In the case of coal, it is not a very difficult task to trace the stored-up heat to its source. But what a surprising truth it is, which becomes apparent when the task has been performed. The heat is, so to speak, _bottled-up sunshine_! Coal is dug up from deep mines hollowed out in the earth. But at one time it was wood, growing on the outer surface of the globe, and covered with foliage which was spread out into the genial air. Traces of the leaves and stems from which it has been made, are still discovered in its substance. Long centuries ago, the vast forests containing these trees, were overthrown by some tremendous earthquake, and swept away by strong floods of water, and so the tree-stems were at last deposited in hollow basins, and were there buried up by millions and millions of tons of heavy rock and soil. There, where they were buried, they have remained, turning more and more black and dense through the process of slow decay, until they have been dug up piece-meal to feed the furnaces and fires of the existing generation of men. Now, you know very well that trees only grow in warm weather, and in sunshine. In winter time their branches stick out stiff and bare, and do not increase in the slightest degree. But in summer time they clothe themselves with beautiful masses of foliage, and suck in from both the air and the soil large quantities of vapor, of liquid food, and of sunshine. All these they combine together into fresh layers of timber. All these therefore were buried in the ground as timber, when those old forests were overthrown which form the coal-beds. Timber cannot be made in cold weather, because heat is one of its necessary ingredients. But as all the warmth of the weather comes from the sun, it is the sun’s warmth which is stored away in the coal, and which is set free and made useful when the coal is burned.