The use of coal for heating purposes is so familiar to every one nowadays that probably few have ever thought about the time when it was unknown. Coal was as plentiful three thousand years ago as it is now. Layers and beds of the fuel existed just under the surface of the ground, and in many places cropped out through it. But the stones were merely "black rocks," and the idea that rocks would burn was too absurd to occur to any one. We may well wonder how it was first discovered that coal would burn. Professor Greene suggests a possible explanation of this discovery. "There is in coal a hard, yellow, brassy mineral which flies in the fire and not infrequently startles the circle that has gathered around its cheerful blaze. When exposed to damp air this mineral undergoes chemical change, and during the process heat is given out, sometimes in sufficient quantities to set the coal alight. In this way it occasionally happens that seams of coal, when they lie near the surface, take fire of their own accord. One day a savage on a stroll was startled by finding the ground warm beneath his feet, and by seeing smoke and sulphurous vapors issuing from it. He laid it first to a supernatural cause; but curiosity getting the better of superstition, he scraped away the earth to find whence the reek came. Then he saw a bed of black stone, loose blocks of which he had already noticed lying about; parts of this stone were smouldering, and as soon as air was admitted burst into a blaze." Whether coal was thus discovered or not, its first discovery must have occurred early in the history of the world. More than twenty centuries ago the Greek scholar, Theophrastus, wrote of the coals which were used by blacksmiths. There are indications that coal was mined in England before that country was conquered by the Romans. But not until the twelfth century was enough of the mineral mined in Newcastle, the great coal region of England, to warrant its being carried to London. As this coal was brought in vessels to the metropolis it received the name of "sea-coal," and it was thus called for several centuries.