II. Physical Properties or Material Condition. The group of physical properties now to be considered, and on which the mechanical properties are directly dependent, are of considerably more importance than the mere outward appearance in determining the general value of different kinds of wood for technical purposes. These properties all constantly act and react on each other. The density or specific weight is directly affected by the moistness or dryness of the wood, which also produces changes in its volume; and as the shrinking or the swelling thus caused does not take place equally in all directions, change in the quantity of water contained likewise produces change in form (warping). So long, therefore, as the wood remains in sound condition, any alteration in density, moistness, volume, or form is bound to affect all the other three conditions to some greater or less extent.

1. Density and Weight. The proportion of the wood-fibres forming the framework, and of the other elementary substances contained in wood, varies in different parts of the tree, as well as in different kinds of trees, and also according to age, soil, situation, and sylvicultural method of treatment. Many other substances are present besides the woody tissue, and all kinds of wood contain, to a greater or less extent, lumina and hollow spaces filled with air or water and distributed throughout it in groups or otherwise.

The only convenient standard for the density of wood is its specific gravity— i.e., its weight as compared with an equal volume of pure water at 4° C. The independent investigations of Sachs and R. Hartig agreed in determining the facts that no great difference is noticeable in the specific weight of the pure woody substance (exclusive of the lumina and other hollow spaces) of our chief timber-trees; that it may be taken as 1:56 for Oak, Beech, Birch, Scots Pine, and Spruce; and that there is no perceptible difference between the specific weight in the substance of the sapwood and the duramen in the same stem. According to Exner (Die Technischen Eigenschaften der Hölzer, in Lorey's Handbuch, vol. ii. p. 118), it is 1-53 for Beech, 1.52 for Elm, 1:48 for Lime, Birch, and Poplar, and 146 for Maple, Sycamore, and Silver Fir, or about 15 on a general average. Such scientific investigations will of course vary in their results, according to the state of the atmosphere in which they are carried out.

It is not, however, the specific weight of the actual woody substance which is of technical importance, but the specific gravity of the wood per cubic foot, or per load or ton. This of course varies considerably according to the amount of sap or water still in the tree, the specific weight of green wood being greater than that of seasoned or air-dried timber, and greater still than that still further dried by artificial processes; because even in seasoned air-dried wood on the average from 8 to 12 per cent of the weight still consists of water, which can only be driven off by subjecting the wood to a temperature of 105° to 110° C. (221° to 230° Fahr.) until no further loss of weight is observable, and the wood becomes as thoroughly dessicated as is practicable (see p. 533).

Whereas the specific weight of green wood shows whether it is floatable or not, it is only that of seasoned wood which is of practical use in estimating the technical properties of different kinds of timber for constructive and other purposes. And while the specific weight varies for each kind of wood, both when green and after seasoning, with soil, local climate, aspect, part of tree, and season of the year, even in seasoned wood variations in weight are caused by hygroscopicity according to the amount of moisture in the atmosphere. The results of many investigations into the specific gravity of European woods are as follows: 1

1 These data are taken from Gayer's Forstbenutzung, and Exner, op. cit.

1 British Beech is denser and heavier than German Beech (see p. 452). 2 Several such discrepancies are noticeable between Exner's and Gayer's tables. The former always, and the latter nearly always, has evidently taken as the average the arithmetical mean between the two extreme limits of range. Nothing shows that in any one case a true average of all the specimens weighed has been taken.

Note. As the weight of water is 62 lb. per cubic foot, the average weight avoirdupois of any kind of wood per cubic foot may be found by multiplying its specific gravity by 624. And in the same way, the actual dead-weight of a 40 cub. ft. load, or a 50 cub. ft. ton, or any other quantity of timber, can easily be found for estimating the haulage necessary; and the weight of beams and scantlings may be similarly calculated for structural purposes.

The dead-weight of common British coniferous wood has been found by Mr. D. F. Mackenzie (Transactions of the Highland and Agricultural Society of Scotland, 1901, p. 143) to be as follows:

One ton by weight is contained in the following quantities of wood :—

The tables on pages 438-439 show the results of investigations made many years ago by Mr. Rait, forester at Castle Forbes, regarding the weight of home-grown timber in the north of Scotland.

The specific weight of any wood not only determines its floatability and the cost of haulage, but for one and the same kind of wood it also gives a clear indication of its hardness, heating-power, and general durability. A short spring and a long warm summer generally tend, and especially in ring-pored trees (Oak, Ash, Elm, Sweet-Chestnut, Robinia, &c.), to increase the weight of the wood produced, owing to the greater density of the summer zone. But this is not always the case, as in woods with evenly distributed pores (Beech, Maple, Sycamore, and softwoods) the density hardly varies with the breadth of the ring. And this is further evidenced by the superior quality of the Scots Pine and Larch grown in the north of Scotland, as compared with the milder and warmer southern districts of England; and, as a matter of fact, narrow-ringed coniferous wood is usually heavier than that showing broad rings. But there can be no hard-and-fast rule about the matter, as so many factors concerning the climate, elevation, aspect, physical properties of soil, &c., come into operation with regard to the energy of nutrition and the amount of woody substance actually produced each year. In Conifers, the greater the quantity of resin contained in the wood of any given species, the higher is its specífic gravity.

No definite rule seems to hold with regard to the specific gravity of the wood in the upper or the lower, or the inner and the outer portions of a stem. In the case of the trees with a very distinct difference in colour between heartwood and sapwood (Oak, Larch, and Scots Pine: trees which have all a deep tap-root, and are also decidedly light-demanding), the heartwood is heavier than the sapwood; but in most other cases nothing like any constant variation is traceable, and in Birch the younger outer wood is heavier than the older inner portion. The wood in young trees is usually closer and heavier than in old trees, because of the relatively larger space required to be provided in large trees for the upward conveyance of sap; and at the same time the annual rings formed are broader, owing to the trees then being in their most vigorous period of growth. This effect of the age of trees on the specific gravity of their wood showed itself clearly in R. Hartig's investigations concerning the Beech.

Branchwood, however, is usually heavier than the wood of the bole, while this is (except in very resinous Conifers) heavier than that of the roots. The starchy reserve-nutrients and the mineral salts do not much affect the specific gravity of the wood, as they are mostly contained in the bark, cambium, and youngest wood (see vol. i. p. 306).

Kind of tree

or without bark.

weight of a piece

per cubic

feet to a ton weight

cut