requiring but simple apparatus for demonstration on a small scale. To
take them in order the processes are--

(l) The formation of gas in a retort;

(2) The condensation of the tar;

(3) The condensation of steam;

(4) The removal of the ammonia gas;

(5) The removal of the sulphuretted hydrogen and carbonic acid.

The last two processes are, in a real gasworks, usually separated, but for
simplicity's sake we will combine them. Finally, the storage of the gas has
to be provided for.

The Retort.--To get very good results, the retort should be of cast iron,
and have a removable air-tight cover; but, to keep down expense, we will
use an ordinary 2-pound self-opening coffee tin. A short piece of brass
pipe is soldered into the lid near one edge to carry off the gas as it is
generated. To get a fairly gas-tight joint, red-leaded asbestos string
should be rammed tightly between the lid and the tin. The tin may be laid
on an open fire on the slant, the lid end uppermost, and the pipe at the
top, where the gas will collect; or, if you wish to make things more
realistic, you may easily construct an oven with sides and back of
fire-brick, and front of sheet iron, through the hole in which the tin is
pushed horizontally, so that only half an inch projects. This is a.
suitable arrangement for out of doors.

[Illustration: FIG. 191.--Vertical section of condenser.]

The Hydraulic Main.--This is represented in Fig. 190 by a double-necked
bottle, B, standing in a bowl of cold water. The pipe from the retort
passes through the cork in one neck and dips half an inch below the surface
of the water inside. The gas, on meeting the water, is cooled, and some of
the steam in it is condensed, also most of the tar present, which floats on
the top of the water. From the bottle the gas passes on to the Condensers,
where the process of cooling is completed gradually. The condenser (Fig.
191) is so designed as to cause the gas to pass through several pipes in
succession. The base consists of a tin box, 6 inches long, 4 wide, and
1-3/4 deep. This is divided longitudinally down the centre by a 1-1/2-inch
partition, soldered to the bottom and sides; and the two divisions are
again subdivided, as shown in Fig. 192, by shorter cross partitions.

[Illustration: FIG. 192.--Plan of condenser.]

For the condensing pipes, "compo" tubing of 1/2-inch outside diameter is
convenient. The amount required will, of course, depend on the number of
pi