=323. Dependence of modern industry on coal.=—It would be a great mistake to consider coal necessary now only in its most common application, that of generating steam for engines. The chemical industry depends largely, though not entirely, on the heat obtained from coal, to break down its raw materials and build up its finished products. Metallurgical industries would shrink almost to infinitesimal proportions if they were denied the use of coal. It has been estimated that the manufacture of a ton of pig iron requires the use of two tons of coal or more; while an equal quantity of steel requires six to eight tons. Still, the use of coal for the steam-engine is undoubtedly its most important application; and we can gain some conception of the place that coal has taken in the world’s economy by considering the growth of steam power. =324. Importance of coal, estimated in steam horse-power.=—A horse-power, the technical unit adopted for measuring the working capacity of an engine, is for practical purposes equal to the force that can be got from several (perhaps three) horses, or from a number of men variously estimated at ten to twenty-four. Now in round numbers the steam horse-power of the world was a million and a half in 1840, and had increased, at the end of the century, to _fifty_ times that amount. A simple operation in arithmetic will show the amount of work, in human equivalent, now done by steam. Taking, for example, a modern country, Germany, we find engaged in industry and transportation slightly over ten million people, while we find engaged beside them another population of mechanical iron slaves (steam-engines), variously estimated as equivalent to one hundred to two hundred and fifty million people. These slaves cost for food (coal), attendance, doctor’s bills (repairs), and burial expenses (including the cost of replacing them once in twenty-five years), only about $2.50 a year apiece. Admit some exaggeration in the figures, and still the contrast with the cost of human labor is most striking.