Chapter III.) of seasoning and of hastening the drying process. Wood is

sometimes soaked in water before being seasoned. This assists in removing the soluble elements of the sap, but it is doubtful whether the process improves the quality of the wood. Smoking and steaming are also resorted to. Small pieces can readily be smoked, which hardens the wood and adds to its durability,--a method which has been known for centuries,--but care must be taken not to burn, scorch, or crack the wood.

=Decay and Preservation.=--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 unventilated 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.

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 wood-work.

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.

=Effects of Expansion and Contraction.=--Cracks, curling, warping, winding, or twisting are due to nothing but irregular and uneven swelling and shrinking. Some kinds of wood shrink much in drying, others but little. Some, after seasoning, swell or shrink and curl and warp to a marked degree with every change in temperature and dryness. Others, once thoroughly air-seasoned, alter much less in shape or size under ordinary circumstances.

We have already seen that the heart side of a board tends to become convex in seasoning, owing to the shrinkage of the other side, and that if one part swells much more than another the wood becomes out of shape,--warped, curled, or twisted. If one part shrinks much faster than another, cracks usually result in the quicker shrinking portion. If you stick one end of a green board into the hot oven of the kitchen stove, the heated end will crack and split before the rest of the board has fairly begun to dry. We have seen illustrations of this in the seasoning process, as shown in Chapter III.

Exposure of one side of a seasoned piece to either dampness or heat will thus cause the piece to curl. The dampness swells the side affected or the heat shrinks it so that the convexity will be on the dampened side, or the concavity on the heated side, as the case may be.

If lumber were of perfectly uniform texture, hung up where it would be entirely unconfined and free to swell or shrink in all directions, and equally exposed all over the surface to exactly the same degrees and changes of heat and cold, dryness and moisture, it would simply grow larger or smaller without changing its form or shape. There would then be no curling, warping or winding. As a matter of fact, however, wood is not uniform in texture, but exceedingly varied, some pieces being extremely complex in structure; neither is it always free to expand and contract in every direction, nor equally exposed on all sides to the alternations of heat and cold, moisture and dryness.

To come to the practical application of these facts, we have seen (in