s labour, the steel is first annealed. In this case it is
brought to a low red heat, and allowed to cool away from the air. It can
then be machined with comparative ease and be subsequently hardened or
tempered. When a metallic structure has endured long service a state of
fatigue results. Annealing is, where practicable, resorted to in order
to restore the original strength. A familiar illustration is that of
chains which are specially liable to succumb to constant overstrain if
continued for only a year or two. This is so well known that the
practice is regularly adopted of annealing the chains at regular
intervals. They are put into a clear hot furnace and raised to a low red
heat, continued for a few hours, and then allowed to cool down in the
furnace after the withdrawal of the source of heat. Before the annealing
the fracture of a link would be more crystalline than afterwards.

In these examples, and others of which these are typical, two conditions
are essential, one being the grade of temperature, the other the
cooling. The temperature must never be so high as to cause the metal to
become overheated, with risk of burning, nor so low as to prevent the
penetration of the substance with a good volume of heat. It must also be
continued for sufficient time. More than this cannot be said. Each
particular piece of work requires its own treatment and period, and
nothing but experience of similar work will help the craftsman. The
cooling must always be gradual, such as that which results from removing
the source of heat, as by drawing a furnace fire, or covering with
non-conducting substances.

The chemical kind of annealing is specifically that employed in the
manufacture of malleable cast iron. In this process, castings are made
of white iron,--a brittle quality which has its carbon wholly in the
combined state. These castings, when subjected to heat for a period of
ten days or a fortnight, in closed boxes, in the presence of substances
containing oxygen, become highly ductile. This change is due to the
absorption of the carbon by the oxygen in the cementing material, a
comparatively pure soft iron being left behind. The result is that the
originally hard, brittle castings after this treatment may be cut with a
knife, and be bent double and twisted into spirals without fracturing.

The distinction between _hardening_ and _tempering_ is one of degree
only, and both are of an opposite character to annealing. Hardening,