the high temperatures and pressures)
continue, there is a progressive trans-
formation of the deposit: the propor-
tion of carbon relative to oxygen
rises and volatile substances and
water are driven out. The various
stages in this process are referred to
as the ranksof the coal. In ascending
order, the main ranks of coal are: lig-
nite(or brown coal), which is soft,
brown, and has a high moisture con-
tent; subbituminous coal, which is
used chieÛy by generating stations;
bituminous coal, which is the most
abundant rank of coal; semibitumi-
nous coal; semianthracite coal, which
has aÜxed carbon content of be-
tween 86% and 92%; and anthracite
coal, which is hard and black with a
Üxed carbon content of between 92%
and 98%.
Most deposits of coal were formed
during the Carboniferous and Per-
mian periods. More recent periods of
coal formation occurred during the
early Jurassic and Tertiary periods.
Coal deposits occur in all the major
continents; the leading producers in-
clude the USA, China, Ukraine,
Poland, UK, South Africa, India, Aus-
tralia, and Germany. Coal is used as a
fuel and in the chemical industry; by-
products include coke and coal tar.
coal gasA fuel gas produced by the
destructive distillation of coal. In the
late-19th and early-20th centuries
coal gas was a major source of energy
and was made by heating coal in the
absence of air in local gas works. Typ-
ically, it contained hydrogen (50%),
methane (35%), and carbon monoxide
(8%). By-products of the process were
coal tar and coke. The use of this
type of gas declined with the increas-
ing availability of natural gas, al-
though since the early 1970s interest
has developed in using coal in mak-
ing SNG.
coal tar A tar obtained from the de-
structive distillation of coal. For-
merly, coal tar was obtained as a by-
product in manufacturing *coal gas.
Now it is produced in making coke
for steel making. The crude tar con-
tains a large number of organic
compounds, such as benzene, naph-
thalene, methylbenzene, phenols,
etc., which can be obtained by distil-
lation. The residue is pitch. At one
time coal tar was the major source of
organic chemicals, most of which are
now derived from petroleum and
natural gas.cobalamin(vitamin B 12 )See vita-
min b complex.cobaltSymbol Co. A light-grey
*transition element; a.n. 27; r.a.m.
58.933; r.d. 8.9; m.p. 1495°C; b.p.
2870 °C. Cobalt is ferromagnetic
below its Curie point of 1150°C.
Small amounts of metallic cobalt are
present in meteorites but it is usually
extracted from ore deposits worked
in Canada, Morocco, and Zaïre. It is
present in the minerals cobaltite,
smaltite, and erythrite but also asso-
ciated with copper and nickel as sul-
phides and arsenides. Cobalt ores are
usually roasted to the oxide and then
reduced with carbon or water gas.
Cobalt is usually alloyed for use. Al-
nico is a well-known magnetic alloy
and cobalt is also used to make stain-
less steels and in high-strength alloys
that are resistant to oxidation at high
temperatures (for turbine blades and
cutting tools).
The metal is oxidized by hot air
and also reacts with carbon, phos-
phorus, sulphur, and dilute mineral
acids. Cobalt salts, usual oxidation
states II and III, are used to give a
brilliant blue colour in glass, tiles,
and pottery. Anhydrous cobalt(II)
chloride paper is used as a qualitative
test for water and as a heat-sensitive
ink. Small amounts of cobalt salts are
essential in a balanced diet for mam-131 cobalt
c