Chap. XXII.

TIMBER AND ITS PROPERTIES.

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CIIAP. XXII.

TIMBER AND ITS PROPERTIES.

By the civil engineer this production is required to be sound and properly seasoned ;its durability deperfds upon the absence of those natural juices which possess a saccharinequality, undergo the process of fermentation, and destroy the substance as well as strengthof the timber.

The various trees of the forest are composed of bark, wood, and pith, and spring fromthe acorns or seed which each bears : early in the spring a tender shoot breaks theground, which by the end of the year has extended, enlarged, and hardened into a ligneoussubstance, terminated by a bud, which in the following year throws out a second shoot,but of greater strength and larger than that of the preceding year ; this process continuingyear after year enables the tree to acquire its full growth.

A cylindrical ring encloses the pith in its early stage, which is surrounded by the bark,and if a longitudinal section of the plant be made, we perceive medullary rays traversingthe wood ; the bark and the alburnum or woody matter have each a layer deposited everyyear, whilst the tree is in a growing state ; these layers are both formed from a secretioncalled Cambium, deposited by the succulent vessels of the bark, and the medullary raysbetween the wood and the bark, which secretion is prepared by the leaves and transmittedthrough the bark; the new liber and alburnum formed by this gelatinous fluid are bya process of nature transformed into cells and layers, the diverging as well as concentricbeing produced simultaneously; these layers of wood obtain in a few years greater density,from a deposition of woody matter in the cells, which increases until the tree has arrived atmaturity : each succeeding year of growth a hollow elongated cone may be imagined tocover the ligneous productions of the former years ; the circles, which are distinguishablewhen the tree is cut down at its base, exhibit the number of these cones, as well as theincrease of growth each year.

Hie pith and the bark in young shoots arc in contact; and the former, composedentirely of cellular tissue, is at first filled with a watery fluid, and afterwards with air,becoming dry before the first layer of wood is perfected : the form of its cells is usuallyhexagonal; but as the age of the plant increases, they undergo some change ; a horizontalcommunication of the most perfect kind continues to be maintained between the pith andthe bark, although their separation is annually increased by the succession of concentricrings deposited. Medullary rays, or the silver grain, which are flattened masses of cellulartissue, consisting of oblong cells horizontally arranged, effect this by separating intowedge-shaped vertical plates the rings of zones of wood as they form, and thus keep up aconnection between the pith and the woody tissue and vessels they are combined with onthe outside.

The bark, or outer cuticle, is a thin membrane extending over the entire surface of theplant, spreading its members like a net, the form of which varies in'different plants ; this isdivided into four distinct parts : the epidermis, the cutis, the cellular, and the liberor inner bark ; the cutis consists of two layers, the outer of which is the epidermis, andthe inner a composition of transverse cells, of a varied structure. The longitudinalfibres of the bark vary, but are formed of hollow tubes, which convey the sap downwards,for the purpose of increasing the solid contents of the tree. The bark itself is annuallyreproduced, and the old layers pushed outwards : there are no annular or spiral vessels;the cells contain occasionally a secreted fluid, which, in some instances, is useful to the arts.

Elasticity, extensibility, contractibility, permeability to fluids and gaseous matters, arethe chief elementary organs of trees; which, by their cellular tissue, transmit their food inall directions : by it the sap is circulated, and the medullary rays formed, which conveythe juices from the bark to the centre of the stem; the bark itself is composed of it, aswell as the leaf, which exposes to evaporation, light, and atmospheric action the parenchymain which the sap is diffused. The woody tissue, destined to convey the fluids upwards anddownwards, gives firmness and elasticity to the various parts : and the sap-wood next thebark, containing all the nutritive matter to be converted into leaves, buds, and heartwood,may easily be supposed to be acted upon by a variety of causes, and be most subject todecay: when its vessels have lost their juices, and it is compressed into heartwood, its useis then to give support to the tree.

Wood is formed, according to Linneeus, from the pith, and by other naturalists it issupposed to be produced by a change of the liber of the previous season : by DuHamel , that it is deposited by the secretion called Cambium, which is found between

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