Among minerals of economic importance carbon minerals hold the unique position of being at the same time of the most common and the most rare occurrence. As far as external appearance indicates, a piece of common coal and the most brilliant diamond are widely separated; with regard to chemical composition they are closely related. Intermediate between the coal of the stoke furnace and the "brilliant" of the jewelry shop is still another well-known form of carbon, the graphite of the lead pencil. These three substances comprise the far greater part of carbon-containing minerals. In so far as our mind's picture of a mineral is that of an aggregation of crystals of fairly perfect form our consideration of coal as a mineral is erroneous. We must yield to a broader interpretation of the essential characteristics of a mineral and modify our idea so as to include any homogeneous substance (solid, with the single exception of mercury) of fairly definite chemical composition "occurring in nature but not of apparent organic origin." Organic substances are those that are alive or have lived. Vegetation is, undoubtedly, the origin of all coal, but often much more than a cursory examination is necessary to prove such origin. In the less altered coals the vegetable origin is readily proved by the actual presence of seeds, plant fibers, and other equally apparent organic remains. A microscopic study is necessary for finding the presence of woody fiber in the more metamorphosed form. The word metamorphose comes from the Greek; _meta_ means after or over; _morphe_ is form. A metamorphosis is a change of form or a forming over. The history of the discovery of the value of coal as a means of producing heat and of the development of the coal-mining industry covers a comparatively recent period. Coal occurs in such quantities near the surface of the earth's crust and its outcrops are so numerous that it cannot have failed to attract the attention of the most ancient of peoples. Indeed, that coal could be used as a fuel is mentioned by a writer, Theophrastus, who lived 300 years B. C. The ancient Celts of Britain are reputed to have evidenced knowledge of the industrial value of coal. It was not until near the middle of the thirteenth century, however, that coal became so important an economic product as to result in statutes granting to certain places the privilege of mining it. After a long period of trial in England the superiority of coal over other fuels was recognized, and stone coal, as the harder form was commonly known, came into general use. In America bituminous, or soft coal, was mined to a slight extent in the latter half of the eighteenth century. The form now commonly used in house-heating furnaces, anthracite, for a long time baffled the colonists in their efforts to make it burn. The knowledge that an anthracite fire is most effective if not continually poked is said to have been acquired generally by accident.