MECHANICS AND USEFUL ARTS. 73 



against dry rot in damp situations, like mines, vaults, and the 

 basements of buildings. 



The surfoce of all timber exposed to alternations of wetness 

 and dryness gradually wastes away, becoming dark colored or 

 black. This is really a slow combustion, but is commonly called 

 wet rot, or simply rot. Other conditions being the same, the 

 most dense and resinous Avoods longest resist deeomiDosition. 

 Hence the superior durability of the heart wood, in which the 

 pores have been partly filled with lignin, over the open sapwood ; 

 and of dense oak and lignumvita? over light poplar and willow. 

 Density and resinousness exclude water; therefore our preserva- 

 tives should increase those qualities in the timber. Fixed oils 

 fill up the pores and increase the density ; the essential oils resin- 

 .ify, and furnish an impermeable coating ; but pitch or dead oil 

 possesses advantages over all known substances for the protection 

 of wood against clianges of humidity. According to Professor 

 Letheby ("Civil Engineers' Journal," vol. 23), dead oil, 1st, 

 coagulates albuminous substances ; 2d, absorbs and apjiropri- 

 ates the oxygen in the pores, and so protects from eremacausis ; 

 3d, resinifies in the pores of the wood, and thus shuts out both air 

 and moisture ; and 4th, acts as a poison to lower forms of animal 

 and vegetable life, and so protects the wood from all jsai-asites. 

 These properties specially fit it for impregnating timber exposed 

 to alternations of wet and dry states, as, indeed, some of them do 

 for situations constantly damp and wet. Dead oil is distilled from 

 coal tar, of wliich it constitutes about .30, and boils between 390° 

 and 470° Fahr. Its antiseptic quality resides in the creosote it 

 contains. One of the components of the latter, carbolic acid 

 (phenic acid, phenol) C^ H*^ 0^, the most powerful antiseptic 

 known, is able at once to arrest the decay of every kind of organic 

 matter. Professor Letheby estimates this acid at one-half to six 

 per cent, of the oil. BethelPs process subjects the timber and 

 dead oil, enclosed in huge iron tanks, to a pressure varying from 

 one hundred to two hundred pounds per squai'c inch, about 

 twelve hours : from eight to twelve pounds of oil are thus in- 

 jected into each cubic foot of wood. Lumber thus prepared is 

 not aftected by exposure to air and water, and requires jio paint- 

 ing. Four pence the cubic foot is estimated as the probable 

 exj)ense of this process. 



Though we have not to guard against decay, when timber is 

 constantly wet in salt water, the Teredo navalis, a mollask of tlie 

 family Tubicolaria (Lam.) soon reduces to ruin any unprotected 

 submarine construction of common woods. None of our native 

 timbers are exempt from these inroads. The teredo never perfo- 

 rates below the surface of the sea-bottom, and probably does little 

 injury below low-water mark ; its food is the borings of the wood. 

 Poisoning the timber does not protect from the teredo, the con- 

 stant motion of sea-water soon diluting and washing away the 

 small quantity of soluble poison with Avhieh the wood has been 

 injected. Thorough creosoting the wood, with ten pounds of 

 dead oil per cubic foot, is a complete protection against the 

 teredo. 



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