or Pit-Coal, is a black or brown, laminated, bituminous substance; not very easily inflammable, but, when once inflamed, burns longer and more intensely than any other substance. Of this substance three kinds are distinguished by authors. The residuum of the first after combustion is black; the residuum of the second is spongy, and like pumice-stone; and the residuum of the third is whitish ashes. Some fossil coal, by long exposure to air, falls into a greyish powder, from which alum may be extracted. Fossil coal by distillation yields, 1. a phlegm or water; 2. a very acid liquor; 3. a thin oil like naphtha; 4. a thicker oil, resembling petroleum, which falls to the bottom of the former, and which rises with a violent fire; 5. an acid concrete salt; 6. an unflammable earth remains in the retort. These constituent parts of fossil-coal are very similar to those of amber and other bitumens. For the exciting of intense heats, as of furnaces for smelting iron-ore, and for operations where the acid and oily vapours would be detrimental, as the drying of malt, fossil-coals are previously charred, or reduced to coals; that is, they are made to undergo an operation similar to that by which charcoal is made. By this operation coals are deprived of their phlegm, their acid liquor, and the greatest part of their fluid oil. Coaks therefore consist of the two most fixed constituent parts, the heavy oil and the earth, together with the acid concrete salt, which tho' volatile is detained by the oil and earth.