A definition of wood in the economic sense requires that it be distinguished principally from fiber, because of the especially close similarity between them. Fibers, we have seen, are sometimes woody, while all true woods, as will presently appear, are fibrous. Cellulose is the main constituent of each. Woods and woody fibers contain in addition to cellulose more or less of a substance (or mixture of substances) known as lignin. This is of uncertain chemical composition though known to consist of the same elements as cellulose. Like that substance it permits water and gases to pass readily through it. It is distinguished from cellulose by turning yellow instead of blue when treated with sulphuric acid and iodine. It is the fact that wood is used in comparatively large, firm masses which chiefly distinguishes it from fibers; while it is the fibrousness of wood that most readily distinguishes it from cork and other massive materials to be presently studied. Let us then for our present purpose define wood as the comparatively hard mass of fibrous material which serves mainly for mechanical support in plants and in various artificial structures.

From earliest times wood has been the most widely useful material of construction. Our civilization has been developed largely upon its possibilities. In prehistoric times wherever it was abundant, wood was used almost exclusively for buildings, utensils, and implements; though in regions less favorably situated various substitutes of course had to be found. Even before skill in metal-working had been acquired men were able to shape wood by means of their rude stone tools into many highly useful forms. Thus, only the rudest means are necessary for making from a single log a "dugout" canoe capable of holding many men: a fire kept alive along the top of a fallen trunk burns or chars the wood so that it may be scraped away till the desired form is reached. With the coming of metal tools and their improvement from time to time, more extensive use could be made not only of wood, but also, and for the same reason, of stone and other hard

materials. With still further mastery over metals both wood and stone have lately come to be replaced rather extensively in building by iron and steel. Nevertheless, in spite of the increased facilities for obtaining and working its various rivals, wood is now being used more than ever. During the past fifty years, in this country, each decade has shown a large and steady increase in the amount of wood used proportional to the population. The reason for this must be sought in the remarkable advantages which wood possesses over all other materials for a wide range of uses.

The economic superiority of wood is well shown for example, by comparing it with metals such as iron and steel.

(1) The supply of wood under proper forest management is practically inexhaustible and very widespread, while mines are not only exhaustible but strictly local.

(2) Wood is cheap, and metals are dear because of the much greater labor required in metal-working. Even as lumber, after longdistance transportation, wood rarely costs more than 50 cents a cubic foot, the price of iron being from $5 to $10; while the much greater ease with which wood may be shaped, reshaped, and combined in structures makes it much less expensive to manufacture.

(3) Wood is stronger than is commonly supposed. In tensile strength, i. e., resistance to a pull lengthwise of the grain, a bar of hickory exceeds a similar bar of iron or steel of the same weight. Similarly the resistance to compression parallel to the grain (i. e., against the ends of a stick) is found to be greater in a selected piece of hickory or hard pine than in a rod of wrought iron of the same weight and height. Though under certain conditions iron appears to be much stiffer than wood, it is found that a ten-foot beam of hard pine requires considerably more load to bend it by one inch than a similar bar of iron of same weight and length.

(4) Wood endures a far greater distortion than metal without losing its power to recover the original form.

(5) Wood does not rust or crystallize like metal, and,

(6) as wood is a poor conductor of heat it is not only pleasanter to touch but when used as the chief material of dwellings and ships has none of the injurious effects of iron and steel.

(7) Wooden beams though combustible, are often safer in case of fire than iron ones because the latter twist out of shape at high temperature in a way to wreck the entire structure.

(8) Being unaffected by wines or other weak acids, and imparting no disagreeable flavor, certain woods may be used for casks where metal would be objectionable or even poisonous.

(9) Woods have an organic beauty unrivaled by metals.

(10) The peculiar elasticity of certain woods render them incomparably superior to any metal as material for the resonant parts of violins and similar musical instruments.

(11) Pieces of wood may be easily and strongly united simply by glueing, while metals require the more difficult operation of welding or soldering.

As against wood it must be said

(1) that it cannot be melted and cast or rolled; though by steaming, rods or sheets may be readily bent into curves of small radius; and when reduced to pulp, as we have seen, it can be pressed into almost any shape.

(2) It shrinks or expands with variations of moisture, more than metals do under ordinary variations of temperature.

(3) It decays unless proper precautions are taken to prevent, though under water wood lasts longer than steel or iron.

(4) It is more easily crushed than iron and therefore is not so well suited for bearing the greatest weights or for resisting very heavy blows.

(5) Finally, the greater hardness of many metals gives them obvious advantages over wood for sharp implements and a large variety of objects that have to stand severe wear. A great deal is often gained by combining wood and metal because the properties of one so largely complement those of the other.