ain grade of softness or hardness, with all that is involved in and
follows from those conditions. The effect may be mechanical only, or a
chemical change may take place also. Sometimes the causes are obvious,
in other cases they are more or less obscure. But of the actual facts,
and the immense importance of this operation as well as of the related
ones of tempering and hardening in shop processes, there is no question.

When the treatment is of a mechanical character only, there can be no
reasonable doubt that the common belief is correct, namely, that the
metallic crystals or fibres undergo a molecular rearrangement of some
kind. When it is of a chemical character, the process is one of
cementation, due to the occlusion of gases in the molecules of the
metals.

Numerous examples of annealing due to molecular rearrangement might be
selected from the extensive range of workshop operations. The following
are a few only:--when a boiler-maker bends the edges of a plate of steel
or iron by hammer blows (flanging), he does so in successive stages
(heats), at each of which the plate has to be reheated, with inevitable
cooling down during the time work is being done upon it. The result is
that the plate becomes brittle over the parts which have been subjected
to this treatment; and this brittleness is not uniformly distributed,
but is localized, and is a source of weakness, inducing a liability to
crack. If, however, the plate when finished is raised to a full red
heat, and allowed to cool down away from access of cool air, as in a
furnace, or underneath wood ashes, it resumes its old ductility. The
plate has been annealed, and is as safe as it was before it was flanged.
Again, when a sheet of thin metal is forced to assume a shape very
widely different from its original plane aspect, as by hammering, or by
drawing out in a press--a cartridge case being a familiar example--it is
necessary to anneal it several times during the progress of the
operation. Without such annealing it would never arrive at the final
stage desired, but would become torn asunder by the extension of its
metallic fibres. Cutting tools are made of steel having sufficient
carbon to afford capacity for hardening. Before the process is
performed, the condition in which the carbon is present renders the
steel so hard and tough as to render the preliminary turning or shaping
necessary in many cases (e.g. in milling cutters) a tedious operation.
To lessen thi