequently called Box White Oak, and Iron Oak. It also grows in the forests of the Southern and Western States, but is rarely seen. farther north than the mouth of the Hudson River.

Chestnut White Oak (Quercus Prinus Palustris). The timber of this tree is strong and durable, but inferior to the two preceding species. The tree is abundant from North Carolina to Florida.

Rock Chestnut Oak, (Quercus Prinus Monticola.) The timber of this tree is mostly in use for naval constructions, for which it is esteemed inferior only to the white oak. The tree is found in the Middle States, and as far north as Vermont.

Live Oak (Quercus Virens). The wood of this tree is of a yellowish tinge; it is heavy, compact, and of a fine grain; it is stronger and more durable than any other species, and on this account it is considered invaluable for the purposes of ship-building, for which it is exclusively reserved.