Fuel, the material used for producing heat by combustion. Wood, the most universally known variety of fuel, presents itself in forms and qualities varying with the tree, and to some extent with the part from which it is obtained. It is made up of several compounds -the woody tissue or lignine, the sap, and the alkaline and earthy matters which remain after combustion, as its ash. It also contains a variable proportion of water. The first two named are its combustible ingredients, upon which its value as fuel depends; and of these the lignine is of chief importance, often constituting in thoroughly dried wood 95 per cent. or more of its weight. Yet it is not the ingredient which gives to the wood its distinctive character, except so far as this depends on its density, for pure lignine, freed from the matters soluble in water, alcohol, or alkalies, is of uniform composition in all woods and leaves. The sap and the matters it brings with it differ in the different woods; on those of the pine family the sap bestows their resinous properties, on the oak its tannin, and on all the peculiar extractive matters which distinguish them. Its proportion is small in the mass of the wood, and varies at different seasons.

Schubler found that the ash tree felled in January contained of water 28.8 parts, while that cut in April contained 38.6 parts; the sycamore, 33.6 in January, and 40.3 in April; the white fir, 52.7 and 61.0. As the expulsion of the water present involves the consumption of a portion of the carbon of the wood, the more thoroughly this is air-dried or seasoned, the greater is its heat-producing power. As it dries it loses sometimes one fifth of its weight, yet from 20 to 25 per cent. of that which remains is moisture. If this be all expelled, the wood will absorb from the air 10 per cent. or more of moisture. The mean quantity of hygrometric water in 100 parts of various specimens of wood is thus given in the treatise of Richardson and Ronalds; in cord wood the seasoning would not have been so effectual as in the specimens employed:

SIX MONTHS AFTER FELLING.

Resinous woods.

Non-resinous woods.

Trunk wood.......

29

26

Brush wood..........

32

34

Young branch wood........

33

36

IN THE DRIEST STATE.

Trunk wood.......

15

17

Brush wood.........

15

20

Young branch wood..............

15

9

The gravity of wood varies greatly with the different species, and also with its condition as to dryness. Though the solid fibre is heavier than water, the air contained in the cells causes it commonly to float. As the fibre is the heaviest ingredient, a greater weight in dry wood indicates a greater proportion of woody or combustible matter. The experiments of Marcus Bull upon American woods were conducted with great nicety, the specific gravity of each being taken by coating the dry sample with a varnish of the same weight as water, thus retaining the air in the cells. The table on p. 517 is contained (except changes in some of the names) in his original memoir, read April 7, 1826, and published in the Transactions of the American Philosophical Society" (vol. iii., new series, pp. 1-60). This gives the weight of a cord of wood as it should be put up, the interstitial matter even then amounting to 44 parts in 100 of the whole bulk; as it often much exceeds this, the measure affords an estimate of the quantity of woody matter even more uncertain than would be the estimate by weight, variable as this has been shown to be.

The arrangement of the columns is as follows: A, specific gravity; B, lbs. avoirdupois in one cord; C, charcoal in 100 parts of dry wood by weight; D, specific gravity of dry coal; E, lbs. of dry coal in one bushel; F, lbs. of dry coal from one cord of dry wood; G, bushels of coal from one cord of dry wood; IT, time in hours and minutes during which 10° of heat were maintained in the room by the combustion of 1 lb. of each wood; I, value of specified quantities of each wood compared with shell-bark hickory as the standard.-When wood is exposed to the action of heat, its more volatile ingredients, as the hygrometric moisture, first escape; its gaseous elements are next disturbed from their state of equilibrium, and the hydrogen and oxygen when set free from one combination enter into new ones; portions of these gases combine to produce water; other portions seize upon the carbon and form with this a multitude of unstable compounds, varying with the degree of temperature and the proportions of the elements present.

If the process be conducted in close vessels away from the action of air or oxygen, the volatile ingredients may be driven off in the form of inflammable gases, and of vapors of water holding in solution numerous combustible principles, and last of all the vapors of the resins and ethereal oils constituting tar. When wood is consumed in the air, heat is first applied to drive out the volatile elements. The hydrogen eliminated in the pores of the fuel at a heat below that of redness takes hold of a portion of the solid carbon, and meeting the air they rapidly enter into combination with oxygen. New supplies of the volatile ingredients are disturbed further within the mass of the burning body, and there by their ignition serve to keep up the process. If the supply of oxygen is sufficient, the combustion is complete, and the volatile products of the distillation process, if generated at all, pass immediately into the stable compounds of carbonic acid and water. The carbon attacked at its surface by the oxygen of the air yields more slowly, and a portion of it is left behind after the flame and rapid chemical action caused by the combustion of its volatile associates have disappeared.

When concentration of heat is required, as in the smelting of ores, a condensed form of fuel like charcoal is more effective than one containing gaseous elements, which in their combustion dispense a very uncertain amount of heat, as they flit, perhaps but partially consumed, past the points where the effect is wanted, carrying with them a portion of the carbon of the fuel, and also more caloric rendered latent than the product of combustion of an equal weight of carbon is capable of absorbing. The difference in the pyrometrical effect of wood and charcoal would be still greater than it is, were it not for the property of charcoal of rapidly absorbing moisture from the air. When it is desirable to apply the heat generated by combustion at a distance from the fire, as in rever-beratory furnaces, fuel is preferred that burns with a flame.-As charcoal is obtained from wood by charring, so from peat this fuel is obtained in a condensed form called peat charcoal, and from the bituminous coals the mine-