ABSTRACT
By classifying resources according to their properties, for example, as metals, semiconductors and insulators, one can obtain broad groups of substitutable materials that are present in the earth’s crust, including the water of the oceans, in quantities that probably exceed any foreseeable requirement by man. If one can assume that the use of a resource will rarely result in its being removed from the earth into space, then the term usage will at worst imply dilution to the average concentration of the resource in the earth’s crust. Energy resources have a particular role in man’s activities. The earth receives a continuous flow of energy from the sun, and from solar radiation are derived other energy forms such as wind, currents and sea temperature gradients.
