, iron and nickel, are by themselves
attracted by the magnet. With 36 per cent. nickel and 5 per cent.
manganese we get the alloy known as "invar," because it expands and
contracts very little with changes of temperature. A bar of the best
form of invar will expand less than one-millionth part of its length for
a rise of one degree Centigrade at ordinary atmospheric temperature. For
this reason it is used in watches and measuring instruments. The alloy
of iron with 46 per cent. nickel is called "platinite" because its rate
of expansion and contraction is the same as platinum and glass, and so
it can be used to replace the platinum wire passing through the glass of
an electric light bulb.

A manganese steel of 11 to 14 per cent. is too hard to be machined. It
has to be cast or ground into shape and is used for burglar-proof safes
and armor plate. Chrome steel is also hard and tough and finds use in
files, ball bearings and projectiles. Titanium, which the iron-maker
used to regard as his implacable enemy, has been drafted into service as
a deoxidizer, increasing the strength and elasticity of the steel. It is
reported from France that the addition of three-tenths of 1 per cent. of
zirconium to nickel steel has made it more resistant to the German
perforating bullets than any steel hitherto known. The new "stainless"
cutlery contains 12 to 14 per cent. of chromium.

With the introduction of harder steels came the need of tougher tools to
work them. Now the virtue of a good tool steel is the same as of a good
man. It must be able to get hot without losing its temper. Steel of the
old-fashioned sort, as everybody knows, gets its temper by being heated
to redness and suddenly cooled by quenching or plunging it into water or
oil. But when the point gets heated up again, as it does by friction in
a lathe, it softens and loses its cutting edge. So the necessity of
keeping the tool cool limited the speed of the machine.

But about 1868 a Sheffield metallurgist, Robert F. Mushet, found that a
piece of steel he was working with did not require quenching to harden
it. He had it analyzed to discover the meaning of this peculiarity and
learned that it contained tungsten, a rare metal unrecognized in the
metallurgy of that day. Further investigation showed that steel to which
tungsten and manganese or chromium had been added was tougher and
retained its temper at high temperature better than ordinary carbon
steel. Tools made from it