lutions in which the solute molecules
are large and have an afÜnity for the
solvent. Starch in water is an exam-
ple of such a system. Association col-
loidsare systems in which the
dispersed phase consists of clusters
of molecules that have lyophobic and
lyophilic parts. Soap in water is an as-
sociation colloid (see micelle).
Emulsionsare colloidal systems in
which the dispersed and continuous
phases are both liquids, e.g. oil-in-
water or water-in-oil. Such systems
require an emulsifying agent to stabi-
lize the dispersed particles.
Gelsare colloids in which both dis-
persed and continuous phases have a
three-dimensional network through-
out the material, so that it forms a
jelly-like mass. Gelatin is a common
example. One component may some-
times be removed (e.g. by heating) to
leave a rigid gel (e.g. silica gel).
Other types of colloid include
aerosols (dispersions of liquid or
solid particles in a gas, as in a mist
or smoke) and foams (dispersions of
gases in liquids or solids).
colophony See rosin
colorimetric analysis Quantita-
tive analysis of solutions by estimat-
ing their colour, e.g. by comparing it
with the colours of standard solu-
tions.
colour centre A defect in a crystal
that changes the way in which it ab-
sorbs light or other electromagnetic
radiation. Impurities in the crystal af-
fect the bind structure and allow
transitions in different regions of the
spectrums. Impurity colour centres
are responsible for the characteristic
colours of many gemstones. A par-
ticular type of colour centre is an F-
centre. This is a missing negative ion
in an ionic crystal, where the overall
charge neutrality occurs by trapping
an electron in the vacancy. The elec-
tron has energy levels similar to
those of a particle in a box. F-centres
can be produced by chemical activity
or by irradiation.colour index (CI)A list, regarded as
deÜnitive, of dyes and pigments,
which includes information on their
commercial names, method of appli-
cation, colour fastness, etc.colour photographyAny of vari-
ous methods of forming coloured
images onÜlm or paper by photo-
graphic means. One common process
is a subtractive reversal system that
utilizes aÜlm with three layers of
light-sensitive emulsion, one re-
sponding to each of the three pri-
mary colours. On development a
black image is formed where the
scene is blue. The white areas are
dyed yellow, the complementary col-
our of blue, and the blackened areas
are bleached clean. A yellowÜlter
between this emulsion layer and
the next keeps blue light from the
second emulsion, which is green-
sensitive. This is dyed magenta
where no green light has fallen. The
Ünal emulsion is red-sensitive and is
given a cyan (blue-green) image on
the negative after dying. When white
light shines through the three dye
layers the cyan dye subtracts red
where it does not occur in the scene,
the magenta subtracts green, and the
yellow subtracts blue. The light pro-
jected by the negative therefore re-
constructs the original scene either
as a transparency or for use with
printing paper.columbiumA former name for the
element *niobium.column chromatography See
chromatography.combinatorial chemistryA tech-
nique in which very large numbers
of related compounds are formed in135 combinatorial chemistry
c