Energy is unique among economic resources in that once fully used it is lost for ever. The intense heat that converts iron ore into iron and then steel can be used for that purpose once only. The metal cools, the heat is dissipated, raising the temperature of the air slightly, but not enough for useful work to be derived from it. If too much energy is applied to the blast furnace and it threatens to escape in the form of hot waste gases, an efficient firm will recover and use this surplus energy, but once again after useful work has been extracted from it the low-grade heat that remains has no economic value. For all practical purposes the energy has been lost. This would not present problems if the incoming energy from the sun could be converted into wood, coal, oil, alcohol, or natural gas as fast as existing sources of energy are being used up. But since this is not happening, the world's reserves of energy are being consumed at a rate that is unsustainable in the long run. Other resources, unlike energy, are not lost to the economic process for ever. Soil that is eroded comes to rest in river deltas if water-borne; if wind-blown there is a good chance that it will setde on land rather than at sea. In either case it is potentially available for cultivation. Scrap metals when available in large quantities (or in small quantities if they are of high value), are brought back into use. Even if dumped on a rubbish tip, they are recoverable with the application of enough energy. The sea itself is a repository of metals in extreme dilution, and is steadily enriched as further traces are washed into it or seep into it from below the seabed. In principle and given large enough supplies of cheap energy the sea could provide many if not all the metals in economic demand. Glass and rubber readily come to mind as materials that are easily reusable, because they have favourable physical or chemical properties — a low melting point, or solubility. Other materials like bricks or pottery offer litde but their bulk to the recycler. Unpromising though they are, they can at least act as hardcore or landfdl material; they are not quite as useless as the low-grade heat that remains after the expenditure of energy.