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Petroleum began to be a supplemental fuel of importance when the internal-combustion engine for small mobile powerplants started to proliferate. And uranium became a source of energy, first for warfare and then for electricity, in the second half of the 20th century.

In the United States, the mix of energy-source inputs (Fig. 5), to the economy changed within 100 years from dependence upon renewable resources (fuel wood and animal power) to nonrenewable resources (petroleum, natural gas, coal, uranium). During this period, metallic resources such as iron, copper, lead, zinc, silver, and gold and nonmetallic resources such as cement, rock, sand and gravel, phosphate rock, salt, and sulfur have been mined at rapidly increasing rates.

Importance of nonrenewable resources to an industrial society

The per capita consumption of nonrenewable resources is directly related to the standard of living in a society. Gross energy consumption per capita varies in approximately linear fashion with per capita income (Fig. 6), as does per capita consumption of minerals and metals. Consumption of a mineral resource consists of dispersion and fixation. An example of dispersion is the use of lead in anti-knock gasoline; the lead comes out of millions of exhaust pipes in highly dispersed form and is irretrievably lost from the stock available for re-use. An example of fixation is the use of iron in structural steel, where the material is concentrated and available for recycling when the structure it helps support is torn down. Both per capita dispersion and per capita fixation increase as the material level of living advances.

It may be difficult for the average person in modern society to perceive how dependent he is on nonrenewable mineral resources. In

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the morning he sleepily stills an alarm clock made of steel and glass, turns on lights made of glass and metals and powered by electricity generated by burning a fossil fuel and moved along metal wires, brushes his teeth with a toothbrush made from a petroleum product while standing on tile made from clay, looking into a mirror made from glass sand and mineral fluxes. His stove, refrigerator, dishwasher, air conditioner, washer, dryer, and disposal are made of metal and run by fossil-fueled electricity. The bricks and roofing of his home, the tile, bathtubs, washbasins, and windows are made from mineral resources as are the walks, patio, and paved driveway. Increasingly his clothing, carpeting, and draperies are made from nonrenewable resources. Of course, his automobile, his children's bicycles, and the buses and airplanes that take him to work or on business trips are made of metal and run on nonrenewable energy resources. His working day is spent in structures made of reinforced concrete, a combination of sand, gravel, crushed rock, portland cement, and steel. All the machines that serve him, either directly or indirectly, are mainly of mineral origin. The communications networks that link him to most of the rest of the world's people, are built largely from mineral materials.

The concepts of natural subsidy and net work profit

Important to an understanding of the role played in human society by energy and mineral resources are the concepts of natural subsidy and net work profit.

Life exists on earth by the grace of solar radiation, which provides the ambient temperature range within which man can live. It drives the winds and powers the hydrologic cycle. It provides the energy for plant