The principal constituents of coal are carbon, hydrogen, water, a little sulphur, ashes and clinker, the latter two substances consisting generally of silica, alumina, iron, lime, magnesia, and oxide of manganese. The principal combustible constituent of anthracite is fixed, uncombined carbon. The free-burning or semi-bituminous coals contain a considerable amount of hydrocarbon or volatile combustible matter, and bituminous caking coals have a larger percentage of volatile combustible.

If a mass of coal is brought to a sufficiently high temperature (probably something above 1000° Fahr.), the combustible materials enter into chemical combination, and as much heat is given out as would be required to decompose the resulting products into their elements. When coal is burned the water is first expelled; then the sulphur, if any is contained, is consumed, forming sulphurous oxide; after this the hydrogen in the volatile combustible matter unites with oxygen, forming water; and the carbon set free unites with oxygen, forming carbonic dioxide, if the temperature is sufficiently high and enough oxygen is present, or, under less favorable circumstances, either forming carbonic oxide or passing off unconsumed, as soot. The combustion of the fixed carbon next begins, the product of the combustion being carbonic dioxide or carbonic oxide, so that finally nothing is left except the ashes and clinker.

It may be well to trace the effect of these various combinations: The water contained in the coal is expelled in the form of steam, so that it carries off some heat, and is a positive disadvantage. The complete combustion of a pound of sulphur produces about 4000 units of heat, but the amount of sulphur in coal is usually so slight that its heating qualities scarcely deserve to be regarded. The action of the sulphur on the material of the boiler is, however, a very serious matter. It has not yet been determined by experiment what per cent of sulphur is sufficient to render a coal unfit for use in a furnace, but it is well known that many of the Western coals produce very bad effects when employed in locomotive boilers. A pound of hydrogen combining with oxygen, forming 9 pounds of water, has a heating power of 62, 032 units. It seems doubtful, however, whether this amount of heat is available from the combustion of hydrogen in a boiler. The experiments by which this value was determined were made upon hydrogen in the gaseous state, and the steam resulting from the combustion was condensed. Now the hydrogen in coal is ordinarily combined with carbon, and frequently with nitrogen, so that it must be separated from the combination before it can be united with oxygen, and heat is required for this separation. Again, in a boiler the products of combustion usually pass into the chimney at such a high temperature that the water, which is the result of the combustion of the hydrogen, passes off in the form of steam, and thus carries off a considerable quantity of what is commonly known as latent heat. This subject is one which has been but little considered by experimenters, and is worthy of more extended investigation. The volatile combustible matter of coal generally contains oxygen in combination, and this must be changed into the gaseous state before being united with the hydrogen, an operation that requires as much heat as results from the new combination. Hence it is certain that the amount of hydrogen contained in coal must be diminished by one eighth of the weight of the oxygen before attempting to form any estimate of its heating qualities. The carbon of the coal, as has been already stated, will unite with oxygen, forming carbonic dioxide, and may afterwards take up more carbon, and he converted into carbonic oxide. Now the result of the complete combustion of a pound of carbon is 3-2/3 lbs. of carbonic dioxide, and the combustion produces 14, 500 units of heat. But a pound of carbon Imperfectly burned produces 2-1/3 pounds of carbonic oxide, and only 4400 units of heat. In a furnace where the combustion is imperfect, the action is usually as follows: A pound of carbon is at first completely burned, forming 3-2/3 pounds of carbonic dioxide, and then takes up another pound of carbon, producing 4-2/3 pounds of carbonic oxide and 8800 units of heat. This carbonic oxide, however, if supplied with a sufficient amount of air, will burn and again form carbonic dioxide, so that the full effect of the combustion of the carbon will be realized. A practical application of this principle is seen in the combustion-chambers in boilers, which are designed to complete the combustion of the gases after they leave the furnace.

Having disposed of the materials of the coal which escape into the chimney of a boiler, the ashes and clinker that remain should be considered. The effect of these substances is injurious in several ways: they choke up the furnace, preventing free access of the air to the combustible materials, and instead of entering into combinations and producing heat, they require to be heated to the temperature of the furnace, and are then removed, without having produced an equivalent for the heat expended upon them.

In ordinary boiler-furnaces, the amount of air required for the combustion of 1 pound of coal is about 24 pounds, or between 280 and 300 cubic feet.

For a table of the qualities of American coal from various localities, compiled from Prof. Johnson's Report, see page 95. B.