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 pipes used and the length of the individual pipes.  The design shows 6 pipes, each 3 feet long, bent to a semicircular curve (Fig. 191) at the middle to form very long, narrow horse-shoes.  The pipes are supported at the curve by the crossbar, S (Fig. 191), of a frame, and their ends enter short pieces of brass tubing soldered into holes in the bottom of the tin box.  Rubber bands make the joints air-tight.

[Illustration:  Fig. 193.—­Vertical section of purifier.]

The base is stood bottom upwards in a larger tin containing an inch and a half of water.  The water acts as a seal, preventing the passage of the gas from one compartment to another through the pipes which it traverses, in the order indicated by the arrows and numbers in Fig. 192, to reach the outlet.  On its way the gas is deprived of any water and of any traces of tar.  The condensed water and tar fall from the open ends of the pipes into the base.

The Purifier is made of a large tin with overlapping lid.  Near the bottom is soldered on an inlet pipe; just below the lid an outlet pipe.  Cut out two discs of perforated zinc or sheet tin to fit inside the tin easily, but not loosely. (If tin is used, make a number of small holes in it.) The lower of the discs (Fig. 193, Bl) has three wire legs, aa, soldered to it, to support the upper disc, B. Three short supports keep it clear of the bottom.