Oak of a good quality is more durable than any other wood that attains a like size. Vitruvius says it is of eternal dura tion when driven into the earth : it is ex tremely durable in water ; and in a dry state it has been known to last nearly 1000 years. The more compact it is, and the smaller the pores are, the longer it will last ; but the open, porous, and foxy-col ored oak, which grows in Lincolnshire and some other places, is not near so durable. Besides the common British oak (Quer elm robur), the sessile-fruited bay-oak (Quercus sessiliflora) is pretty abundant in several parts of England, particularly in the north. The wood of this species is said by Tredgold to be darker, heavier, harder, and more elastic than the com mon oak ; tough, and difficult to work ; and very subject to warp and split in sea soning. Mr. Tredgold seems disposed to regard this species as superior to the com mon oak for shipbuilding. But other, and also very high authorities, are opposed to him on this point ; and, on the whole, we should think that it is sufficiently well established that for all the great practical purposes to which oak timber is appliebl, and especially for shipbuilding, the wood of the common oak deserves to be pre ferred to every other species.
The oak is among the most useful pro ductions of temperate climates, with the exception of a few on the mountainous party of the equatorial regions. More than eighty species are known, of which one half inhabit North America, either within the territory of the United States, or on the mountains of Mexico. The white oak (Q. alba) is one of the most valuable. It extends from lat 46° to Flor ida, and from the Atlantic to a little west of the Mississippi. It attains the height of seventy or eighty feet, with a trunk six or seven in diameter. It is usual, after stripping the oak of its bark, to leave it standing for three or four years before it is cut for use. This species, and the atillata, are the species which furnish staves for casks, of which the consump tion is immense. White oak timber is imported in immense quantities, from the ports of the Northern and Middle States ; and that brought from Quebec is procur ed chiefly on the borders of Lake Cham plain, in the states of New York and Ver mont. It is also used for making the keels and knees of ships. The Q. macro carioa is remarkable for the large size of the leaves and acorns but the wood is of little value. The Q. lyrata is exclusively confined to wet swamps. The acorns are nearly covered by the cups. The timber is large and highly esteemed.
The live oak (Q. 'arms) is a tree of the very first importance. It is found grow ing along the Atlantic shores of the Unit ed States for 1600 miles, from Norfolk southwards. The leaves are evergreen, coriaceous, and entire. It does not usu ally attain greater height than forty or forty-five feet, with a trunk one or two in diameter, but the wide and branching summit furnishes knees of vessels. The
wood is used for the naves and felloes of heavy wheels, for which purposes it is far superior to the white oak, as well as for screws and the cogs of mill-wheels. In the Southern States it is used for the frame-work of ships, and is looked on to be as durable as any European variety. The bark, too, is excellent for tanning. The black or quercitron oak (Q. tinctorza) is a large tree, found throughout the United States south of latitude 43°, and abundant in the Middle States. It is re cognized by the yellow stain which it gives to the saliva on being chewed. The wood is reddish and coarse-grained, and is frequently substituted for white oak in building. It furnishes a large proportion of the red oak staves which are exported to the West Indies, and the bark Is ex tensively employed in tanning. From the cellular integument quercitron is ob tained—an article extensively employed in dyeing wool, silk, and paper-hangings, and which forms an important article of export from Philadelphia. The cork oak (Q. suber) furnishes the cork of com merce, which substance is the outer, thick, fungous covering of the bark, and is detached, at intervals of ten or twelve years, for as . many as twelve or fifteen times, but after the fifth or sixth the quality degenerates. If not removed af ter a certain period, it splits and falls off, and is replaced by a new growth beneath. In some countries, where cork is abun dant, the inhabitants use it for lining or covering their houses. When burnt iu close vessels, a black powder is obtained, known under the name of Spanish black. The cork oak is cultivated in Spain, Por tugal, and the south of France. It is best adapted to a dry, sandy, mountainous soil, and is never found in limestone dis tricts.
To ascertain the strength of New For est oak (English), a seasoned stick of tim ber was selected in April. Flom about midway between the centre and circum ference of the tree, and beginning at about four feet from the ground end, a piece of very good and perfectly sound timber was cut, and reduced to the dimensions of five inches square, and eleven feet long. It was laid across two uprights ; and a rough scale-like platform to contain the weight, formed of a very large plank, was sus pended from the centre by a strong tim ber chain. Upon this platform, piece after piece was laid of hard Purbeck stone, until it became evident that there was sufficient to effect the fracture, and in a few seconds the whole fell to the ground. The stones employed were then weighed, and the weight of the platform and chain being added, it was found that the aggre gate weight by which the object had been obtained, was 9061 pounds, or 4 tons, 3 quarters, and 17 pounds.
Oak bark, in the inner cortical of young trees, contains 77 of 111 of the tannin principle. The cellular, or middle, only 19 of 43, and the external part scarcely any tannin. In spring, the tannin is more than in winter. See TAN.