quantity (the polarizability) and a
macroscopic quantity (the dielectric
constant); it was derived using
macroscopic electrostatics by the Ital-
ian physicist Ottaviano Fabrizio
Mossotti (1791–1863) in 1850 and in-
dependently by Rudolf *Clausius in
- It works best for gases and is
only approximately true for liquids
or solids, particularly if the dielectric
constant is large. Compare lorentz–
lorenz equation.
Claus process A process for obtain-
ing sulphur from hydrogen sulphide
(from natural gas or crude oil). It in-
volves two stages. First, part of the
hydrogen sulphide is oxidized to sul-
phur dioxide:
2H 2 S + 3O 2 →2SO 2 + 2H 2 OSubsequently, the sulphur dioxide re-
acts with hydrogen sulphide to pro-
duce sulphur:
SO 2 + 2H 2 S →3S + 2H 2 OThe second stage occurs at 300°C and
needs an iron or aluminium oxide
catalyst.
clay AÜne-grained deposit consist-
ing chieÛy of *clay minerals. It is
characteristically plastic and virtually
impermeable when wet and cracks
when it dries out. In geology the size
of the constituent particles is usually
taken to be less than 1/256 mm. In
soil science clay is regarded as a soil
with particles less than 0.002 mm in
size.
clay minerals Very small particles,
chieÛy hydrous silicates of alu-
minium, sometimes with magne-
sium and/or iron substituting for all
or part of the aluminium, that are
the major constituents of clay ma-
terials. The particles are essentially
crystalline (either platy orÜbrous)
with a layered structure, but may be
amorphous or metalloidal. The clay
minerals are responsible for the plas-
tic properties of clay; the particles
have the property of being able to
hold water. The chief groups of clay
minerals are:
kaolinite, Al 4 Si 4 O 10 (OH) 8 , the chief
constituent of *kaolin;
halloysite, Al 4 Si 4 (OH) 8 O 10 .4H 2 O;
illite, KAl 4 (Si,Al) 8 O 18 .2H 2 O;
montmorillonite, (Na,Ca)0.33(Al,Mg) 2 -
Si 4 O 10 (OH) 2 .nH 2 O, formed chieÛy
through alteration of volcanic ash;
and
vermiculite, (Mg,Fe,Al) 3 (Al,Si) 4 -
O 10 (OH) 2 .4H 2 O, used as an insulat-
ing material and potting soil.cleavageThe splitting of a crystal
along planes of atoms in the lattice.Clemmensen reduction A
method of reducing a *carbonyl
group (C=O) to CH 2 , using zinc amal-
gam and concentrated hydrochloric
acid. It is used as a method of ascend-
ing a homologous series. The reac-
tion is named after Erik Clemmensen
(1876–1941).clinalSee torsion angle.clock reaction See b–z reaction;
oscillating reaction.closed chainSee chain; ring.close packingThe packing of
spheres so as to occupy the mini-
mum amount of space. In a single
plane, each sphere is surrounded by
six close neighbours in a hexagonal
arrangement. The spheres in the sec-
ond planeÜt into depressions in the
Ürst layer, and so on. Each sphere has
12 other touching spheres. There are
two types of close packing. In hexag-
onal close packing the spheres in the
third layer are directly over those in
theÜrst, etc., and the arrangement of
planes is ABAB.... In cubic close pack-
ingthe spheres in the third layer oc-
cupy a different set of depressions
than those in theÜrst. The arrange-129 close packing
c