Wood is a product of vegetable growth, found in the trunks and branches of trees. It contains hydro-carbons in a solid form. It consists of slender fibres or tubes closely packed together. When first formed these are hollow, and contain the sap or vegetable juices; but gradually they become hardened and consolidated, and by their successive layers or rings indicate the age of the tree.

The fibres in hard woods are more densely packed and are of a purer quality than those in soft woods. When freshly cut, wood contains a large amount of water or sap, and soft wood contains more than hard. On exposure to the air this water is lost by evaporation. Wood should be well dried to be useful and economical as fuel.

Charcoal is obtained by heating wood in close vessels, or in covered pits, with a limited supply of air,- enough to decompose the wood, but not enough to consume, or entirely burn it, - a kind of partial or half-smothered burning. The gaseous elements in the wood are all expelled, and the coal or charred wood that remains is nearly pure carbon.

Anthracite coal is 90 to 98 per cent carbon. It is found in immense layers, deeply embedded in various parts of the earth's crust. Ages ago the vast forests and luxuriant forms of vegetation were submerged; and by the action of pressure, heat, and other causes, they have been changed to their present form. The gaseous substances have nearly all been expelled, and the carbon that remains forms the hardest kind of coal.

Other forms of ancient vegetation thus buried had less charring, and much of the hydrogen, or gaseous element, remains. These are called bituminous coals, from the bitumen or pitch which they contain.

Petroleum, from which kerosene oil is made, contains liquid compounds of hydrogen and carbon. It is obtained from wells in the bituminous coal regions.

Illuminating gas is made by distilling or heating bituminous coal with entire exclusion of air. Coke is the black, porous mass left after the volatile gases have been driven off, and is nearly pure carbon.

Carbon is the chief element in all these forms of fuel. In burning, the oxygen unites with the carbon and hydrogen, forming, with the carbon, carbonic acid gas, and with the hydrogen, watery vapor. Both escape into the air, and the gas is absorbed by plants. Some of the carbon is not consumed, and passes off as smoke.

Any fuel that burns with a flame must be at that moment in a gaseous state. In burning gas we simply have to heat the gas to its kindling-point, and we have a bright flame. We light the wick in a candle and at first it burns slowly ; the wax in the wick must first melt, then change to a vapor, and when the vapor is heated to its kindling-point it burns with a flame. Wood burns with a flame because it is first decomposed by the heat. Gases are formed ; and the burning of these gases, and not of the solid wood, produces the flame. Hard coal is made up almost entirely of solid carbon, which no furnace heat can change into gas. As there are no gases first made by the heat, so there can be no flame produced in the burning. Hard coal burns with a steady glow without flame, provided there is plenty of air to burn the carbon ; but when the coal is densely packed in the grate and the supply of air is insufficient, a poisonous gas is formed which burns with a blue flame. It disappears when the coal burns freely. - Cooky's Chemistry, page 45.

Wood charcoal, being light and porous, ignites readily, burns rapidly with little or no flame, and gives out more heat than an equal weight of any other fuel.

Anthracite coal is next in heating power. Owing to its density it kindles slowly, but when once thoroughly ignited it burns with an intense heat, without flame, smoke, or soot, and for a long time.

Bituminous coal ignites readily, burns with much flame and smoke, but yields less heat than anthracite.

Soft woods kindle quickly, burn with much flame, produce intense heat, and leave but few coals.

Hard woods kindle and burn slowly, with less flame, but afford a large mass of coals, which retain the heat a long time.