INDUSTRIAL APPLICATIONS OF SOLAR HEAT. 559 



The world will not perish for want of coal, yet the coal-supply 

 will fail, and that much sooner than Ericsson estimated, for the pro- 

 duction 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 cen- 

 turies 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 kilo- 

 grammes 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 conclu- 

 sion 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 disso- 

 ciate 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 eco- 

 nomical motors and the fuel of the future. 



As for directly storing up solar heat in good conductors or absorb- 

 ents 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 sub- 

 stances which retain heat. We might suiTound with a double en- 

 velope 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. 



