[AS.] A black substance (consisting mainly of carbon), dug out of the earth, which burns and gives heat. Coal is of compact but brittle structure, and found in seams and beds, and is the remains of a luxuriant vegetation which flourished on the earth's surface during the Carboniferous age, and to some extent in other geological periods. It is supposed to be due to the action of heat and pressure upon great accumulations of this material. There are many varieties of coal, distin-guished from one another by the varying proportions of the different constituents. Bituminous coal, such as is in gen-eral use, contains from 75 to 80 percent, of carbon, 5 to 6 of hydrogen, and 10 to 12 of oxygen. Anthracite is the most completely mineralized variety, and contains about 90 per cent. of carbon, the gases oxygen and hydrogen having been driven off. Cannel coal and lignite contain less carbon and yield more ashes than those named. The present yearly output of coal in Great Britain and the United States is about 200,000,000 tons in each. Other countries yield much lesst Some of the mines are very deep.
COAL UNDER THE MICROSCOPE.