These changes are somewhat similar to those taking place
in the destructive distillation of coal. In the carbonisation
of coal and bone, although practically carried on in the
same way, very different ends are sought for. The gas-maker
looks to the production of a rich illuminating gas as his
primary object, — the coke, tar, and ammoniacal liquor are of
secondary importance ; on the other hand, the contents of the
retort form the chief aim of the char-maker, — the illuminating
gases, with the tar and ammoniacal liquor, are bye-products
only.
The bones are carbonised in a series of retorts placed
horizontally in a furnace, and heated from a fire underneath.
They are made of iron, with one end closed, each being 12
feet long, and in shape are similar to the letter D. At the
open end is fixed the frame or mouthpiece which carries the
door swung on a hinge. The door has a slight projecting rim
some two inches wide, which, with the surface of the frame, are
ground perfectly true ; on closing, the joint is made perfectly
gastight by a lever arrangement. Fixed to the frame on the
upper surface is the " up-take " pipe leading to the hydraulic
main. Five retorts are generally bedded in each furnace,
and the setting is so arranged that, by the aid of dampers
and pigeon-holes, the flame may be made to sweep equally



BONE PEODUCTS AND MANURES



19



round each retort, so that each shall receive its full comple-
ment of heat. The spent heat finally passes to an under-
ground flue connected with the chimney. The furnaces are
all internally lined with fire-brick, and have an outer 14-inch
red brick wall, the whole being bound by 1-inch tie-rods.
