ABSTRACT
The oldest and most important energy sources for humanity are those which represent stored solar energy. Fire, as the early energy-supplying chemical process of oxidation in air, was always based on plants, bushes, and trees which had grown by photolysis in the sun. Coal burning initiated the Industrial Age and enabled the growth of energy-dependent human activities. It was intimately connected to the beginning of the "Iron Age" and later to steel production as well as to the first mass-transport activities. The extraordinary increase in the use of oil is a consequence of its high energy density. The development of modern transportation is intimately connected to the availability of oil as a high energy density liquid. Switzerland and the Scandinavian countries have supplied a large portion of their electrical energy by hydroelectric generating stations. In the United States a fixed but relatively small percentage of the electric energy is supplied by hydroelectric plants.
