about 15 lb. to the square inch. With the baryta so disposed in the
retorts as to present as large a superficies as possible to the action
of the air, it is found that in 1½ to 2 hours--during which time about
12,000 cub. 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° 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 1½ 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.

The cost of producing oxygen at Westminster, under specially expensive
conditions, is high--about 12s. per 1,000 cub. ft. When we consider,
however, that the cost should only embrace attendance, fuel, wear and
tear, and a little lime and soda for the purifiers, that the
consumption of fuel is small, the wear and tear light, and that the
raw material--air--is obtained for nothing, it ought to be possible to
produce the gas for a third or fourth of this amount in most of our