pes 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.

The tin must be charged with a mixture of two parts green sulphate of iron
and one part lime. The lime should be slaked a short time before use. The
sulphate, lime, and sufficient water to moisten the whole are ground into a
pulp and left to dry. The dry mixture, which has a reddish-yellow colour,
is broken up fine. Put tray B1 into place and spread half the chemical over
it; then lay B on the top and cover it with the remainder. The lid joint is
sealed by a broad rubber band.

While passing through the tin, the ammonia, sulphuretted hydrogen and
carbonic acid gases all combine with the chemical, and fairly pure gas
issues from the outlet.

The Gasholder.--As the gasometer is an important feature of a gasworks,
our small plant should contain its counterpart, as it serves to regulate
the pressure of the gas, and, therefore, the steadiness of the flame, as
well as affording storage room.

As a gasometer, one may use a container made on the principle of the
lung-testing apparatus described on p. 361; or the gasholder of a lantern
acetylene apparatus, which must, of course, b