ft of air have been passed through the retorts--the gas at
the outlet fails to extinguish a glowing chip, indicating that oxygen
is no longer being absorbed. The pumping now ceases, and the
temperature of the retorts is raised to about 800 deg. C. The workman is
able to judge the temperature with sufficient accuracy by means of the
small inspection holes, H, fitted with panes of mica, through which
the color of the heat in the furnace can be distinguished. The pumps
are now reversed and the process of exhaustion begins. At Westminster
the pressure in the retorts is reduced to about 11/2 in. of mercury. In
this partial vacuum the oxygen is given off rapidly, and if forced by
the pumps through another pipe and away into an ordinary gas holder,
where it is stored for use. With powerful pumps such as are used in
the plant under notice the whole of the oxygen can be drawn off in an
hour, and from one charge a yield of about 2,000 cub. ft. is obtained.
With a less perfect vacuum the time is longer--even as much as four
hours. The whole operation of charging and exhausting the retorts can
be completed in from three to four hours. As soon as the evolution of
oxygen is finished, the doors, K, and ventilators, L, may be opened
and the retorts cooled for recharging.
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