1290

THEORY AND PRACTICE OF ENGINEERING.

Rook II.

of carbon anil water; moist woody fibre in a state of decay evolves one volume of carbonicacid for every volume of oxygen absorbed ; woody fibre containing carbon and the elementsof water, the result of the action of oxygen upon it is the same as if pure charcoal hadcombined with oxygen ; the elements of water are not, however, contained in the woodyfibre in the form of water, but in their separate states.

When wood in water, therefore, putrefies or rots, carbon and oxygen are separated fromit in the form of carbonic acid, and hydrogen in the form of carburetted hydrogen ; when thisdecay takes place in the air its hydrogen does not combine with carbon but with oxygen,for which it lias a much greater affinity at common temperatures, and being convertedinto vegetable mould, and in a state to be acted upon, is taken up by the plants whichgrow within its influence, whilst the oxygen which it contained is gradually given outto the atmosphere as its decomposition is carried on.

This decomposition may proceed, whether the air has free access* or is excludedfrom it; under water or in dry air timber will endure for ages without much apparentchange, but when it has been immersed, if the air be admitted freely, the oxygen sur-rounding it is converted at once into carbonic acid, and the woody fibre is decomposed :240 parts of dry oak sawdust would convert 10 cubic inches of oxygen into as manyof carbonic acid, which contains 3 parts by weight of carbon, whilst the weight of the saw-dust is diminished by 15 parts, and 12 parts by weight of water are separated from thewood.

The decay of wood bears a strong resemblance to the slow combustion of those bodiescontaining a quantity of hydrogen, which is oxidised at the expense of the oxygen of theair, while carbonic acid is produced from the elements of the wood : carbonic acid is not,however, formed at a common temperature.

Wood in a state of decay sets free two atoms of oxygen and one of carbon for every twoequivalents of hydrogen oxidised; this decay is not effected in a short space of time atcommon temperatures, or when humidity is not present: when timber not in a perfectlydry state is in contact with any of the alkaline earths, its decay is accelerated, the presenceof an alkali enabling it to absorb oxygen, which alone it has not the power to do, whihtan acid would produce a different effect and retard decay. In a loamy soil timber willendure some time, because it is kept from contact with air : but if this earth contain muchwater, then decay will be produced; in moist situations, where the soil is composed of sandand carbonate of lime, decay is very rapid, the alkaline lime assisting much to hasten it.

When woody fibre or sawdust is moistened with water and excluded from the air forany time carbonic acid gas is evolved in the same manner as when the air is admitted, anda putrefaction takes place, converting it into a rotten state; when exposed to a certaindegree of moisture and a temperature not sufficiently high to evaporate the water, decom-position takes place suddenly, and as this increases, carbonic acid and hydrogen are givenout. Sapwood is consequently more perishable than the heartwood, as it contains thesejuices which more readily ferment; hence it must be obvious that unless they are abstractedby seasoning, mischief arises from confining them in the wood, by covering the surface withpaint or pitch, or even bedding their ends solidly in newly constructed walls ; when timberis quite immersed in water, and kept so, it is by no means so liable to perish as when exposedto moisture only, aided by a slight degree of warmth.

The well-seasoned timber employed by the carpenters of the middle ages, in the old hallsparticularly, as well as in our churches and chapter-houses, needed neither paint nor dressingof any kind to preserve them ; and in many instances they are found sound after four orfive centuries. The weather-boarding of our English barns, which in many instances is ofundressed elm timber, owes its duration to a hard external surface, which it acquires bytime, and which resists almost the edge of a sharp tool, or the point of a nail, apparently acoating of silica, quite impervious to water, or even the action of moisture; this silica maybe derived from the decomposition of the wheat straw with which the barns are usuallythatched.

"When the engineer or the carpenter selects timber for the purposes of construction dueregard must be paid to its firmness, density, and strength, without which qualities nosolid or durable works can be executed. The great weight with which timber is sometimesloaded, placed vertically or horizontally, shows at once the caution which should be usedin its selection, and when it is exposed to the action of the air, the necessity of having it soseasoned and protected that it should not be subject to decay : when employed in theformation of centres to the arch of a bridge, durability for a time only is required, but inpermanent roofs and floors it should be capable of resisting all efforts which may destroyits utility or strength ; its warping and shrinking in edifices entirely formed of it lead some-times to the greatest inconveniences, as the strength of a truss may be entirely destroyed bythis cause.

The oak and the fir are the species most in use, and are the best for the purposes of con-struction : the first has the advantage of duration over the other, and will bear either tobe exposed to the action of the air, the water, or to be buried in the earth ; in water it willendure to a time unknown, and it has the advantages also of strength over all other timber.