The relative terms hard and soft, elastic or non-elastic, and the proportions of resins, gums, etc, as applied to the woods, appear to be in a great measure explained by their examination under the microscope, which develops their structure in a very satisfactory manner.
The fibres of the various woods do not appear to differ so materially in individual size or bulk, as in their densities and distances: those of the soft woods, such as willow, alder, and deal, appear slight and loose; they are placed rather wide asunder, and present considerable intervals for the softer and more spongy cellular tissue between them; whereas in oak, mahogany, ebony, and rosewood, the fibres appear rather smaller, but as if they possessed a similar quantity of matter, just as threads containing the same number of filaments are larger or smaller accordingly as they are spun. The fibres are also more closely arranged in the harder woods, the intervals between them are necessarily less, and the whole appears a more solid and compact formation.
The very different tools used by the turner for the soft woods and hard woods respectively, may have assisted in fixing these denominations as regards his art; a division that is less specifically entertained by the joiner, who uses the same tools for the hard and soft woods, excepting a trifling difference in their angles and inclinations; whereas the turner employs for the soft woods, tools with keen edges of thirty or forty degrees, applied obliquely, and as a tangent to the circle; and for the hard woods, tools of from seventy to ninety degrees upon the edge, applied as a radius, and parallel with the fibres, if so required. The tools last described answer very properly for the dense woods, in which the fibres are close and well united; but applied to the softer kinds, in which the filaments are more tender and less firmly joined, the hard-wood tools produce rough, torn, and unfinished surfaces.
In general the weight or specific gravity of the woods may be taken as a sure criterion of their hardness; for instance, the hard lignum-viaae, boxwood, ironwood, and others, are mostly so heavy as to sink in water; whereas the soft firs, poplar and willow, do not on the average exceed half the weight of water, and other woods are of intermediate kinds.*
The density or weight of many of the woods may be increased by their mechanical compression, which may be carried to the extent of fully one third or fourth of their primary bulk, and the weight and hardness obtain a corresponding increase. This has been practised for the compression of tree-nails for ships, by driving the pins through a metal ring smaller than themselves directly into the hole in the ship's side;† at other times, (for railway purposes,) the woods have been passed through rollers, but this practice has been discontinued, as it is found to spread the fibres laterally, and to tear them asunder;‡ an injury that does not occur when they are forced through a ring, which condenses the wood at all parts alike, without any disturbance of its fibrous structure, § even when tested by the
* The most dense wood I have met with is in Mr. Fincham's collection; it is the Iron Bark wood from New South Wales: in appearance it resembles a close hard mahogany,but more brown than red; its specific gravity is 1.426, - its strength, (compared with English oak, taken as usual at 1.000,) is 1*557. On the other hand the lightest of the true woods is probably the Cortica, or the Anona palustris, from Brazil, in Mr. Mier's collection; the specific gravity of this is only 0.206, (whereas that of cork is 0.240,) it has only one-seventh the weight of the Iron Bark wood. The Cortina resembles ash in colour and grain, except that it is paler, finer, and much softer; it is used by the natives for wooden shoes, etc.
The Pita wood, that of the Fourcroya gigantea, of the Brazils, an endogen almost like pith, (used by the fishermen of Rio de Janeiro, as a slow match, for lighting cigars, etc.; also like cork for lining the drawers of cabinets for insects,) and the rice paper plant of India and China, which is still lighter and more pithy, can hardly be taken into comparison.
† Mr. Annersley's Patent, 1821, for building vessels of planks only, without ribs.
‡ Dublin and Kingston Railway.
§ The mode at present practised by the Messrs. Ransoms of Ipswich, (under their patent,) is to drive the pieces of oak into an iron ring by means of a screw press, and to expose them within the ring to a temperature of about 180° for twelve or sixteen hours before forcing them out again.
The tree-nails may be thus compressed into two-thirds their original size, and they recover three-fourths of the compression on being wetted; they are used for microscope; after compression the wood is so much harder, that it cuts very differently, and the pieces almost ring when they are struck together; fir may be thus compressed into a substance as close as pitch-pine.
In many of the more dense woods, we also find an abundance of gum or resin, which fills up many of those spaces that would be otherwise void: the gum not only makes the wood so much the heavier, but at the same time it appears to act in a mechanical manner, to mingle with the fibres as a cement, and to unite them into a stronger mass; for example, it is the turpentine that gives to the outer surface of the annual rings of the red and yellow deals, the hard horny character, and increases the elasticity of those timbers.*
Those woods which are the more completely impregnated with resin, gum, or oil, are in general also the more durable, as they are better defended from the attacks of moisture and insects.
Timbers alternately exposed to wet and dry, are thought by Tredgold and others, to suffer from losing every time a certain portion of their soluble parts; if so, those which are naturally impregnated with substances insoluble in water may, in consequence, give out little or none of their component parts in the change from wet to dry, and on that account the better resist decay: this has been artificially imitated by forcing oil, tar, etc, through the pores of the wood from the one extremity.†
Many of the woods are very durable when constantly wet; the generality are so when always dry, although but few are suited to withstand the continual change from one to the other state; but these particulars, and many points of information respecting timber-woods that concern the general practice of the builder, or naval architect, such as their specific gravities, relative strengths, resistances to bending and compression, and other characters, are treated of in Tredgold's Elements of Carpentry, at considerable length. ‡ railway purposes, but appear equally desirable for ship-building, in which the treenails fulfil an important office, and in either case their after-expansion fixes them most securely. - See Minutes, Inst. Civ. Eng., 1841, p. 83-7.
* See the treatment of the Firs in Norway, article Firs, in Catalogue.
† The durability of pitch pine, when "wet and dry," is however questioned.
‡ The work contains a variety of the most useful tables: the reader will likewise find a set of tables of similar experiments on American timbers, by Lieut. Denison, Royal Engineers, F.R.S., etc, in the Trans. Inst. Civ. Eng., vol. ii. p. 15, and also and other timbers for ship-building, the lance-wood shafts for carriages, the staves of casks, and various other works.