英文科学读本 第四册·Lesson 14 Timber

Lesson 14 Timber

The different varieties of wood furnished by our timber trees owe their value to special characteristics, such as their hardness and durability, the beauty of their grain, or the ease with which they may be worked and polished. As a rule we may judge of the hardness of wood by its action when it is put into water. Some, such as ebony and lignum-vitae, are so heavy that they sink in the water. Spanish mahogany and oak float, but the upper edge of the wood is on a level with the surface of the water. Yellow pine-wood floats higher out of the water than these, larch higher than the pine, and common poplar higher still. What is the explanation of this?

The ebony and lignum-vitae are the heaviest of these woods, poplar the lightest.

But why are some bodies heavier than others? Because the force of cohesion binds their molecules more closely together. There is more matter in them."

Exactly. The molecules of the ebony and lignum-vitae are very densely packed; the substance of the poplar-wood is loose compared with them. But what has this to do with the hardness of the wood?

It is a general rule that when the molecules of a body are densely packed, owing to a strong force of cohesion, the body is not only heavy but hard. This explains why we are able to judge as to the hardness of different woods by placing them in water. Ebony and lignum-vitae represent very hard woods; poplar is a very soft wood. The mahogany, oak, pine, and larch show varying steps between the two extremes.

The strength and durability of timber depend mainly upon the part of the tree from which it is cut, but the age of the tree must also be taken into account. It is important to remember that trees, like animals, grow till they reach maturity, but after that they begin to decay. The age for reaching maturity depends on the nature of the tree, as well as on the soil and climate. The tree which is felled at its maturity yields the best timber. At this period the whole of the trunk, except of course the new sapwood, is equally good. In a young tree the heartwood is always the best; but after the tree has passed maturity, this heartwood is the first to decay. In the young tree, on the other hand, the sapwood is always the worst part of the timber, and is liable to decay quickly. But after the tree has reached its maturity, even the sap-wood becomes good useful timber.

The usefulness of timber, moreover, depends upon the proper seasoning of the wood. Newly-felled timber is unfit for use. It is not only weak, but it is liable to warp, twist, and change its form. In this state it is known as green wood. The outer surfaces, acted upon by the air, crack and split more than the inside. All timber requires to be well seasoned to make it durable. It must be cut up by the saw while green, and exposed to the air for a long time. The result of this exposure is to gradually dry the whole of the wood. The surrounding air first absorbs all moisture from the surface, and then as the outside dries, the air by degrees finds its way to the inside, and the whole becomes uniformly dry. With some woods this seasoning or drying is done in an artificial way. The wood is placed in a drying-room, and a current of warm dry air is passed over it. This hot dry air soon absorbs all the moisture from the wood, and seasons it as effectually as years of exposure would have done."