come less
satisfactory. This limit is probably somewhat under 8 in. Each retort
is closely packed with baryta in lumps about the size of a walnut. The
baryta is a heavy grayish porous substance prepared by carefully
igniting the nitrate of barium; and of this each retort having the
above dimensions holds about 125 lb. The retorts so charged are closed
at each end by a gun metal lid riveted on so as to be air tight. From
the center of each lid a bent gun metal pipe, B, connects each retort
with the next of its series, so that air introduced into the end
retort of any row may pass through the whole series of six retorts.
Suppose now that the operations are to commence.

The retorts are first heated to a temperature of about 600 deg. C. or
faint redness, then the air pumps, C C, are started. Air is drawn by
them through the purifier, D, where it is freed from carbon dioxide
and moisture by the layers of quicklime and caustic soda with which
the purifier is charged. The air is then forced along the pipe, E,
into the small air vessel, F, which acts as a sort of cushion to
prevent the baryta in the retorts being disturbed by the pulsation of
the pumps. From this vessel the air passes by the pipe, G, and is
distributed in the retorts as rapidly as possible at such a pressure
that the nitrogen which passes out unabsorbed at the outlet registers
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 11/2 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 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