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 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 imbedded in various parts of the HEAT AND FUEL 37 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 of coal. The gaseous substances have nearly all been expelled and the carbon that remains forms the hardest kind of coal. This is also known as hard coal and has a glossy black color. Anthracite coal is sometimes named from the place where it is mined, but there are only two varieties, — the red ash and the white ash. The red ash burns more freely than the white ash, and although it costs more it is better for cooking purposes. Coal of a dull slaty color often contains pieces of slate and other incombustible compounds. Bituminous Coal, so called from the bitumen or pitch which it contains, is derived from other forms of vegetation which were buried and had less charring than the hard coal, and much of the hydrogen or gaseous element remains. It is more abundant than hard coal and therefore cheaper. niuminating Gas is made by distilling or heating bitumi- nous coal with entire exclusion of air. Water Gas is made by passing steam through heated coal. Coke is the black, porous mass left after the volatile gases have been driven off from certain kinds of coal and is nearly pure carbon. As it burns out quickly, coke fires need fre- quent replenishing ; but it is light, easy to handle, and does not smoke. Petroleum, from which kerosene oil is made, contains liquid compounds of hydrogen and carbon, and is obtained from wells in the bituminous coal regions. Its location is often discovered by finding the coal oil oozing from crevices in the rocks. The petroleum goes through various refining processes, and when of the best quality it is nearly colorless. We burn fuel on a small scale in candles, gas, and oil in lamps to secure the light from the flame. We burn it on a large scale in stoves and furnaces to obtain heat to warm our houses and to cook our food. Carbon is the chief element in all these forms of fuel. In burning, the oxygen unites with the carbon and hy- drogen, forming, with the carbon, carbon dioxide gas, and 38^ SCHOOL KITCHEN TEXTBOOK 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. The Heating Power of Fuels. 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.

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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 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 imbedded in various parts of the HEAT AND FUEL 37 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 of coal. The gaseous substances have nearly all been expelled and the carbon that remains forms the hardest kind of coal. This is also known as hard coal and has a glossy black color. Anthracite coal is sometimes named from the place where it is mined, but there are only two varieties, — the red ash and the white ash. The red ash burns more freely than the white ash, and although it costs more it is better for cooking purposes. Coal of a dull slaty color often contains pieces of slate and other incombustible compounds. Bituminous Coal, so called from the bitumen or pitch which it contains, is derived from other forms of vegetation which were buried and had less charring than the hard coal, and much of the hydrogen or gaseous element remains. It is more abundant than hard coal and therefore cheaper. Illuminating Gas is made by distilling or heating bituminous coal with entire exclusion of air. Water Gas is made by passing steam through heated coal. Coke is the black, porous mass left after the volatile gases have been driven off from certain kinds of coal and is nearly pure carbon. As it burns out quickly, coke fires need frequent replenishing ; but it is light, easy to handle, and does not smoke. Petroleum, from which kerosene oil is made, contains liquid compounds of hydrogen and carbon, and is obtained from wells in the bituminous coal regions. Its location is often discovered by finding the coal oil oozing from crevices in the rocks. The petroleum goes through various refining processes, and when of the best quality it is nearly colorless. We burn fuel on a small scale in candles, gas, and oil in lamps to secure the light from the flame. We burn it on a large scale in stoves and furnaces to obtain heat to warm our houses and to cook our food. 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, carbon dioxide 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. The Heating Power of Fuels. 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.

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