tion quite compatible with the absence of mineral fuel, at all events in regions where the climate is tolerably mild. We must also remember, as has often been pointed out, that there are vast stores of energy available elsewhere. The radiation from the sun, if it could be suitably garnered up and employed both directly as heat and indirectly as a source of power, would be quite capable of supplying all conceivable wants of humanity for ages. It is also to be noted that we live on the outside of a globe the inside of which is filled with substances that appear, from all we can learn, to have a temperature not less than that of molten iron. If the crust could be pierced sufficiently far, vast indeed is the quantity of heat that might be available. We see the operation of tapping the internal heat going on in nature. Every volcanic outbreak, every spring of hot water, every geyser are but indications of the internal heat of our globe. It may indeed be hard to see how a practical method for drawing on this vast reserve of heat can be devised, but it is at least conceivable that it may be rendered available when the coal and other more accessible sources have become exhausted, or even when their yield has considerably lessened.
The coal of England may last a century or two; the coal in other parts of the globe may supply our cellars for a few centuries more, but the exhaustion of this truly marvellous product is proceeding at an accelerated pace. Doubtless the end of the coal, at least as an article of a mighty commerce, will arrive within a period brief in comparison with the ages of human existence. In the history of humanity from first to last the few centuries through which we are now passing will stand out prominently as the coal-burning period.
