ansmitted through the sides of the
latter which are comparatively thin. The arrangement is somewhat similar
to that of a gas-works retort, whence the name of "retort ovens" is
sometimes applied to them. The difference between the second and third
classes is founded on the treatment of the gases. In the former the gas
is fired in the side flues immediately upon issuing from the oven, while
in the latter the gases are first subjected to a systematic treatment in
condensers, similar to those used in gas-works, to remove tar, ammonia
and condensable hydrocarbons, the incondensable gases being returned to
the oven and burned in the heating flues. These are generally known as
"by-product ovens."

Beehive oven.

The simplest form of coke oven, and probably that still most largely
used, is the so-called "beehive oven." This is circular in plan, from
7 to 12 ft. in diameter, with a cylindrical wall about 2-1/2 ft. high
and a nearly hemispherical roof with a circular hole at the top. The
floor, made of refractory bricks or slabs, is laid with a slight slope
towards an arched opening in the ring wall, which is stopped with
brickwork during the coking but opened for drawing the finished
charge. The ovens are usually arranged in rows or banks of 20 to 30 or
more, with their doors outwards, two rows being often placed with a
longitudinal flue between them connected by uptakes with the
individual ovens on either side. A railway along the top of the bank
brings the coal from the screens or washery. The largest ovens take a
charge of about 5 tons, which is introduced through the hole in the
roof, the brickwork of the empty oven being still red hot from the
preceding charge, and when levelled fills the cylindrical part nearly
to the springing of the roof. The gas fires as it is given off and
fills the dome with flame, and the burning is regulated by air
admitted through holes in the upper part of the door stopping. The
temperature being very high, a proportion of the volatile hydrocarbons
is decomposed, and a film of graphitic carbon is deposited on the
coke, giving it a semi-metallic lustre and silvery grey colour. When
the gas is burned off, the upper part of the door is opened and the
glowing charge cooled by jets of water thrown directly upon it from a
hose, and it is subsequently drawn out through the open door. The
charge breaks up into prisms or columns whose length correspo