DRY ROT, a kind of decay, often very rapid, to which timber is subject, without the presence of much moisture. It has proved ruinous to many valuable edifices, and has been the cause of many serious accidents. The ends of joists are often affected by it, so that upon burdened with even a slight additional load, they are ready to break off by the wall; and the process of destruction has often gone far without a suspicion being entertained of anything wrong. Dry rot is occasioned by fungi, the 'mycelium of which diffuses itself through the substance of the timber, destroying its texture, and reducing it to a fragile or even friable mass. Merulius lacrymccns, ll vastator, and polyporus des tructor (see A,rADou), are species very commonly productive of this mischief; the first being the most common and formidable dry-rot fungus in Britain, and the last having the same' pre-eminence in Germany. Its German name is hausschwamm . Other fungi, however, produce the same effects where none of these are present; but besides the species which are well ascertained, there are some forms of mycelium not unfrequently occurring as dry rot, of which it is uncertain to what fungus they ought to be referred, as they have not been observed to develop themselves in any perfect form, whilst also it is not known what different modifications of appearance the mycelium of the same fungus may exhibit in different circumstances. Very destructive ravages have been ascribed to different species of sporotrichum, particularly in the naval yards of Britain; but the genus is altogether a doubtful one, and not improbably consists of mere forms of undeveloped mycelium. Several species of fungi are often present together in timber affected with dry rot. Some of them penetrate deeply into the substance of the timber, others spread more superficially, but attract moisture from the atmosphere, which hastens decay. This is the case with merulius lacrymans, which first appears in small white points; a filamentous substance, radiating from these, gradually forms broad patches, sometimes many feet in diameter; from these, long creeping shoots often proceed, and a net-work of filaments penetrates into every crevice. The species of polyporus more generally fill the whole mass of the timber with delicate filaments, which destroy the cohesion of its fibers. Thvdalea quercina appears in the form of leathery often
in the 'strongest oak, and the delicate threads of mycelium penetrate every duct and cavity, reducino. the whole to a fungous mass. Beautiful orange tufts sometimes appear, supposed to be the mycelium of species of coprinus.
Of the causes of dry rot, stagnation of air, as behind a wainscot or under a floor, is Certainly one of the chief, and a knowledge of it suggests means of prevention which may often be easily and most advantageously employed. Another principal cause is insufficient drying of the timber itself; and much of the prevalence of dry rot is not improbably due to the practice of felling oak in for the sake of the bark, when the wood is full of sap. Any circumstance which rilaytend to render the sap acidulous, greatly increases the liability to dry rot. The production of fungi takes place with unusual rapidity when by fermentation or otherwise an acidulous condition of organic substances is produced. A fermentation and chemical change in the althimitious con stituents of the wood, is not improbably the immediate cause of dry rot, providing a soil suitable for the vegetationnf fungi.
For the prevention of dry-rot, various processes have recently begun to be employed, the object of which is to fill the pores of the wood with some chemical substance, sir William Burnett used chloride of zinc; Mr. Bethel] used creosote; and Mr. Payne, lime, with silicate of potash. The process most generally approved, and apparently most cessful, is that of Ryan, called Kyanizing (q.v.), in which a solution of corrosive subli• mate is introduced into the pores and cells of the timber by means of an air-pump. The salts of mercury have been found to be more unfavorable to the development of fungi than any other chemical substances.—But without the use of any such means, we have abundant evidence that well-seasoned timber, in favorable circumstances, may remain unassailed by fungi for many centuries. England contains structures of which the tim ber is known to be nearly 1000 years old; wood in a state of perfect preservation was brought by lord Elgin from behind the frieze of the Parthenon, where it must have been placed more than 2,000 years ago; and the British museum contains a block of charred wood found by Mr. Layard in his excavations at Nineveh.