The following timbers are elastic: elm, ash, aspen, oak, spruce fir, birch, maple, and poplar.
Hornbeam, alder, and Scotch fir are less elastic.
5. The texture, colour and smell of timber. Knowledge of these qualities is very important in connection with the recognition of different kinds of timber, and in estimating their value.
By texture is understood the way in which the vessels, fibres, medullary rays and annual layers are woven or connected together. (See fig. 1).
Wood as it appears in cross section is said to be end way of the grain; as it appears in radial and tangential section- parallel with the fibres - it is said to be length way of the grain, or with the grain; and as it appears when we look across the fibres at right angles to their length, it is said to be across the grain.
We distinguish between coarse and fine texture according to the quality of the fibres, vessels, medullary rays and annual layers, which, taken all together, give to wood its characteristic appearance. Similarly we speak of long-fibred and of short-fibred texture, according as the wood " works " with long or short shavings.
Coarse and fine texture
The colour of wood varies from white to deep black, with many intermediate shades of yellow, red, brown, etc., depending on the kind of tree. It varies not only in different kinds of timber, but in the same kind of timber, and even in the same tree. As has been said above, the heart-wood is always darker than the sap-wood. Certain kinds of timber, again, e.g., oak and mahogany, become darker with time.
Our ordinary timbers are whitish, yellowish, brownish or reddish, and are not so highly coloured as tropical timbers, some of which are very striking in colour.
Different colours of wood.
The smell peculiar to many kinds of timber is a mark by which they may sometimes be recognised. This characteristic smell does not proceed from the wood itself, for it has none. It is due to the sap, and is always strongest in fresh sappy wood ; though seasoned timber sometimes has a very decided smell, which is often quite unlike that of the unseasoned wood. Needle-leaved trees have a strong smell of turpentine, and certain broad-leaved trees, e.g., the oak, often smell of tannic acid. Many trees have an agreeable smell, e.g., the cedar, juniper, the camphor-tree, etc. The smell of some
The smell of wood due to the sap timber remains in it for a long time, and communicates itself to food kept in vessels made of it.
A musty smell in timber is a sign of decay.
6. The weight or specific gravity of timber is very variable, depending as it does on a number of different cir-stances. Hence it is impossible to give such definite statistics under this head as can be given in the case of metals and many other substances. We have to take into consideration the closeness or the looseness of the fibres, which determines the hardness or density of the wood ; the presence of more or less sap; the climate and soil in which the tree has grown ; its age ; its different parts ; the degree of seasoning, etc.
The specific gravity of wood properly so called, i.e., of the cellular tissue which composes it, is very similar in all timbers, and even in the lightest kinds it is greater than that of water. Nevertheless, most timbers, owing to their porous nature, are lighter than water, and float in it. This is the case with all our indigenous trees after seasoning. A warm climate produces heavy timber; and the heaviest timbers, such as ebony and lignum vitse, are found in the tropics.
The presence of water is the circumstance which most affects the weight of timber. All timbers are heavier when newly felled than after seasoning. Hence, in determining the specific gravity of different kinds of timber, we must assume that the timber is fully seasoned.
The average specific gravity of the most common kinds of timber is given as follows by competent authorities:-
Specific gravity of the cellular tissue.
The Hornbeam ..
The Common Alder
The Common Ash
The Common Beech
The Com. Juniper
The Scotch Fir ..
The Spruce Fir
The Lime ...
The Common Larch
The White-beam ..
The Pear ..
The Common Walnut
The absolute weight percubic foot in any given timber is ascertained by multiplying the specificgravity given above by 62.5 = the number of pounds in a cubic foot of water.
7. The durability of timber. This and the circumstances which favour it have been touched on in connection with the sap, with seasoning, with decay, and the means of its prevention.
The conditions under which timber is used have the greatest influence on its durability. Thus, timber which is kept under cover and protected from moisture is very durable, and may last for many centuries. Some kinds of timber are extremely durable if kept under water. Thus, the oak used in ancient lake-dwellings and bridges, or found in bogs, has been preserved for thousands of years.
The dura-. biiity of wood under water.
If timber is exposed to alternations of moisture and dryness, its durability is diminished ; and yet, in most cases, it is precisely in these unfavourable conditions it has to be used.
When wood is least durable.
Hence it follows that it is impossible to give precise details regarding the durability of timber. Under this head all that can be done is to mention the trees which in all circumstances give the most durable timbers, viz.: the oak, and resinous, close-grained Scotch fir and larch. The elm comes next to these. If exposed to alternations of moisture and dryness, oak is said to last one hundred years, birch fifteen years, and beech not more than ten. Durability is also mentioned in the description of different kinds of timber, which follows.
The most durable timbert.