ects. It is further purified by
heating in closed vessels, but even then it still contains a certain
amount of mineral matter and more or less hydrocarbons. It is used in
the manufacture of printer's ink, in the preparation of black paint and
in calico printing. _Gas carbon_ is produced by the destructive
distillation of coal in the manufacture of illuminating gas (see GAS:
_Manufacture_), being probably formed by the decomposition of gaseous
hydrocarbons. It is a very dense form of carbon, and is a good conductor
of heat and electricity. It is used in the manufacture of carbon rods
for arc lights, and for the negative element in the Bunsen battery.

_Charcoal_ is a porous form of carbon; several varieties exist. _Sugar
charcoal_ is obtained by the carbonization of sugar. It is purified by
boiling with acids, to remove any mineral matter, and is then ignited
for a long time in a current of chlorine in order to remove the last
traces of hydrogen. _Animal charcoal_ (bone black) is prepared by
charring bones in iron retorts. It is a very impure form of carbon,
containing on the average about 80% of calcium phosphate. It possesses a
much greater decolorizing and absorbing power than wood charcoal. A
variety of animal charcoal is sometimes prepared by calcining fresh
blood with potassium carbonate in large cylinders, the mass being
purified by boiling out with dilute hydrochloric acid and subsequent
reheating. _Wood charcoal_ is a hard and brittle black substance, which
retains the external structure of the wood from which it is made. It is
prepared (where wood is plentiful) by stacking the wood in heaps, which
are covered with earth or with brushwood and turf, and then burning the
heap slowly in a limited supply of air. The combustion of the wood is
conducted from the top downwards, and from the exterior towards the
centre; great care has to be taken that the process is carried out
slowly. The disadvantage in this process is that the by-products, such
as pyroligneous acid, acetone, wood spirit, &c., are lost; as an
alternative method, wood is frequently carbonized in ovens or retorts
and the volatile products are condensed and utilized.

Charcoal varies considerably in its properties, depending upon the
particular variety of wood from which it is prepared, and also upon
the process used in its manufacture. It can be made at a temperature
as low as 300 deg. C., and is then a soft, very friable material
possessing