nd to show how their abundance, shape and
arrangement contribute to the strength or weakness of the specimen. The
last of these constituents, cementite, is a definite chemical compound,
an iron carbide, Fe_{3}C, containing 6.6 per cent. of carbon, so hard as
to scratch glass, very brittle, and imparting these properties to
hardened steel and cast iron.

With this knowledge at his disposal the iron-maker can work with his
eyes open and so regulate his melt as to cause these various
constituents to crystallize out as he wants them to. Besides, he is no
longer confined to the alloys of iron and carbon. He has ransacked the
chemical dictionary to find new elements to add to his alloys, and some
of these rarities have proved to possess great practical value.
Vanadium, for instance, used to be put into a fine print paragraph in
the back of the chemistry book, where the class did not get to it until
the term closed. Yet if it had not been for vanadium steel we should
have no Ford cars. Tungsten, too, was relegated to the rear, and if the
student remembered it at all it was because it bothered him to
understand why its symbol should be W instead of T. But the student of
today studies his lesson in the light of a tungsten wire and relieves
his mind by listening to a phonograph record played with a "tungs-tone"
stylus. When I was assistant in chemistry an "analysis" of steel
consisted merely in the determination of its percentage of carbon, and I
used to take Saturday for it so I could have time enough to complete the
combustion. Now the chemists of a steel works' laboratory may have to
determine also the tungsten, chromium, vanadium, titanium, nickel,
cobalt, phosphorus, molybdenum, manganese, silicon and sulfur, any or
all of them, and be spry about it, because if they do not get the report
out within fifteen minutes while the steel is melting in the electrical
furnace the whole batch of 75 tons may go wrong. I'm glad I quit the
laboratory before they got to speeding up chemists so.

The quality of the steel depends upon the presence and the relative
proportions of these ingredients, and a variation of a tenth of 1 per
cent. in certain of them will make a different metal out of it. For
instance, the steel becomes stronger and tougher as the proportion of
nicked is increased up to about 15 per cent. Raising the percentage to
25 we get an alloy that does not rust or corrode and is non-magnetic,
although both its component metals