the energy difference between the
two states. Some caesium nuclei ab-
sorb this radiation and are excited to
the higher state. These atoms are
deÛected by a further magneticÜeld,
which causes them to hit a detector.
A signal from this detector is fed
back to the radio-frequency oscillator
to prevent it drifting from the reso-
nant frequency of 9 192 631 770
hertz. In this way the device is locked
to this frequency with an accuracy
better than 1 part in 10^13. The cae-
sium clock is used in the *SI unit
deÜnition of the second.caffeine (1,3,7-trimethylxanthine)
An alkaloid, C 8 H 10 N 4 O 2 ; m.p. 235°C;
sublimes at 176°C. It is a stimulant
and diuretic and is present in cof-
fee, tea, and some soft drinks. See
methylxanthines.cage compoundSee clathrate.cage effect An effect occurring in
certain condensed-phase reactions in
which fragments are formed and
their diffusion is hindered by a sur-
rounding ‘cage’ of molecules. The ini-
tial fragments are consequently more
likely to recombine or to react to-
gether to form new products.Cahn–Ingold–Prelog system See
cip system.calamineA mineral form of zinc
carbonate, ZnCO 3 (smithsonite), al-
though in the USA the same name is
given to a hydrated zinc silicate
(hemimorphite). The calamine used
medicinally in lotions for treating
sunburn and other skin conditions is
basic zinc carbonate coloured pink
with a trace of iron(III) oxide.
calcinationThe formation of a cal-
cium carbonate deposit from hard
water. See hardness of water.calciniteA mineral form of *potas-
sium hydrogencarbonate, KHCO 3.calciteOne of the most common
and widespread minerals, consisting
of crystalline calcium carbonate,
CaCO 3. Calcite crystallizes in the
rhombohedral system; it is usually
colourless or white and has a hard-
ness of 3 on the Mohs’ scale. It has
the property of double refraction,
which is apparent in Iceland spar –
the transparent variety of calcite. It is
an important rock-forming mineral
and is a major constituent in lime-
stones, marbles, and carbonatites.calciumSymbol Ca. A soft grey
metallic element belonging to
*group 2 (formerly IIA) of the peri-
odic table; a.n. 20; r.a.m. 40.08; r.d.
1.54; m.p. 839°C; b.p. 1484°C. Cal-
cium compounds are common in the
earth’s crust; e.g. limestone and mar-
ble (CaCO 3 ), gypsum (CaSO 4 .2H 2 O),
andÛuorite (CaF 2 ). The element is ex-
tracted by electrolysis of fused cal-
cium chloride and is used as a getter
in vacuum systems and a deoxidizer
in producing nonferrous alloys. It is
also used as a reducing agent in the
extraction of such metals as thorium,
zirconium, and uranium.
Calcium is an essential element for
living organisms, being required for
normal growth and development. In
animals it is an important con-
stituent of bones and teeth and is
present in the blood, being required
for muscle contraction and other
metabolic processes. In plants it is a
constituent (in the form of calcium
pectate) of the middle lamella.
A- Information from the WebElements site
calcium acetylideSee calcium di-
carbide.calcium bicarbonateSee calcium
hydrogencarbonate.calcium carbideSee calcium di-
carbide.caffeine 92c