the fumes are poisonous in most all cases, and if not poisonous, are
extremely disagreeable.

The degree of heat that a piece of steel must be brought to in order that
it may be hardened depends on the percentage of carbon in the steel. The
greater the percentage of carbon, the lower the heat necessary to harden.

[Illustration: Figure 5.--Cooling the Test Bar for Hardening]

To find the proper heat from which any steel must be cooled, a simple test
may be carried out provided a sample of the steel, about six inches long
can be secured. One end of this test bar should be heated almost to its
melting point, and held at this heat until the other end just turns red.
Now cool the piece in water by plunging it so that both ends enter at the
same time (Figure 5), that is, hold it parallel with the surface of the
water when plunged in. This serves the purpose of cooling each point along
the bar from a different heat. When it has cooled in the water remove the
piece and break it at short intervals, about 1/2 inch, along its length.
The point along the test bar which was cooled from the best possible
temperature will show a very fine smooth grain and the piece cannot be cut
by a file at this point. It will be necessary to remember the exact color
of that point when taken from the fire, making another test if necessary,
and heat all pieces of this same steel to this heat. It will be necessary
to have the cooling bath always at the same temperature, or the results
cannot be alike.

While steel to be hardened is usually cooled in water, many other liquids
may be used. If cooled in strong brine, the heat will be extracted much
quicker, and the degree of hardness will be greater. A still greater degree
of hardness is secured by cooling in a bath of mercury. Care should be used