The world will not perish for want of coal, yet the coal-supply will fail, and that much sooner than Ericsson estimated, for the production doubles every ten or fifteen years. It will not take thousands of years to exhaust the European coal-mines, but only hundreds, and not very many hundreds either. In England, as appears from recent calculations, the supply will have been consumed in two or three centuries at the farthest. Belgium, Germany, France, and the other countries of Europe, are no better off. The United States of America and the north of China have coal enough to last for one thousand years, and that is all. We must then have recourse to the sun.
It will perhaps be said, "There is electricity." Electricity, as a mechanical agent, is too costly; to produce electricity we have to consume copper, zinc, and acids. Now, one kilogramme of copper, zinc, or acid represents several kilogrammes of coal expended in procuring it. In reducing copper-ore according to the Welch method, sixteen kilogrammes of coal is consumed for each kilogramme of copper obtained. Hence it were reasoning in a vicious circle to suppose that electrical or electro-magnetic machines can usefully or economically take the place of steam-engines. There is only one case in which this conclusion would be weakened; namely, if with a thermo-electric pile we should succeed in decomposing water into its elements, oxygen and hydrogen, at little or no expense. The problem would then be solved, for this would place in the hands of all the two greatest sources of light, heat, and force—oxygen and hydrogen. But, even then, to what should we owe this unexpected solution? To the sun, for it is only by the aid of a thermo-electric pile (wherein we suppose electricity to be produced by solar heat) that we could economically decompose oxygen and hydrogen; else it would require at least as much heat to dissociate them as they would yield on recombining—a petitio principii overlooked by those simple inventors who persist in attempting, by means of ordinary electric piles, to solve the great problem of economical motors and the fuel of the future.
As for directly storing up solar heat in good conductors or absorbents of heat which are then to be insulated—for instance, receiving the heat in porous black stones which are first exposed to the sun and afterward thrown into a great reservoir, just as snow is piled up in the ice-house—it involves no impossibility. These stones could be thrown into water, if needs were, and in this way we might easily attain or surpass the temperature of boiling water.
Straw, sawdust, wool, feathers, confined air, are insulating substances which retain heat. We might surround with a double envelope of this kind the reservoir holding the sun-heated stones, and in this way we might have our store of solar heat, as now we have our store of ice. It is one problem whether we have to retain cold or to retain heat. Now, ice keeps very well even when stowed in the hold of a vessel; a little sawdust and careful stowage do the whole work.
