The heat lost is not dependent upon the amount of carbon alone, but also upon the invisible but combustible gases, hydrogen and carbonic oxide; so that while the color may indicate the amount of carbon in the smoke, it does not indicate the amount of the heat lost; hence, the smokeless locomotive burning coke may lose more heat in this way than that arising from the imperfect burning of coal under the stationary engine boiler.
A practical and familiar instance of imperfect combustion is exhibited when a lamp smokes and the unconsumed carbon is deposited all about in the form of soot. When the evolving or disengagement of the carbon is reduced by lowering the wick to meet the supply of oxygen, the carbon is all consumed and the smoke ceases. What takes place in a lamp also occurs in a furnace, so that the proper supply of air is a primary thing, relating to economy, both as regards its quantity and its mode of admission to a fire.
The economical generation of heat is one thing, the use made of that heat afterwards is another. Combustion may be perfect, but the absorption of heat by a boiler may be inferior.
The chief agents operating in the furnace are carbon, hydrogen and oxygen, and their union in certain proportions produces other bodies, as water or steam, carbonic acid, besides others of less practical importance.
Oxygen is an invisible gas, has no smell, and remains permanently in receptacles, unchanged by time. It can be obtained in an experimental quantity by heating the chlorate of potash, and collecting the gas given off in a bladder or jar. It is a trifle heavier than common air, i.e., 1.106 times and a cubic foot at 32° temperature weighs 1.428 ounces. It is one of the most abundant bodies in nature, and is combined with many others in a great variety of ways.
Carbon is one of the most interesting elementary substances in nature. It is combustible and forms the base of charcoal, and enters largely into mineral coal. It is a mineral capable of being reduced to a feathery powder, and is found in many different forms. It is obtained by various processes: from oil lamps as lamp-black; from coal as coke, and from wood as charcoal; the mineral particles of carbon in a state of combustion render flame luminous from either gas, oil or candles.
Carbon unites with iron to form steel, and with hydrogen to form the common street gas. Carbon is considered as the next most abundant body in nature to oxygen. In the furnace the carbon of the fuel unites with the oxygen of the air to produce heat; if the supply of air is correctly regulated, there will be perfect combustion, but if the supply of air be deficient, combustion will be imperfect.
Hydrogen is an invisible gas, and the lightest known body in the world, being many times lighter than oxygen. It is combustible and gives out much heat. In our gas establishments it is made in large quantities and combined with carbon for illuminating streets, shops and dwellings. It is the source of all common flame. When united with sulphur in coal mines it becomes explosive. By passing a current of steam through a hot iron tube partly filled with filings, hydrogen gas is given off and burns with a pale yellow flame.
The more hydrogen, therefore, there is in the fuel, the greater in general is its heating power. But it must be borne in mind that the element of hydrogen is, nevertheless, to a greater or less degree neutralized by the other element, oxygen, when it is present as a constituent of the fuel; since the affinity of hydrogen for oxygen is superior to that of carbon, and the oxygen saturated with hydrogen is converted into steam and rises in this form from the fuel bed without producing heat. Thus it is that the more oxygen there is in the fuel the less is its power for developing heat by combustion.