Wood, in common with other vegetable products, when exposed to a certain degree of moisture, and at a temperature not much under 45°, nor too high to evaporate suddenly all the moisture, gradually decomposes. This decomposition is called putrefaction by chemical writers, but is called the "rot" in common language. It proceeds with most rapidity in the open air, but the contact of air is not absolutely necessary. Water is in all cases essential to the process; indeed, it is a principal agent in all processes of decomposition.

* Evelyn's 'Silva,' vol. ii., p. 105.

† Dr. Murray's ' System of Chemistry,' vol. iv., p. 321.

As the decay goes on, certain gaseous matters are given out, chiefly carbonic acid gas and hydrogen gas.*

Pure woody fibre, alone, undergoes this change slowly, but its texture is soon broken down, and it is easily resolved into new elements when mixed with substances more liable to change. "Any process," observes Sir H. Davy, "that tends to abstract carbonaceous matter from it must bring it nearer in composition to the soluble principles," and this is done by fermentation, † Hence it is that the sap-wood is of a more perishable nature than the heart-wood; for the sap-wood abounds more in saccharine and fermentable principles, and consequently sooner ferments. Dr. Darwin took part of the branch of an oak tree, cut in January, and divided it carefully into three parts, the bark, the alburnum or sap-wood, and the heart-wood. These were each shaved, or rasped, and separately boiled in water, and then set in a warm room to ferment. The decoction of the sap-wood passed into rapid fermentation, and became acid; but not the others ‡

Duhamel tried some experiments to ascertain the effect of confining the sap in green timber, and found that the pieces thus confined soon exhibited signs of rapid decay; and therefore he strongly recommends a free space for the circulation of air round the ends of joists and beams, instead of building them in the wall.§ Duhamel also tried the effect of covering the external surface of timber with paint, tar, and pitch; and found that it contributed much to the durability of dry or well-seasoned timber, but hastened the decay of green and unseasoned timber. || effect in hastening the decomposition of wood, in consequence of its abstracting carbon. Mild lime (carbonate of lime) has not this effect.* But mortar requires a considerable time to bring it to the state of mild lime; therefore, bedding timber in mortar, or building it in walls where it will long remain in a damp state in contact with mortar, is very injurious, and often the cause of rapid decay. Wood in a perfectly dry state does not appear to be injured by dry lime; of this we have examples in plastering laths, which are generally found sound and good in places where they have been dry. Lime also protects wood from worms.

Quicklime, when assisted by moisture, has a powerful

* Dr. Thomson's ' System of Chemistry,' vol. iv., p. 396.

† ' Agricultural Chemistry,' p. 249, 4to edit.

‡' Phytologia,' p. 33.

§ ' Transport des Bois,' pp. 52 and 53.

|| Ibid., p. 60; also Chapman on ' Preservation of Timber,' p. 121.

Volatile and fixed oils, resins, and wax are equally as susceptible of decay as woody fibre under the same circumstances;† hence we see the impropriety of attempting to protect wood in any situation where the coat of paint, etc, cannot be renewed from time to time; and also, that woods abounding in resinous matter cannot be more durable than others.

Decay sometimes commences in the growing tree, for when it has stood beyond a certain age, decay at the heart has generally made some progress (see Art. 442). This has often been observed in large girders of yellow fir, which have appeared sound on the outside, but by removing some of the binding joists have been found completely rotten at the heart. ‡ It is on this account that the practice of sawing and bolting girders is recommended (see Art. 198. Sect. III.).

It is usual to divide the rot into two kinds, the wet rot and the dry rot. The former may take place while the tree is standing, whereas the latter takes place only when the wood is dead. Any part of the substance of a tree or piece of limber may be affected with the wet rot, whereas the dry rot commences chiefly in the alburnum or sap-wood, and is most rapidly, developed in warm, moist situations, where the ventilation is imperfect. It is said that the decay of a post placed in the ground, or in water, is an example of the wet rot; and it is sometimes assumed that the parts undergoing he process of decay are alternately wet and dry; but the fact is, they are constantly supplied with that degree of dampness which is essential to putrefaction. For "timber being composed longitudinally of an assemblage of pipes or tubes, it is only necessary that one end of a log of wood should be placed in a damp or wet situation, to occasion the moisture to be conveyed to the opposite end by capillary attraction."* Prevent a free change of atmospheric air, and a post so circumstanced, it is well known, would be affected with the dry rot,' otherwise it is not easy to discern any real difference between these two modes of decay.

* ' Agricultural Chemistry,' p. 278. † Ibid., p. 238.

‡The author observed an instance of this kind in the repairs at Kenwood (the seat of the Earl of Mansfield) in 1815; and similar ones at some other places since that time.

When the external part of a beam only has been seasoned, and the sap has never been evaporated from the internal part, the rot will be an internal disease; and where an internal decay of this kind is found, it proves that the timber has never been properly seasoned. Mr. Bowden, in the plate to his ' Treatise on the Dry Rot,' gives figures of such a piece; it appears to be of common occurrence; and it is evident from such examples of decay, that the want of due time and attention in seasoning is one of the chief causes of the rapid decay of ships. In Mr. Bowden's specimen the exterior part, to the depth of 2 inches from the outside, was as sound as any wood could possibly be; but the central part was filled with a fine, white, thread-like vegetation, extending within the above-mentioned 2 inches of the exterior surface, and uniting in a thick coat of fungus at the end of the piece,* thus showing the depth to which the timber had been seasoned.

* ' Encyclopaedia Britannica,' art. Dry Rot.