BUILDING MATERIALS. The mate rials used for structural engineering and archi tectural purposes may be conveniently divided into two general classes — "materials of con struction,* such as the woods, stone, metals, cements, etc., and •materials of consumption," such as coal, water, oil, etc., which are con sumed or transformed while being used.
In this article, the materials of construction will be briefly considered according to their physical and chemical properties, and their adaptability for various purposes, leaving the consideration of the materials of consumption to the sphere of chemistry and physics where they properly belong.
Apart from their chemical composition, the principal properties of building materials im portant to the engineer are the (density" or specific gravity of the substance; its (resist ance" or capacity to withstand strains and stresses; the •hardness* or power to oppose penetration; its (toughness" or capacity to elongate under tension without rupturing; its 'brittleness,' which is the opposite of tough ness; and its behavior under conditions of varying temperature, or when worked in the many ways required by structural operations.
Timber or Wood.— The following general facts relative to the physical properties of wood have been determined by experiment: (1) Bleeding has not much effect on the strength of wood, but increases its flexibility slightly, and it is probable that bled timber will stand expo;ure to the weather fully as well as the unbled. (2) In general, moisture absorbed in the form of sap, or in the form of water after seasoning, reduces the strength of wood. Well seasoned wood or that which contains not more than 12 per cent of moisture is from 75 to 100 per cent stronger than green timber. (3) In artificially-dried. timber any remaining moisture exists in a uniform percentage throughout the mass, a condition which requires months, and sometimes years, to attain in heavy air-dried timber. (4) The strength per square inch of section of large timbers is in every way equal to that of small timbers, provided they are equally sound and contain the same percentage of moisture. (5) In general, the strength of woods of uniform structure increases with their specific gravity, that is, the heavier wood is generally the stronger. Oak, however, appears
to be an exception to this rule. (6) Seasoned wood will increase in weight to the extent of 5 to 15 per cent if exposed to the weather. This excess of weight can be easily reduced by keeping the timber in a warm dry place for a week or 10 days.
In Table 1 below is given the physical prop erties of some of the woods suitable for structural, interior finishing, decorative and other similar purposes. The °elastic limit' given in the table is a relative quantity, as there is no definite 'elastic in woods similar to that in metals. In Table 2, the strengths of green woods of various kinds un der different stresses. When well dried their strength is increased from 50 to 75 per cent. It is considered that only when wood is green can a test be accurately made, as it is quite impos sible to gauge the exact degree of .moisture re maining in seasoned wood.
In selecting and preparing timber or wood for structural purposes, a careful considera tion of the following facts in addition to those already stated is quite important: (1) That timber grown in moist soils is lighter, and decays more quickly than that grown in dry, sandy soil, and that, usually, the best timber is that grown in a dark soil intermixed with gravel, with the exception of those various kinds, such as poplar, cypress, willow and all others that naturally grow best in a wet soil. (2) That the wood of trees grown upon the plains or in the centre of forests is less dense than that of those grown upon the edge of a forest or upon the side of a hill. (3) That in temperate latitudes, as in the United States, standing timber should be selected in the latter part of July or the first part of August, when the sound and healthy trees are indicated by fresh green leaves, in contrast to the unsound and unhealthy trees, the leaves of which begin to turn yellow at that season of the year. caying branches, a scarcity of leaves and the tendency of the bark to become rough and to separate from the wood, are positive indi cations that the physical properties of the wood are impaired.