TIMBER AND TIMBER-TRADE. It is the purpose of this article to treat briefly of several matters relating to timber-trees, wood-working, and the thnber-trade, and to refer to such portions of the subject as Lave been noticed in other articles.
Timber-Tree ; Wood.—The botanical characteristics of timber-trees are given under the scientific names of the several trees in the NAT. lion. Dn.. The economical uses of many of them are described in the present Division, under Asu, IIer.cu, Bincn, Rum, FIR, OAK, &c. But the characteristics of timber-trees, and of wood generally, may be rapidly glanced at in this place.
Wood is that part of a plant that exists between the pith and the bark. Amongst the various kinds of wood yielded by the different families, there are groat differences of character depending on the mode and rapidity of its growth, the size of the fibres of which it is composed, and their relation to the medullary rays which pass through them, and also on the character of the secretions deposited in it. Endogens have no bark, and are generally hollow in the middle, and their wood does not permit of being worked into many shapes; but its cylindrical form affords great facility for constructing a variety of utensils, and for application to the simple wants of man in tropical climates. The stems of Exogens are solid, and the older the tree becomes the more solid is the wood. Hence a distinction is made between the centre of the wood of the trunk and its circumference, the one being called heart-wood, the other sap-wood. The heart-wood is the scat of the deposition of the peculiar secretion of the tree, and is frequently separated from the sap-wood by a distinct line. It is the secretion in the heart-wood that renders it darker, harder, and more durable than the sap-wood ; and for practical purposes it is of import ance to distinguish between the one and the other. There is much differ ence between the relative sizes of the ultimate woody fibres of which wood is composed ; and the durability and tenacity of wood frequently depend on the fineness of its fibres. It is to the secretions deposited in the wood, probably more than to the fibres themselves, that wood is indebted for its varying degrees of density. Thus, although it has been ascertained that woody fibre itself has a specific gravity of about water being 1, yet there are many woods whose specific gravity is lighter than water on account of the mode in which their fibres are arranged. The conducting power of wood in relation to heat is a
matter of importance in the construction of buildings and other pur poses. In some experiments performed by Delarive and De Condone on prisms of different kinds of wood, to ascertain their power of con ducting heat, they found that the direction of the fibres materially interfered with their conducting power. Thus it appeared that the obstruction to the passage of caloric was greater when the current was at right angles to the woody fibre than when it flowed longitudinally in the direction of the fibres. This difference also appeared to increase in proportion as the wood was a bad conductor of heat. The cooling power of these woods is another important point, and this is not at all in relation to their conducting power : thus fir-wood being 100, the cooling power of oak-wood is only 30.38, whilst that of beech-wood is whereas, in conducting power, if fir-wood be taken at 100, then beech-wood is 83.19, and oak-wood Another important point of inquiry with regard to the physical properties of wood, as to its value in building, &c., is its relation to moisture. The less the specific gravity of the wood the greater is its capacity for moisture : fir absorbs more and teak less than most other kinds of timber.
It is the peculiar resinous, gummy, oily, or other secretions, that give to the various woods their different colour, smell, and taste. The colouring matter is sometimes deposited in such abundance as to render it useful for dyeing, as seen in log-wood, red sanders-wood, and other woods used as dyes. Some woods have volatile oils deposited in them, which, being slowly given out, render them odoriferous ; and this is the case with sandal-wood, rose-wood, the wood of cedar, fir, and other trees. Frequently bitter and other secretions are deposited in wood, giving it a peculiar taste, and rendering it useful in medicine. The wood of the quassia, as well as of the sassafras, are examples of this kind of use. The wood of trees frequently contains in small quantities the secretions which are deposited in other parts of the plant.