tion states. Selenium was discovered
in 1817 by Jöns Berzelius.
A
- Information from the WebElements site
selenium cellEither of two types
of photoelectric cell; one type relies
on the photoconductive effect, the
other on the photovoltaic effect (see
photoelectric effect). In the photo-
conductive selenium cell an external
e.m.f. must be applied; as the sele-
nium changes its resistance on expo-
sure to light, the current produced is
a measure of the light energy falling
on the selenium. In the photovoltaic
selenium cell, the e.m.f. is generated
within the cell. In this type of cell, a
thinÜlm of vitreous or metallic sele-
nium is applied to a metal surface, a
transparentÜlm of another metal,
usually gold or platinum, being
placed over the selenium. Both types
of cell are used as light meters in
photography.
self-assemblySee self-organiza-
tion.
self-consistentÜeld(SCF)A con-
cept used toÜnd approximate solu-
tions to the many-body problem in
quantum mechanics. The procedure
starts with an approximate solution
for a particle moving in a single-
particle potential, which derives
from its average interaction with all
the other particles. This average in-
teraction is determined by the wave
functions of all the other particles.
The equation describing this average
interaction is solved and the im-
proved solution obtained is used in
the calculation of the interaction
term. This procedure is repeated for
wave functions until the wave func-
tions and associated energies are
not signiÜcantly changed in the
cycle, self-consistency having been
attained. In atomic theory the
*Hartree–Fock procedure makes use
of self-consistentÜelds.
self-organizationThe sponta-
neous order arising in a system when
certain parameters of the system
reach critical values. Self-organiza-
tion occurs in many systems in
physics, chemistry, and biology. It
can occur when a system is driven
far from thermal *equilibrium. Since
a self-organizing system is open to its
environment, the second law of
*thermodynamics is not violated by
the formation of an ordered phase, as
entropy can be transferred to the en-
vironment. Self-organization is re-
lated to the concepts of broken
symmetry, complexity, nonlinearity,
and *nonequilibrium statistical me-
chanics. Many systems that undergo
transitions to self-organization can
also undergo transitions to *chaos. In
chemistry, self-assemblyis one of the
features of *supramolecular chem-
istry.Seliwanoff’s test A biochemical
test to identify the presence of ke-
tonic sugars, such as fructose, in so-
lution. It was devised by the Russian
chemist F. F. Seliwanoff. A few drops
of the reagent, consisting of resorci-
nol crystals dissolved in equal
amounts of water and hydrochloric
acid, are heated with the test solu-
tion and the formation of a red pre-
cipitate indicates a positive result.semicarbazonesOrganic com-
pounds containing the unsaturated
group =C:N.NH.CO.NH 2. They are
formed when aldehydes or ke-
tones react with a semicarbazide
(H 2 N.NH.CO.NH 2 ). Semicarbazones
are crystalline compounds with rela-
tively high melting points. They are
used to identify aldehydes and ke-
tones in quantitative analysis: the
semicarbazone derivative is made
and identiÜed by its melting point.
Semicarbazones are also used in sep-477 semicarbazones
s