The first operation is the preparation of the coal, to reduce it to the state of coke, which is accomplished either in kilns, or in the open air. The latter plan has been, and is still, the most extensively adopted; we shall therefore describe it the first. An oblong square hearth is prepared, by heating the earth to a firm" flat surface, and paddling it over with clay. On this the pieces of coal are piled up, inclining toward one another; and those of the lower strata are set upon their acutest angle, so as to touch the ground with the least surface possible. The piles are usually from 30 to 50 inches high, from 9 to 16 feet broad, and contain from 40 to 100 tons of coal. A number of vents are left, reaching from top to bottom, into which the burning fuel is thrown; and they are then immediately closed with small pieces of coal beaten hard in. Thus the kindled fire is forced to creep along the bottom, and when that of all the vents is united, it rises gradually, and bursts out on every side at once.

If the coal contain pyrites, the combustion is allowed to continue a considerable time after the disappearance of the smoke, to extricate the sulphur, part of which will be found in flowers on the surface.

If it contain none, the fire is covered up soon after the smoke disappears, beginning at the bottom and proceeding gradually to the top. In 50, 60, or 70 hours, the fire is in general completely covered with ashes of char, formerly made, and in twelve or fourteen days the coke may be removed for use. In this way a ton of ordinary bituminous coal commonly produces from 700 to 1,100 lbs of coke.