nds to
the depth of the charge, and as a rule is uniform in character and
free from dull black patches or "black ends." The time of burning is
either 48 or 72 hours, the turns being so arranged as to avoid the
necessity of drawing the ovens on Sunday. The longer the heat is
continued the denser the product becomes, but the yield also
diminishes, as a portion of the finished coke necessarily burns to
waste when the gas is exhausted. For this reason the yield on the coal
charged is usually less than that obtained in retort ovens, although
the quality may be better. Coals containing at most about 35% of
volatile matter are best suited for the beehive oven. With less than
25% the gas is not sufficient to effect the coking completely, and
when there is a higher percentage the coke is brittle and spongy and
unsuited for blast furnace or foundry use. The spent flame from the
ovens passes to a range of steam boilers before escaping by the
chimney.

Retort oven.

The retort oven, which is now generally displacing the beehive form in
new installations, is made in a great variety of forms, the
differences being mainly in the arrangement of the heating flues, but
all have the central feature, the coking chamber, in common. This is a
tubular chamber with vertical sides and cylindrical roof, about 30 ft.
long, from 17 to 20 in. wide, and 6 or 7 ft. high, and closed at both
ends by sliding doors which are raised by crab winches when the charge
is to be drawn. The general arrangements of such an oven are shown in
fig. 1, which represents one of the earliest and most popular forms,
that of Evence Coppee of Brussels. The coking chambers A B connect by
rectangular posts at the springing of the roof, where the gas given
off from the top of the charge is fired by air introduced through _c
c_. The flames pass downwards through the parallel flues _f f_ along
the bottom flue of one oven, and return in the opposite direction
under the next to the chimney flue, a further part of the heat being
intercepted by placing a range of steam boilers between the ovens and
the chimney stack. The charging of the oven is done through the
passages D D in the roof from small wagons on transverse lines of
rails, the surface being raked level before the doors are closed and
luted up. The time of coking is much less than in the beehive ovens
and may be from 24 to 36 hours, according t