Timber decays fastest when alternately wet and dry, as in the piles of a wharf, fence-posts, and the like, or when subjected to a hot, moist, close atmosphere, as the sills and floor-timbers situated over some damp and unven-tilated cellar. Fig. 694 shows the decay caused by alternate wetness and dryness, while the parts above and below are still sound.

Wood lasts the best when kept dry and well ventilated. When kept constantly wet it is somewhat softened, and will not resist so much, but it does not decay. Recently, upon cutting a slab from the outside of a large log taken from the bed of a river, where it had lain for one hundred years or more, the interior proved as sound and clear as could be found in any lumber-yard. Undoubtedly, however, such long submersion lessens the elastic strength of timber after it is dried. That is not, however, an extreme example of durability. Wood has been taken from bogs and ancient lake-dwellings after being preserved for ages. Piles were taken from the Old London Bridge after about 650 years of service. Piles placed in the Rhine about 2000 years ago have been found quite sound during the present century; and piles are now regularly used, as you doubtless know, for the support of the most massive stone buildings and piers, but only where they are driven deep in the ground or below the low-water line. Many examples of the durability of wood kept dry are found in European structures. Timbers put into the roof of Westminster Abbey in the reign of Richard II. are still in place, and the roof-timbers of some of the older Italian churches remain in good condition.

Fig. 694.

Thorough seasoning, protection from the sun and rain, and the free circulation of air are the essentials to the preservation of timber.

Many preparations and chemical processes have been tried for the preservation of wood.

Creosote is one of the best preservatives known. Insects and fungi are repelled by its odour. The modern so-called "creosote stains" are excellent, not very expensive, and easily applied. They are only suitable for outside work, however, on account of the odour.

Coal-tar and wood-tar or pitch, applied hot in thin coats, are also good and cheap preservatives for exposed woodwork.

Charring the ends of fence-posts by holding them for a short time over a fire and forming a protecting coating of charcoal is another method which has been extensively used.

Oil paint will protect wood from moisture from without, and is the method most commonly in use.

In the case of any external coating, however, which interferes with the process of evaporation, as tar or paint, the wood must be thoroughly dry when it is applied, or the moisture within will be unable to escape, and will cause decay.

Lumber as well as the living tree has enemies in the form of insects and worms, but the conditions best for the preservation of the wood, as referred to above, are also the least favourable for the attacks of animal life and of fungi.

As soon as the tree has been felled and dies, decomposition begins, as in all organic bodies, and sooner or later will totally destroy the wood. The woody fibre itself will last for ages, but some of the substances involved in the growth soon decay. The sap is liable to fermentation, shown by a bluish tint, and decay sets in. Fungi are liable to fasten upon the wood. Worms and insects also attack it, preferring that which is richest in sap. Thus we see that the danger of decay originates chiefly in the decomposition of the sap (although in living trees past their prime decay begins in the heartwood while the sapwood is sound), so the more the sap can be got rid of the better. There are, however, some substances found in various trees, aside from those elements especially required for their growth, which render the wood more durable, like tannic acid, which abounds in oak and a number of trees, particularly in the bark. There is no advantage in getting rid of the turpentine and other volatile oils and the resinous deposits found in needle-leaved trees, particularly in the case of those woods in which they abound. Care should be taken, however, not to use a piece of pine badly streaked or spotted with resinous deposits in a place where it will be exposed, as the turpentine or resinous matter will be apt to ooze out and blister the paint.

Wet rot is a decay of the unseasoned wood, which may also be caused in seasoned wood by moisture with a temperate degree of warmth. It occurs in wood alternately exposed to dryness and moisture. Dry rot, which is due to fungi, does not attack dry wood, but is found where there is dampness and lack of free circulation of air, as in warm, damp, and unventilated situations, like cellars and the more confined parts of ships, and in time results in the entire crumbling away of the wood. There are several forms of dry rot. One of the most common and worst of dry-rot fungi attacks pine and fir. Fungi also attack oak. Creosote is used as a preventive, to the extent to which it saturates the wood.