464. Timber, when properly seasoned, is strong, tough, and elastic; but it does not long retain those properties. It is generally employed in situations where it is either continually dry, constantly wet, alternately wet and dry, or where it is exposed to heat and continued moisture.

465. Effect Of Continued Dryness

Timber that is constantly dry, or affected only by the small quantity of moisture which it absorbs from the air in damp weather, has been known to last for seven or eight hundred years; but, even in this state, time produces a sensible alteration in its properties, for it is found to lose its elastic and coherent powers gradually, and to become brittle. Hence it is unfit to sustain the action of variable loads, though in a state of rest it may endure an immense length of time.

466. Effect Of Continued Wetness

The wood of trees in its natural state is a compound substance, having a certain portion of its constituents soluble in water, another part capable of being extracted by alcohol, and the part remaining, after being treated with alcohol, is the pure woody fibre, or lignin, of chemists. After water has extracted all the soluble parts from timber, it is obvious that while the timber continues immersed in water it may remain unchanged for an indefinite period; but if it be taken out and dried, it is found to be brittle and effete; or, to use the workman's expression, "its nature is gone;" and, as Dr. Sloane has observed of oak that had been buried in a wet situation, "it dries, splits, becomes light, and soon impairs." * But though oak timber taken from bogs is always found to be brittle and in a state of decay, fir from the same bog is often, if not always, in a much sounder state.

467. Effect Of Alternate Dryness And Moisture

When timber is exposed to the action of alternate dryness and moisture it soon decays. It has been already noticed that repeated steeping and drying removes a sensible portion of the wood at each operation (sec Arts. 451 and 452); and it is evident that at each drying a new portion of soluble matter is formed, which either did not before exist, or which is rendered soluble by a change in its nature. This conclusion is further established by Saussure, who found that wood the most completely freed of its soluble particles furnishes always by maceration in water, with the contact of air, infusions holding extractive matter dissolved.† The effect of this kind of decay may be observed in weather-boarding, fencing, and in any situation where wood is constantly exposed to the vicissitudes of the weather. When the timber has been thoroughly seasoned, painting or any kind of coating that is capable of resisting moisture is the best means of preserving it from this kind of decay (see Art. 473).