Grain. - We have seen that wood is composed of long, hollow wood cells, or fibers, sometimes accompanied by vessels of varying diameters. The character and direction of these fibers constitute what is termed the grain of the wood. As these fibers separate and break more easily lengthwise than across, we say that wood splits with the grain. If the fibers run very straight, the wood is straight-grained; if crooked, then it is called cross-grained. Many causes affect the regularity of the grain: the stem itself may be crooked, it may be straight, but the fibers run spirally around it, or there may be sets of fibers alternating in spiral directions; branches and wounds also cause cross-grain.

If the cells are small and compact, the grain is said to be fine, as in box-wood; if nearly uniform in size and thickness, the wood is even-grained, as in maple. The cells may vary greatly in size and thickness, and have large vessels in the spring growth, which would give rise to coarse-grained wood, as in the oak, ash, and chestnut.

Fig. 15. - Warping of planks cut from an unseasoned log.

The appearance given by the annual rings and medullary rays to the surface of the wood differs very much with the kind of wood and the part of the log from which the board is sawed. Special cuts are made to obtain the best effect of these markings. To show silver-grain, the face of the board should be parallel, or nearly so, with the medullary rays. The birch is an excellent example of this effect. Maple and ash are frequently seen with a wavy or curled grain. For veneers, which are about one sixteenth of an inch in thickness, wood with a very irregular grain is selected, such as walnut roots and knots, and knurls of mahogany. In some old maple-trees an appearance called bird's-eye, due to a small circular inflection of the fibers, gives to the wood a fine effect.

Woods in which the grain runs alternately in different directions, though hard to split and very difficult to work and finish, usually furnish an ornamented grain, such as mahogany.

Density. - This property depends on the more or less complete thickening of the walls of the wood-cells, and also upon the number and size of the vessels. Certain operations, such as turning, carving, and wood-engraving require dense or close-grained woods.

Porosity. - A porous wood has large, thin-walled cells and many open vessels. Its open grain is easily filled with preserving liquids which adapts it for framing and timber work generally; if such a wood is to be finished, the pores must be filled before a good surface can be obtained. As a rule, porous woods are soft and light, while dense woods are hard and heavy.

Weight and Hardness. - It sometimes happens that the entire cell is replaced by the thickened cell-wall, and this, together with deposits of oily and resinous substances, make an exceedingly hard and heavy wood. On the contrary, we have very light woods, even lighter than cork; these are composed of thin-walled cells filled with air. Between these extremes are found many gradations of weight and hardness, but woods are generally spoken of as hard or soft, and heavy or light. The hard and heavy woods are stronger and more durable than the softer and lighter ones.

The weight is expressed by a number, which shows the weight of the wood compared with the weight of an equal bulk of water, taken as the standard.

During the process of drying, wood becomes lighter and harder; thus, lignum-vitse and most of the palms are quite soft and easily cut when green, but after drying are worked with great difficulty.

Strength. - The strength of wood depends on peculiar powers of resisting various forces brought to bear upon it. Thus, lignum-vitse and the oaks are noted for their stiffness, or resistance to bending, which is probably due to the interlacing of their fibers. Young hickory, lance-wood, and others are very elastic, bending readily and returning to their former position without injury to the structure. Black or swamp ash and young white oak split easily into long and strong strips or bands such as those used for making chair-seats or baskets. Very little force is required to break the fibers of whitewood, birch, and mahogany across the grain. Pine, ash, and maple break easily but with a splintered fracture. In some palms this splintering occurs to such a degree, that walking-sticks may be transformed into very dangerous weapons, which has given rise to laws in some countries restricting their use. Rattan, oak, and hickory, when bent short, have the individual fibers unbroken, but separated from each other; and are therefore tough woods. Hard and dense woods resist compression, while soft woods yield to pressure and are indented; and more so when the pressure is applied on the sides than on the ends of the fibers. This compressibility of the softer woods is taken advantage of in gluing up joints, where the pieces are forced into perfect contact by the pressure of the screws. To secure a good joint with hard woods it is necessary to use the greatest care in preparing and cutting the pieces. The cohesion of the particles of the fibers, when strains are applied lengthwise, is very great, several tons being required to fracture pine one inch square.

Color. - As the heart-wood becomes lignified, coloring-matters are deposited within the substance of the cell-wall, giving to each kind its characteristic colors; these are exhibited in great variety, including every shade of color between the white of satin-wood and the black of ebony. In • the same wood there may be variations of tint, or even color, in the annual rings and medullary rays, enhancing the beauty of its appearance. The sap-wood receives none of the color-pigments, and therefore is always light or even white. As a rule, exposed surfaces, whether varnished or not, become darker; and this darkening, besides indicating age, gives to the surface a more agreeable effect than that of new wood. It is for this reason, as well as deception, that new cabinetwork of hard wood is stained to imitate the effects of age. Color combined with a figured grain constitutes the intrinsic ornament of wood.

Durability. - At great age a slow oxidation of the constituents of the cell-wall takes place in the interior of the heart-wood of standing trees, thus rendering the wood softer and brittle, and an easy prey to the fungi and insects. Dampness, by promoting fungus growths, is very destructive to cut timber, few woods withstanding its injurious influence; especially is this so when there are alternating dampness and dryness as seen in those portions of a building or structure in contact with the soil. Most woods if kept dry and protected from insects with paint or varnish, will last for ages, as illustrated by ancient pieces of furniture. Nearly all woods are perfectly preserved if kept immersed in water, which is shown by the wood of vessels that have been sunk for a hundred years or more, and which finds application in laying the foundations of stone for large buildings and bridges upon the tops of piles driven below the water-mark. Many woods like cedar and camphor-wood have within their substance oils and resins which protect them from the fungi and insect life.