This is an absolute seal against the entry of oxygen in
an uncombined state; free oxygen cannot exist at a very high temperature
in the presence of coal gas.
For many trades there is a demand for hardened and tempered steel wire,
either round or flattened, and the production of this has led to many
attempts to obtain a satisfactory continuous process. The common method
now, which is worked as a "secret" process by most firms, is to pass
the wire through a tube to heat it, as already described, and to run it
direct from the tube through a hole in the side of a box filled with
oil, the whole being packed with asbestos, to prevent leakage; from this
it is passed through another similar hole on the opposite side, either
over a plate heated to the right temperature, or over a narrow open
flame of sufficient length and power to give the correct heat for
tempering.
Where absolute precision is necessary, the gas supply must be adapted by
an automatic regulator on the main, to prevent the slightest variation
of heat.
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