are two of the best-known examples of this
modification.

Coke from retort ovens is not so dense or brilliant as that made in
beehive ovens, but the waste being less there is a decided saving,
apart from the value of the condensed products. In one instance the
coke was found to be about 5% less efficient in the blast furnace,
while the yield on the coal charged was increased 10%. In the further
treatment of the condensed products by distillation the tar gives
burning oil and pitch, the benzene is separated from the creosote oil
by steam-heated stills, and the ammoniacal liquor, after some lime has
been added to decompose fixed ammonium compounds, is heated to
vaporize the ammonia, which is condensed in lead or copper-lined tanks
containing strong sulphuric acid to produce a crystalline powder of
ammonium sulphate, which accumulates in the receiver and is fished out
from time to time. The yield of by-products averages about 1% of
ammonium sulphate, about 3-1/2% of tar, and 0.6 to 0.9% of benzene, of
the weight of the coal carbonized. After the ovens have been heated
and steam supplied for the machinery of the condensing plant and the
coke ovens, there is usually a surplus of gas, which may be used for
lighting or driving gas-engines. For the latter purpose, however, it
is necessary to remove the last traces of tar, which acts very
prejudicially in fouling the valves when the gas is not completely
purified. The gas given off during the earlier part of the coking
process is richer in heavy hydrocarbons and of a higher illuminating
value than that of the later period when the temperature is higher.
This property is utilized in several large coking plants in America,
where the gas from the first ten hours' working is drawn off by a
second hydraulic main and sent directly to town gas-works, where it
passes through the ordinary purifying treatment, the gas from the
second period being alone used for heating the ovens.

Coke is essentially a partially graphitized carbon, its density being
about midway between that of coal and graphite, and it should therefore
occupy less space than the original coal; but owing to the softening of
the charge a spongy structure is set up by the escaping gases, which
acts in the other direction, so that for equal bulk coke is somewhat
lighter than coal. It is this combination of properties that gives it
its chief value in iron smelting, th