Timber

The various substances here described collectively as cellulose are carbohydrates, which may be popularly defined as sugars or substances capable of being converted into sugars. Thus cellulose of wood yields for instance a sugar, glucose, which by fermenta tion can produce ethyl alcohol (spirits of wine). Cellulose by ap propriate treatment with nitric or acetic acid produces soluble compounds which when precipitated serve to make artificial silk, dopes, films (collodion, etc.), and in the case of nitric acid, explosives.

The general agreement of all kinds of timbers as regards chem ical composition is illustrated by the fact that all these under destructive distillation yield tar and tar-derivatives and methyl alcohol (wood spirit).

But the various kinds of wood also have additional chemical bodies which differ in the different species, and include tannin-like bodies, resins, scented ethereal oils and colouring substances.

Physical Constitution.

Physically wood-substance is a very stiff jelly or gel, and consequently is comparable with glue, gelatin or gums. Like these, it is hygroscopic and swells as it absorbs water and shrinks as it dries. Wood-substance can take up only a definite amount of water, which lies near 3o% of its dry weight when the wood is in a saturated atmosphere. A piece of wood containing this maximum amount in its walls but none in its cavi ties is described as having reached fibre-saturation point, and has attained its maximum volume : when liquid water is added to it the water remains in the cavities and no additional swelling ensues.

It is a familiar fact that when water is added to dry glue or gelatin these show a decline in hardness, strength and stiffness, and a rise in flexibility and extensibility : as wood-substance is a gel, the same changes in its properties take place when water is added to a piece of dry wood until fibre-saturation point is reached. Moreover, just as when heat is applied together with water to glue this shows increased loss of stiffness (by "melting"), so when moist steam is applied to wood this can readily be bent for the manufacture of furniture and so forth.

The weight or heaviness of wood is recorded as the weight of a unit of volume of the wood, which is the density. The units em ployed in the British Empire and United States are cubic feet and pounds avoirdupois : elsewhere, and in scientific work, they are cubic centimetres and grammes.

Since the fundamental chemical composition of wood-substance approaches identity in all timbers, its specific gravity in these varies but slightly and lies near 1•S5; that is, dry wood-substance weighs slightly more than one and a half times as much as water.

Consequently when two perfectly dry pieces of wood of equal size differ in weight, the one is heavier than the other because it contains more wood-substance ; in other words the density of per fectly dry wood is a measure of the amount of wood in a unit vol ume. The various kinds of timbers differ in their densities, some woods, including ebony and box, even when fully seasoned, sink in water, whereas some balsa-woods are lighter than cork. Conse quently density facilitates identification of different woods.

Water Content.

The amount of water present in a piece of wood is of profound practical importance, since it determines the weight (consequently cost of transport), size, shape, heat raising power, hardness, strength and stiffness of the piece, as well as the vulnerability of the wood to attacks by fungi and insects : it also determines the changes in these characters that will ensue when the wood is transferred to another place.

In commerce the water-content of the wood is recorded as the percentage weight of the wood and contained water, but in scientific work and in this article, as a percentage weight of the wood when it is absolutely dry. According to the latter method of recording, a log of wood is described as containing 200% of water when it contains 200 lb. of water to every ioo lb. of dry wood. Such a high water-content does occasionally occur naturally, for instance in freshly felled swamp (Louisiana) cypress; more fre quent in freshly felled timber is 1 00% in the sap-wood. Heart wood contains much less. Here a popular error must be corrected : in cold-temperate regions the wood of a tree does not contain less water in winter than in summer; often the reverse is the case.

When exposed to the open air, preferably under cover, a freshly felled piece of wood dries and shrinks until the water-content at its surface has a vapour pressure equal to that of the atmosphere. The water-content of pieces of timber thus seasoned in the open air, and not too massive, varies according to the season and site between 15 and 20% in Great Britain but is much less in drier climates, for instance, that of Egypt, in which it may be 6%.