oky and flares, the
point has been passed at which the induced chimney draught can supply
sufficient oxygen to combine with the carbon of the vapour, and the
"free" carbon escapes as smoke.

The blower-plate used to draw up a fire (Fig. 194) performs exactly the
same function as the lamp chimney, but on a larger scale. The plate
prevents air passing straight up the chimney over the coals, and compels
it to find a way through the fire itself to replace the heated air
rising up the chimney.

[Illustration: FIG. 194.--Showing how a blower-plate draws up the
fire.]

GAS AND GASWORKS.

A lamp is an apparatus for converting hydro-carbon mineral oil into gas
and burning it efficiently. The gas-jet burns gases produced by driving
off hydro-carbon vapours from coal in apparatus specially designed for
the purpose. Gas-making is now, in spite of the competition of electric
lighting, so important an industry that we shall do well to glance at
the processes which it includes. Coal gas may be produced on a very
small scale as follows:--Fill a tin canister (the joints of which have
been made by folding the metal, not by soldering) with coal, clap on the
lid, and place it, lid downwards, in a bright fire, after punching a
hole in the bottom. Vapour soon begins to issue from the hole. This is
probably at first only steam, due to the coal being more or less damp.
But if a lighted match be presently applied the vapour takes fire,
showing that coal gas proper is coming off. The flame lasts for a long
time. When it dies the canister may be removed and the contents
examined. Most of the carbon remains in the form of _coke_. It is bulk
for bulk much lighter than coal, for the hydrogen, oxygen, and other
gases, and some of the carbon have been driven off by the heat. The coke
itself burns if placed in a fire, but without any smoke, such as issues
from coal.

[Illustration: FIG. 195.--Sketch of the apparatus used in the
manufacture of coal gas.]

Our home-made gas yields a smoky and unsatisfactory flame, owing to the
presence of certain impurities--ammonia, tar, sulphuretted hydrogen, and
carbon bisulphide. A gas factory must be equipped with means of getting
rid of these objectionable constituents. Turning to Fig. 195, which
displays very diagrammatically the main features of a gas plant, we
observe at the extreme right the _retorts_, which correspond to our
canister. These are usually long fire-brick tubes of D-section, the flat
si