is the characteristic range of radia-
tions it emits when it is heated, bom-
barded by electron or ions, or
absorbs photons. The absorption
spectrumof a substance is produced
by examining, through the substance
and through a spectroscope, a contin-
uous spectrum of radiation. The ener-
gies removed from the continuous
spectrum by the absorbing medium
show up as black lines or bands.
With a substance capable of emitting
a spectrum, these are in exactly the
same positions in the spectrum as
some of the lines and bands in the
emission spectrum.
Emission and absorption spectra
may show a continuous spectrum, a
line spectrum, or a band spectrum. A
continuous spectrum contains an un-
broken sequence of frequencies over
a relatively wide range; it is produced
by incandescent solids, liquids, and
compressed gases. Line spectra are
discontinuous lines produced by ex-
cited atoms and ions as they fall back
to a lower energy level. Band spectra
(closely grouped bands of lines) are
characteristic of molecular gases or
chemical compounds. See also spec-
troscopy.speculumAn alloy of copper and
tin formerly used in reÛecting tele-
scopes to make the main mirror as it
could be cast, ground, and polished
to make a highly reÛective surface. It
has now been largely replaced by sil-
vered glass for this purpose.sphalerite(zinc blende)A mineral
form of zinc sulphide, ZnS, crystalliz-
ing in the cubic system; the principal
ore of zinc. It is usually yellow-brown
to brownish-black in colour and
occurs, often with galena, in meta-
somatic deposits and also in hydro-
thermal veins and replacement
deposits. Sphalerite is mined on
every continent, the chief sources in-cluding the USA, Canada, Mexico,
Russia, Australia, Peru, and Poland.sphalerite structure (zinc-blende
structure) A type of ionic crystal
structure in which the anions have
an expanded face-centred cubic
arrangement with the cations occu-
pying one type of tetrahedral hole.
The coordination number of each
type of ion is 4. Examples of com-
pounds with the sphalerite structure
are ZnS, CuCl, CdS, and InAs.
A- An interactive version of the structure
spherical topSee moment of iner-
tia.sphingolipidSee phospholipid.
spiegel(spiegeleisen)A form of
*pig iron containing 15–30% of man-
ganese and 4–5% of carbon. It is
added to steel in a Bessemer con-
verter as a deoxidizing agent and to
raise the manganese content of steel.
spin(intrinsic angular momentum)
Symbol s. The part of the total angu-
lar momentum of a particle, atom,
nucleus, etc., that can continue to
exist even when the particle is appar-
ently at rest, i.e. when its transla-
tional motion is zero and therefore
its orbital angular momentum is
zero. A molecule, atom, or nucleus in
a speciÜed energy level, or a particu-
lar elementary particle, has a particu-
lar spin, just as it has a particular
charge or mass. According to *quan-
tum theory, this is quantized and is
restricted to multiples of h/2π, where
h is the *Planck constant. Spin is
characterized by a quantum number
s. For example, for an electron s =
±½, implying a spin of + h/4πwhen it
is spinning in one direction and –h/4π
when it is spinning in the other. Be-
cause of their spin, particles also
have their own intrinsic magnetic
moments and in a magneticÜeld thespeculum 498s