The value of a fuel is estimated by determining the amount of moisture, of volatile matter, and of fixed carbon and sulphur it contains. The principal fuels occur in the solid, liquid, and gaseous forms. Chief among the solid fuels are coal, wood, charcoal, and coke. The distribution of coal is general over the United States and Europe. Although the coal fields of the United States are shallow compared with those of Nova Scotia and parts of Europe, they are sufficiently extensive to render them the richest in the world.

Varieties of Coal

Coals are divided in the first place into hard and soft coal. The hard coal is known as "anthracite" or "glance," the soft, as "black" or "bituminous," coal. Each of these groups may be subdivided into several varieties.

Graphite may be placed at the base of the series of coal formations. This represents coal deprived of all its volatile matter and a very large portion of the original carbon, but practically none of the original ash. Graphite is practically incombustible, and is never used as a fuel, and is not listed among coals.

Anthracite coal comes next in the series as regards hardness and amount of carbon. It contains from three to ten per cent, of volatile matter, and sometimes as high, as ninety-five per cent, of carbon. This variety of mineral coal contains a small amount of hydrogen, and consequently burns almost without flame. Anthracite coal is a clean and convenient fuel for household use. Its available heating- power is high. It has great durability no combustion, and it is possible to gain practically complete combustion by sifting the ashes and reusing the partially burned coal. It is a better winter than summer fuel for kitchen use, because it is so much more difficult to kindle a fire with coal than with wood, and it is easier to have a good fire and then extinguish it when using wood. The greatest objection to this coal as a household fuel is its expense. It makes a hot, steady fire, and is pleasant to handle.

Semi-bituminous coal comes next in the line of hardness. It contains from ten to eighteen per cent, of gaseous matter. It kindles more readily than anthracite coal, has a high heating power, and cakes in the fire. As a coal for household purposes, it ranks next to anthracite coal. It burns more freely in an open grate than anthracite coal, but it is less cleanly.

The bituminous or soft coals are divided into coking, furnace, and cannel coals. In bituminous coal, the amount of volatile matter varies from eighteen to fifty per cent, of the entire mass.

The coking coals melt and adhere in burning, and when the gaseous matter has escaped, a mass of coke remains. Most bituminous coals belong to this variety, of which the Pittsburg coal may be taken as a type. Bituminous coals are extensively employed for the generation of steam, and, when coked, for smelting metals. Their tendency to adhere in masses when burning prevents their being used for this in their raw state. This variety of coal is a good heat producer, but on account of the large amount of volatile matter contained, it produces a great quantity of smoke and soot. In some localities it is expedient to use it because it is the cheapest fuel. Extreme care is necessary, when it is burned in the kitchen range, to prevent the light, black, tenacious particles of soot escaping into the room. It is also very unpleasant to handle.

Coking coals sometimes contain much sulphur, and when so contaminated they are not prized as gas coal, but when sufficiently free from this they are much used in the production of illuminating gas. The cannel coals exceed these in the volume and illuminating power of their gas, but the coking coals furnish the most valuable coke.

The furnace coals are those bituminous coals which do not melt or adhere in the fire.

Cannel coals form a third variety of coals, and differ from other bituminous coals in the following particulars: They are more homogeneous in texture, contain less pitch, and are less brilliant; they have a low heating power, but are esteemed in some localities as a household fuel.