Coal is of prehistoric origin, formed from accumulation of vegetable matter, supposed to be the remains of immense forests. In past ages the deposits underwent destructive distillation from great heat and was subjected to pressure, sufficient to compress it into varying degrees of hardness. Coal is composed of carbon, hydrogen and oxygen, with small quantities of nitrogen and varying amounts of sulphur and ash.

The coals from different geological formations vary in quality from the hard dry anthracites to the soft wet lignites, with the intermediate bituminous coals; all of which furnish fuels that when burned will produce amounts of heat, depending on their composition, the quantity of moisture contained and the conditions of their combustion.

Carbon, of which coal is principally composed, exists in different combinations, depending on the condition of its formation. Part of the carbon is combined with hydrogen to form hydrocarbon that may be driven off when heated, and which forms the volatile portion of the coal. The remainder of the carbon appears in the form of coke - when the volatile matter is driven off - and is said to be fixed. The fixed carbon and volatile constituents together make up the combustible.

Other ingredients of coal that require attention are the moisture, and the incombustible matter that forms ash. Moisture varies in quantity from as low as 0.75 per cent. in hard coal to 50 per cent. in lignite. The amounts of ash in different coals vary from 3 to 30 per cent. of the weights of the fuel.

The heating value of coals differs in amount by reason of the variable quantities of fixed and combined carbons, moisture and ash. Different coals are compared in value by the number of B.t.u. per pound of dry coal that can possibly be developed when burned, and with these factors are given the percentages of moisture and ash.

There are no distinct demarkations between different grades of coal. The classifications are made because of their chief characteristics and they commonly are graded as anthracites, semi-anthracites, semi-bituminous and bituminous coals. These classes comprehend the most common commercial coals of the United States. Aside from those named are forms of coal that are occasionally found, such as graphitic anthracite, cannel coal, etc., and the various lignites.

The value of coal as a heat-producing agent is represented by the B.t.u. it is capable of turning to useful account. The price of coal should be based on the amount of heat it is capable of generating when burned. In considering the value of coal for any particular purpose, thought must be taken as to its characteristic properties, for coals that produce excellent results for one purpose may be very unsatisfactory in others. Soft coal containing a large percentage of volatile matter usually produces a great amount of smoke and unless carefully fired this will condense and form accumulations of soot that are objectionable. For reasons of this kind bituminous coals are often sold at a lower price than their rated heating value might indicate.

Anthracite or hard coal possesses bright lustrous surfaces when newly fractured, that when handled do not soil the hands. It contains a high percentage of carbon, a small amount of volatile matter and little moisture. It is greatly in demand as a domestic fuel because it burns slowly with an intense heat, practically without flame and produces no smoke. It invariably commands a higher price than soft coal, but in heating value is not superior to the better grades of soft coal. In furnaces for house heating the use of soft coal often gives better satisfaction than hard coal.

The grades of hard coal found in the market will vary with the demand in any locality but those recognized by the trade are:

Hard coal of stove and chestnut sizes are those most commonly used for domestic heating, because they are well suited for furnaces and heating stoves. Of the two sizes chestnut coal is most largely used and on account of the greater demand, the price for this size is usually somewhat in advance of the others; at the same time the smaller sizes - pea and buckwheat coals - are less in price for the same grade of coal. Under conditions that will permit their use the latter coals are an economical form of fuel.

Bituminous or soft coal represents the chief fuel of commerce. The market prices of these coals are determined largely by reason of their reputation as desirable fuel. The variations in price depend on the physical qualities, rather than on the amount of heat evolved in combustion. The compositions of coals vary markedly in different localities and often in the same locality several grades are produced. It sometimes happens that from different parts of a mine the coal will differ very much in heat value.

Bituminous coals are roughly classified as coking and free-burning. The former is valuable for gas manufacture and for production of coke. The coking coals fuse on being heated, allowing the volatile portion to escape; and when the gas has been all distilled, the residue is coke. When used for gas making, the volatile portion forms the illuminating gas. When burned in a furnace, the gases from soft coal burn with a yellow flame and usually with considerable smoke. The classification of bituminous coals differ somewhat in the East from that of the West. Eastern bituminous coals are commonly graded:

A. Run-of-mine coal = unscreened coal as taken from the mine.

B. Lump coal = that which passes over a bar screen with 1 1/4-inch openings.

C. Nut coal = that which passes through a bar screen with 1 1/4-inch openings and over one with 3/4-inch openings.

D. Slack = all that which passes through a 3/4-inch. bar screen.

Western bituminous coal:

E. Run-of-mine coal = the unscreened coal as taken from the mine.

F. Lump coal - divides as 6-inch, 3-inch and l 1/4-inch according to the diameter of the mesh through which the pieces pass the screens.

G. Nut coal - varying from 1 1/4-inch size to 3/4-inch in diameter.

H. Screening = all coal which passes a 1 1/4-inch screen including the dust.

Heat derived from coal - or any other fuel - in the process of combustion is due to oxidation. Combustion or burning is caused by rapid oxidation. When oxygen combines with carbon in sufficient quantities, carbon dioxide is formed and at the same time heat is liberated. In burning fuel, if the carbon is completely oxidized and changed into carbon dioxide, the greatest amount of heat is produced. The required oxygen is furnished by the air, which through the dampers of the furnace regulates the rate of combustion.