469. When the decay is owing to dry rot the timber at first swells, changes colour, and emits a musty smell. Soon a quantity of white cottony fibres will be found to have penetrated the tissues of the wood, this is the " mycelium " or rootlets of a fungus which derive their nourishment from the wood, and spreading rapidly, collect on the surface into a fine delicate thread-like vegetation, and if a sufficient quantity of heat and moisture be present, they soon form into a compact mass, which ultimately becomes a moist fleshy substance, called the " pileus," of a ferruginous yellow colour, the margin of which is tumid or swollen, downy and white, the folds broad porus and toothed on the surface; lining the sides of the pores of these toothed folds are found the "spores," the dissemination of which reproduces the parent plant. This fungus is known as the Merulius lacrymans of the order Polyporei, or pore-bearing fungi, so called to distinguish them from the Agaricini, or gill-bearing fungi, to which order the common mushroom belongs. The pileus of the Merulius lacrymans varies from a few inches to .several feet in length and breadth, and is usually found dripping with moisture.

Polyporus destructor is another species found on timber affected with dry-rot; it resembles M. lacrymans, but the pileus is neither so fleshy nor so watery, although more fragile.

Oak timber is chiefly attacked by another species called Polyporus hybridus, in which the mycelium attains great density and capacity for moisture.

In the more advanced stages of decay the wood contracts lengthwise, and shows many deep fissures across the fibres, similar to a piece of wood scorched by fire. The woody fibres, however, appear to retain their natural form, but easily crumble into a fine powder. In oak this powder is of a snuff-brown colour.

* ' Treatise on the Dry Rot,' pp. 11 and 15.

The fungus, when it spreads upon the surface of the wood, often becomes of a considerable size, sometimes spreading over the adjoining walls, and ascending to a considerable height.

The decayed state of a barn floor is thus described by Mr. B. Johnson: - "An oak barn floor which had been laid twelve years began to shake upon the joists, and on examination was found to be quite rotten in various parts. The planks, 2 1/2 inches in thickness, were nearly eaten through, except the outside, which was glossy, and apparently without blemish. The rotten wood was partly in the state of an impalpable powder, of a snuff colour; other parts were black, and the rest clearly fungus. No earth was near the wood."*

470. In timber of the same kind, that of the most sappy and rapidly grown trees is the most liable to decay. The wood of trees from the close forests of Germany or America is more subject to it than that of trees grown in more open situations; and it is remarked by Mr. Barrow, that "the timber brought from America in the heated hold of a ship, is invariably covered over, on being landed, with a complete coating of fungus."† Very few cargoes of timber in the log come from America, in which the commencement of the vegetation of the dry rot is not perceptible in some part of each log. Sometimes it will show itself only in a few reddish, discoloured spots on the surface of the log, which, if scratched with the nail, the texture of the timber to a slight depth will be found destroyed, and generally a white fibre will be seen growing on these spots. The red wood timber is less subject to dry rot than the yellow, owing to its containing more turpentine. If the cargo has been shipped in a wet condition, and the voyage has been a long one, a white fibre will most likely be seen growing over nearly every part of the surface of each log, and in cargoes that have been so shipped, all the logs of yellow pine, red pine, and oak, are generally more or less affected on the surface.* It is, therefore, most probable that timber is infected with the seeds of decay before it is brought into use. The custom of floating timber in docks and rivers also injures it very much; it would be better to sink it completely under water, as partial immersion is the worst situation it can be placed in.

* 'Transactions of the Society of Arts,' etc, vol. xxi., p. 294. † ' Encyclopaedia Britannica,' art. Dry Rot.

471. Though moisture is essential to the progress of decay, absolute wetness will prevent it, especially at a low temperature. In ships this has been particularly remarked, for that part of the hold of a ship which is constantly washed by the bilge-water is never affected by dry rot.† Neither is that side of the planking of a ship's bottom which is next the water often found in a state of decay, even when the inside is quite rotten, unless the rot has penetrated quite through from the inside. ‡

472. Warmth and moisture are the most active causes of decay, and provided the necessary degree of moisture is present, the higher the heat the more rapid is its progress. In warm cellars, or close confined situations where the air is filled with vapour without a current to change it, decay proceeds with astonishing rapidity, and the timber-work is destroyed in a very short time. The bread-rooms of ships, behind the skirtings and under the wooden floors of the basement stories of houses, particularly in kitchens, or other rooms where there are constant fires; and, in general, in every place where wood is exposed to warmth and damp air, the dry rot will soon make its appearance, and if the building be inhabited, will also show itself in the altered health of the occupants.

* T. A. Britton: Paper read before the Archit. Association, 1866. † Bowden's ' Treatise on Dry Rot,' p. 62. ‡ Ibid., p. 68.

473. All kinds of stoves are sure to increase the disease if moisture be present. The effect of heat is also evident from the rapid decay of ships in hot climates.* And the warm moisture given out by particular cargoes is also very destructive; such as cargoes of hemp,† pepper, and cotton. ‡