After vital action ceases in a tree, its substance, like that of other organic bodies, undergoes a process of decomposition, which sooner or later terminates in the total decay of the wood. Decay takes place very rapidly if the timber is exposed to alternations of moisture, air, and heat.

The wood fibres themselves have a high degree of durability, especially if the sap, which is the prime cause of decay, has been removed. Some of the constituents of the sap, e.g., starch and sugar, neither hasten decay nor retard it, while others, e.g., tannic acid and resin, counteract it. It is the albuminoids which are the cause of decomposition, and the sap-wood in which they abound is the part which decays most rapidly.

The decay of timber is caused, in the first instance, by the fermentation of the sap, which in this state soon acts injuriously on the wood-fibres. The first sign of this is a bluish tinge on the surface of the wood. Timber which has assumed this bluish tinge is not only less durable and strong, but it is also extremely difficult to work. Though the fermenting elements dry in the wood cells, they do not therefore lose their power. They remain dormant merely, and the application of moisture after the lapse of time is sufficient to wake them into activity. Hence, timber which is exposed to alternations of heat and moisture may very soon acquire a "blue surface," especially if kept where ventilation is deficient.

Blue surface.

If the process of decay goes on further, fungi almost always make their appearance. One of the most destructive forms in which they appear is known as " dry rot."

Dry rot.

Timber is also destroyed by insects or worms, which bore their way through the wood, and often reduce the inner portion completely to dust before any signs of destruction appear on the outside. Wood which is rich in sap, e.g., birch and alder, is most liable to such attacks ; beech is less liable ; while the elm, the maple, and resinous needle-leaved trees, are seldom attacked.

Attacks of insects.

Means of Preventing- Decay.

As the decomposition of the sap is the real cause of the decay of wood, the means taken to prevent decay are directed either towards the retardation of. this decomposition or to the complete expulsion of the sap, e.g. :

1. - The timber is cut down during the season of the year when there is least sap in the stem. 2. - The timber is seasoned as thoroughly as possible, in circumstances which permit free access and circulation of air, and is protected not only during seasoning but afterwards, from alternations of moisture and dryness.

The growth of fungus is prevented by exposure to light, and continuous and uniform ventilation

3. - The wood, after it has been made into articles, is preserved from damp by varnish, oil paint, etc.

4. - The sap is got rid of by steeping the timber in water or steaming it in ovens.

It is to be observed, however, that in this way the constituents of the sap which contribute to the durability of the wood, i.e., resin and tannic acid, are also removed.

5. The timber is impregnated with some substance in solution which neutralises the effects of the sap.

The two last named processes are not used for slojd timber.

In conclusion, it may be added that when the sap is removed entirely, or when the timber is impregnated with some neutralising substance, it does not become worm-eaten. When insects attack wood which has not been treated in one of these ways, it is almost impossible to extirpate them. It has been recommended to apply an acid, e.g. muriatic acid, or a solution of camphor to the worm-eaten holes; but this is, generally speaking, not practicable, and it is, moreover, not a complete cure.