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.

THE AGE OF REPTILES

Two big