thus makes the bed porous. However the third form, with
the traveling grate, accomplishes combustion much more
completely than the others.
The second way of using up the coal of the culm heaps
has met with only partial success. The coal in an impalp
able powder is blown into the combustion chamber. An
ideal furnace for this purpose is one in which the fuel is kept
suspended in the air until entirely consumed, and one from
whose chimney no smoke issues. The original attempts to
make furnaces of this sort failed. Such a furnace requires a
very delicate and complex mechanism, and must execute
rapid and complete combustion. Very few furnaces of this
sort have been constructed.
The third way of using waste coal, which will doubtless
prove to be the most serviceable of the three, consists, as
said above, in mixing the fine coal with some binding mater
ial and compressing portions of the mixture into forms of
suitable size and shape so that they may be used as ordinary
coal. The forms thus made are called briquettes, and were
first made in France. In fact nearly all of the methods for
economy originated in the Old World, as it was there that
the need of economy was first felt. briquettes were made
in France as early as the year 1594, but did not become of
commercial importance until a few years before the middle
of the century. The different substances, "binding mater
ials," that may be used to mix with the coal are very nu
merous, and may be inert or combustible. The principal
inert substances are,—clay, soap, plaster of Paris, hydraulic
lime, slack lime, wood ashes, caustic soda, carbonate of soda
sulphide of ammonia, sulphide of iron, sand, silicate of pot
ash, furnace slag, black oxide of manganese, etc. The
principle combustible substances that may be used are,
bitumenous coal slack, pitch, tar, asphalt, petroleum, dead
oils from distillery petroleum, some one of the hydrocarbons,
etc. Some attempts have been made to agglomerate the
fine coal without a binding material, but have always proved
.-01Thracile Coal IVasie