It is fortunate for us all that, out of the half-dozen so-called useful metals, iron, which is the most useful of them all to the human race, should be also the most plentiful and the cheapest. Aluminum is abundant in the common clay and soil under our feet. But separating it is still an expensive process; so that this metal is not commercially so plentiful as iron is, nor is it cheap.

All we know of the earth's substance is based on studies of the superficial part of its crust, a mere film compared with the eight thousand miles of its diameter. Nobody knows what the core of the earth—the great globe under this surface film—is made of; but we know that it is of heavier material than the surface layer; and geologists believe that iron is an important element in the central mass of the globe.

One thing that makes this guess seem reasonable is the great abundance of iron in the earth's crust. Another thing is that meteors which fall on the earth out of the sky prove to be chiefly composed of iron. All of their other elements are ones which are found in our own rocks. If we believe that the earth itself is a fragment of the sun, thrown off in a heated condition and cooling as it flew through space, we may consider it a giant meteor, made of the substances we find in the chance meteor that strikes the earth.

Iron is found, not only in the soil, but in all plant and animal bodies that take their food from the soil. The red colour in fruits and flowers, and in the blood of the higher animals, is a form in which iron is familiar to us. It does more, perhaps, to make the world beautiful than any other mineral element known.

But long before these benefits were understood, iron was the backbone of civilization. It is so to-day. Iron, transformed by a simple process into steel, sustains the commercial supremacy of the great civilized nations of the world. The railroad train, the steel-armoured battleship, the great bridge, the towering sky-scraper, the keen-edged tool, the delicate mechanism of watches and a thousand other scientific instruments—all these things are possible to-day because iron was discovered and has been put to use.

It was probably one of the cave men, poking about in his fire among the rocks, who discovered a lump of molten metal which the heat had separated from the rest of the rocks. He examined this "clinker" after it cooled, and it interested him. It was a new discovery. It may have been he, or possibly his descendants, who learned that this metal could be pounded into other shapes, and freed by pounding from the pebbles and other impurities that clung to it when it cooled. The relics of iron-tipped spears and arrows show the skill and ingenuity of our early ancestors in making use of iron as a means of killing their prey. The earliest remains of this kind have probably been lost because the iron rusted away.

Man was pretty well along on the road to civilization before he learned where iron could be found in beds, and how it could be purified for his use. We now know that certain very minute plants, which live in quiet water, cause iron brought into that water to be precipitated, and to accumulate in the bottom of these boggy pools. In ancient days these bog deposits of iron often alternated with coal layers. Millions of years have passed since these two useful substances were laid down. To-day the coal is dug, along with the bog iron. The coal is burned to melt the iron ore and prepare it for use. It is a fortunate region that produces both coal and iron.

Bituminous coal is plentiful, and scattered all over the country, while anthracite is scarce. The cheapest iron is made in Alabama, which has its ore in rich deposits in hillsides, and coal measures close by, furnishing the raw material for coke. The result is that the region of Birmingham has become the centre of great wealth through the development of iron and coal mines.

Where water flows over limestone rock, and percolates through layers of this very common mineral, it causes the iron, gathered in these rock masses, to be deposited in pockets. All along the Appalachian Mountains the iron has been gathered in beds which are being mined. These beds of ore are usually mixed with clay and other earthy substances from which the metal can be separated only by melting. The ore is thrown into a furnace where the metal melts and trickles down, leaving behind the non-metallic impurities. It is drawn off and run into moulds, where it cools in the form of "pig" iron.

The first fuel used in the making of pig iron from the ore was charcoal. In America the early settlers had no difficulty in finding plenty of wood. Indeed, the forests were weeds that had to be cut down and burned to make room for fields of grain. The finding of iron ore always started a small industry in a colony. If there was a blacksmith, or any one else among the small company who understood working in iron, he was put in charge.

To make the charcoal, wood was cut and piled closely in a dome-shaped heap, which was tightly covered with sods, except for a small opening near the ground. In this a fire was built, and smothered, but kept going until all the wood within the oven was charred.

This fuel burned readily, with an intense heat, and without ashes. Sticks of charcoal have the form of the wood, and they are stiff enough to hold up the ore of iron so that it cannot crush out the fire. For a long time American iron was supplied by little smelters, scattered here and there. The workmen beat the melted metal on the forge, freeing it from impurities, and shaping the pure metal into useful articles. Sometimes they made it into steel, by a process learned in the Old World.

The American iron industry, which now is one of the greatest in the world, centres in Pittsburg, near which great deposits of iron and coal lie close together. The making of coke from coal has replaced the burning of charcoal for fuel. When the forests were cut away by lumbermen, the supply of charcoal threatened to give out, and experiments were made in charring coal, which resulted in the successful making of coke, a fuel made from coal by a process similar to the making of charcoal from wood. The story of the making of coke out of hard and soft coal is a long one, for it began as far back as the beginning of the nineteenth century.

In 1812 the first boat-load of anthracite coal was sent to Philadelphia from a little settlement along the Lehigh River. A mine had been opened, the owner of which believed that the black, shiny "rocks" would burn. His neighbours laughed at him, for they had tried building fires with them, and concluded that it could not be done. In Philadelphia, the owners of some coke furnaces gave the new fuel a trial, in spite of the disgust of the stokers, who thought they were putting out their fires with a pile of stones. After a little, however, the new fuel began to burn with the peculiar pale flame and intense heat that we know so well, and the stokers were convinced that here was a new fuel, with possibilities in it.

But it was hard for people to be patient with the slow starting of this hard coal. Not until 1840 did it come into general favour, following the discovery that if hot air was supplied at the draught, instead of cold, anthracite coal became a perfect fuel.

At Connellsville, Pennsylvania, a vein of coal was discovered which made coke of the very finest quality. Around this remarkable centre, coke ovens were built, and iron ore was shipped in, even from the rich beds of the Lake Superior country. But it was plain to see that Connellsville coal would become exhausted; and so experiments in coke-making from other coals were still made. When soft coal burns, a waxy tar oozes out of it, which tends to smother the fire. Early experiments with coal in melting iron ore indicated that soft coal was useless as a substitute for charcoal and coke; but later experiments proved that coke of fine quality can be made out of this bituminous soft coal, by drawing off the tar which makes the trouble. New processes were invented by which valuable gas and coal tar are taken out of bituminous coal, leaving, as a residue, coke that is equal in quality to that made from the Connellsville coal. Fortunes have been made out of the separation of the elements of the once despised soft coal. For the coke and each of its by-products, coal tar and coal gas, are commercial necessities of life.

The impurities absorbed by the melting iron ore include carbon, phosphorus, and silicon. Carbon is the chief cause of the brittleness of cast iron. The puddling furnace was invented to remove this trouble. The melted ore was stirred on a broad, basin-like hearth, with a long-handled puddling rake. The flames swept over the surface, burning the carbon liberated by the stirring. It was a hard, hot job for the man at the rake, but it produced forge iron, that could be shaped, hot or cold, on the anvil.

The next improvement was the process of pressing the hot iron between grooved rollers to rid it of slag and other foreign matters collected in the furnace. The old way was to hammer the metal free from such impurities. This was slow and hard work.

Iron was an expensive and scarce metal until the hot blast-furnace cheapened the process of smelting the ore. The puddling furnace and the grooved rollers did still more to bring it into general use. The railroads developed with the iron industry. Soon they required a metal stronger than iron. Steel was far too expensive, though it was just what was needed. Efforts were made to find a cheap way to change iron into steel. Sir Henry Bessemer solved the problem by inventing the Bessemer converter. It is a great closed retort, which is filled with melted pig iron. A draught admits air, and the carbon is all burned out. Then a definite amount of carbon, just the amount required to change iron into steel, is added, by throwing in bars of an alloy of carbon and manganese. The latter gives steel its toughness, and enables it to resist greater heat without crystallizing and thus losing its temper.

When the carbon has been put in, the retort is closed. The molten metal absorbs the alloy, and the product is Bessemer steel. In fifteen minutes pig iron can be transformed into ingot steel. The invention made possible the use of steel in the construction of bridges, high buildings, and ships. It made this age of the world the Age of Steel.