24. All wood is equally durable under certain conditions. Kept dry or submerged it lasts indefinitely, but under other conditions it may decay very rapidly.

* The term "Georgia pine" in this work refers always to the long leaf yellow pine.

The general causes of the decay in timber are the presence of sap, exposure to alternate wet and dryness, or to moisture accompanied by heat and want of ventilation.

Dryness and ventilation are the best preventatives of decay of timber used in general construction, and wood kept dry has been known to last for centuries, although it finally becomes brittle and loses its strength. Water also seems to act as a preservative, and some kinds of timber constantly immersed in water not in motion may endure for an indefinite period. Piles and cribbage resting on them are the only forms of building construction that come under this condition, and it is essential to their preservation that they be entirely below the water line, as nothing produces decay so rapidly as alternations of moisture and dryness.

" Rot in timber is decomposition or putrefaction, generally occasioned by damp, and which proceeds by the emission of gases, chiefly carbonic acid and hydrogen."

There are two kinds of rot to which the woodwork in buildings is subject, dry rot and wet rot. " The chief difference between them seems to be that wet rot occurs where the gases evolved can escape. By it the tissues of the wood, especially the sappy portions, are decomposed. Dry rot, on the contrary, occurs in confined places where the gases cannot escape, but enter into new combinations, forming fungi which feed upon and destroy the timber."

Wet rot occurs only when the wood is kept damp or is subject to alternate dryness and moisture, and cannot take place if the wood is once thoroughly seasoned and the absorption of further moisture prevented. Wet rot communicates itself to the sound portions of the wood only by actual contact, and if all the rotten wood is cut away and the balance of the timber kept dry it will not be further affected.

25. "Dry rot is generally caused by want of ventilation. Confined air, without much moisture, encourages the growth of the fungus, which eats into the timber, renders it brittle and so reduces the cohesion of the fibres that they are reduced to powder. It generally commences in the sap wood.

"An excess of moisture prevents the growth of the fungus, but moderate warmth, combined with damp and want of air, accelerates it. In the first stage of rottenness the timber swells and changes color, is often covered with fungus or mouldiness and emits a musty smell."*

* "Notes on Building Construction," Part III.

When the fungus first appears on the sides and ends of timbers it covers the surface with a fine, delicate vegetation, called by shipwrights mildew. These fine shoots afterward collect together, and the appearance may then be compared to hoar frost, and increases rapidly, assuming gradually a more compact form, like the external coat of a mushroom, but spreads alike over wood, brickwork, stone and plastering in the form of leaves, being larger or smaller, most probably, in proportion to the nutriment the wood affords. The colors of the fungus are various, sometimes white, grayish white, with violet, often yellowish brown, or a deep shade of fine rich brown.*

The positions in which dry rot are most likely to occur are when imbedded solidly in damp plaster or masonry, as the ends of beams built into a wall, bottoms of posts imbedded in concrete, sleepers bedded in damp mortar or concrete, beams surrounded solidly with fireproof materials, beams in damp, close and imperfectly ventilated cellars, and wainscot or other finish fixed to damp walls.

Anything which absorbs moisture and confines it in contact with wood is likely to accelerate decay, particularly if accompanied with heat.

Wet or unseasoned lumber, covered with paint, tar, plaster, or any material which prevents the moisture from drying out, is quite sure to be attacked by rot. Sapwood is also more subject to decay than heartwood, and doubly so where the latter is protected by resinous substances, as in pine and cedar.

Dry rot is especially dangerous, in that it not only eats up the entire timber in which it originates, but the germs of the fungi producing it spread themselves to all adjacent woodwork without necessary contact between the affected and the sound wood. When dry rot is discovered the affected pieces should be immediately removed, if possible, and all adjoining woodwork thoroughly scraped and washed with strong acids and provision made for thorough ventilation. If the rot is on the outside of the timber, and has not penetrated far, it may be scraped away and treated with strong acids, and if kept well ventilated the rot may be stopped. Fortunately dry rot is not very prevalent in our Northern States, but in some of the Southern States great caution has to be exercised to prevent it.