 these wood- and ray-cells. The wood-cells are pitted with oblique, slit-shaped, simple pits.
The vessels have bordered pits, and the septa are perforated each by one large circular opening. The smaller vessels have delicate spirals on their walls as well as bordered pits.
Nördlinger says that pith-flecks occur occasionally.
It is impossible to distinguish between the wood of the varieties pedunculata and sessiliflora.
(2) Its density varies considerably. Taking the weight of a given volume of water as unity, the weight of an equal volume of oak timber may weigh from 0·633 when air-dry to 1·280 when fresh cut. We may take the average density of green—i.e., newly-felled—oak with all its sap present, as about 1·075, and that of the seasoned wood as about 0·78.
It must be borne in mind, however, that these weights refer to the wood as a structure—that is, a complex of vessels and cells, etc., containing air and liquids—and do not give the specific gravity of the wood substance itself. The latter may be obtained by driving off all the air and water from the wood, and is found to be 1·56, compared with an equal volume of water taken as unity. It is the varying quantities of this wood substance, and of air and water in the cavities, which make the density of different pieces of oak vary so much.
(3) The proportion of sap contained in the cavities of the vessels, cells, etc., of course differs at different times. In the spring, just as the buds are opening, the quantity of water increases more and more up to about July, when the maximum is attained; the proportion of water to solids then sinks until October, when the leaves fall; it increases again up to Christmas-tide, and then sinks to the minimum in the coldest part of the winter. The proportion of water to the total weight of the felled wood may vary from 23 to 39 per cent.
(4) Obviously the loss of water on drying causes shrinkage of the wood, and although oak shrinks very little in the direction of its length (0·028 to 0·435 per cent), the effect is very marked in other directions. In the radial direction—i.e., in the direction of the medullary rays—it may shrink from 1 to 7·5 per cent of its measurement when first felled; and in the direction vertical to this—i.e., parallel to a tangent to the cylindrical stem—the variation is from 0·8 to 10·6 per cent. Of course, green oak shrinks much more than seasoned and older wood, the process of seasoning being, in point of fact, the period of chief shrinkage. It is said that wood from the variety sessiliflora shrinks more than that of the variety pedunculata, but it may be doubted how far the difference would hold if sufficiently numerous comparisons were made.
(5) Swelling may be regarded as complementary to shrinkage. It has been found that if oak wood is allowed to absorb water until thoroughly saturated it will increase from 0·13 to 0·4 per cent in length, and be distended radially from 2·66 to 3·9 per cent, or tangentially 5·59 to 7·55 per cent, according to age and condition, young wood swelling more than old. It has also been found that the total volume increased from 5·5 to 7·9 per cent, and the weight from 60 to 91 per cent, on complete saturation.
(6) Elasticity and Tenacity.—Oak is very elastic, and easily bent if steamed, and it does not readily splinter. When pulled in a direction parallel to the length of the structure the absolute tenacity = 2·23 to 14·51 kgr.—i.e., it took a pull equal to this weight per 1 sq. mm. of section to pull the wood asunder.
The limit of elasticity corresponds to a load of 2·72 to 3·5 kgr., according to various authorities, the specimen lengthening th in the former case.
The modulus of elasticity is given as 826 to 1,030 kgr., and the breaking limit as 4·66 to 6·85.
When the pull is in a direction across the length of the fibers, the results differ according as the load is applied so as to act radially or tangentially.
When acting radially the modulus of elasticity is given as 188·7 kgr., and the breaking limit as 0·582 kgr.
When acting parallel to a tangent the modulus of elasticity = 12