In its natural state the durability of wood depends upon the variety of tree from which it is taken, the time of felling, the manner of drying, and the conditions in which it is placed. Natural wood in deep fresh water will last for centuries, as it is not liable to attacks of worms as it is in sea water, and its depth of immersion preserves it from eremacausis. Alternate exposure to air and water, or continued exposure to air and moisture combined, is most favorable to the decay of wood, especially if accompanied by warmth; but different kinds of wood have vastly different powers of resisting such influences. (See Wood.) Pure woody fibre is much less affected by the action of air and moisture than when it contains starchy, gummy, and albuminous matter. If felled in winter, after the sap has been mostly converted into woody fibre, it is more durable than if felled in summer. Kiln drying is the ordinary method of preserving wood which is to be worked by the carpenter or cabinet maker, by which the water is eliminated, and the albuminous and gummy constituents hardened. The constituents of the sap may be removed by boiling the wood in water or subjecting it to the action of steam, alternate vacuum exhaustion and injection, if desired, aiding in the process of removal.

This mode of treatment is practised when it is desirable that the grain and color of the wood be preserved. The constituents of the sap may also be changed by the action of chemicals, and these may be introduced by first exhausting the air and vapor from the pores of the wood and then letting into the receiver a strong solution of the preserving substance. In England several scores of patents have been issued for processes for the preservation of animal and vegetable substances, including timber, of which only four were in the last century, the earliest being in 1737. The first process for preservation by chemicals which was extensively applied was that of Mr. Kyan, which was patented in England in 1832, and some time afterward introduced into the United States by the inventor. The wood was steeped in a solution of corrosive sublimate, or it was placed in an exhausting cylinder and the solution forced in under atmospheric pressure. Its expensive-ness, however, and the difficulty of conducting it, have prevented this process, called kyani-zing, from being widely employed. Of the preparations now in use in England, that of Mr. J. Bethel, consisting of the oily mixtures obtained by a rough distillation of the tarry liquor of gas works, is considered as very efficient.

A solution of pyrolignate of iron is also regarded as a good preservative. Because of the presence of creosote in these liquors and its known antiseptic property, the process is called creoso-ting, and is effected by the method of exhaustion and subsequent injection by intense pressure (150 lbs. or more to the square inch), which is continued for 48 hours or longer. The process is adopted by many of the railway companies of England, and notwithstanding its expensiveness is highly approved of. Wood thus prepared is said to be also protected against the teredo navalis or ship worm. In France the process of Dr. Boucherie has been extensively employed for railway and ship timber. The material used is sulphate of copper dissolved in water so that at a temperature of 60° the density of the solution is about 1*006. A water-tight cap is fastened on one end of the log, and into this is inserted the end of a vertical tube, 30, 40, or more feet in height, into which the solution is poured. The hydrostatic pressure first forces out the sap, and then the solution enters the pores of the wood. Timber thus prepared at Fontaine-bleau was fully impregnated in 24 hours in lengths of 7 ft.; but sticks of the same timber 40 ft. long required 10 days for the completion of the process.

Another process employed in both England and France is that of Mr. Payne, patented in 1841. The timber is introduced into a long iron cylinder, which is then closed air-tight; steam is driven in, expelling the air through a valve, and a cold solution of sulphate of iron is pumped in, which condenses the steam and produces a partial vacuum. This is made more complete by the air pump, and the cylinder is then filled with the solution, which is still forced in under considerable pressure. In a few minutes the solution is let out and the cylinder is again filled with air. This is again expelled by steam, and a solution of another salt is admitted, of such a character as will react upon the sulphate of iron, producing double decomposition and leaving in the pores of the wood an insoluble salt. Chloride of calcium answers this purpose, and the insoluble sulphate of lime remains in the wood, the chloride of iron being dissolved out. Carbonate of soda may be substituted, in which case insoluble carbonate of iron is formed, the soluble salt being sulphate of soda. The process of Sir William Burnett, called " burnettizing," patented in 1838, is often used. This consists in injecting chloride of zinc solution into the timber after the pores have been exhausted of air.

The process was extensively employed in 1850 at Lowell, Mass., for the locks on the Merrimack river. Wood may be preserved by immersing it when thoroughly dried in a solution of paraffine, heated under a pressure of six or eight atmospheres. The solvent, which may be coal naphtha, is then distilled off, to be used again. The See-ley process consists in subjecting the wood to a temperature between 212° and 300° F. while immersed in a bath of creosote for a sufficient time to expel the moisture. The pores are then filled only with steam, and the hot oil is replaced with cold, which creating a vacuum in the pores, they are filled with the oil.