the drawings being divided into two systems of thirty-six. Each system
is fed by the two pumps on the corresponding side of the boiler. Each
set of retorts consists of six rows of six retorts each, one row above
the other. They are heated by a small Wilson's producer, so that the
attendant can easily regulate the supply of heat and obtain complete
control over the temperature of the retorts. The retorts, A, are made
of wrought iron and are about 10 ft long and 8 in. diameter.
Experience, however, goes to prove that there is a limit to the
diameter of the retorts beyond which the results become 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° 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