tar. It is used in making other or-
ganic chemicals.
pyridoxineSee vitamin b complex.
pyrimidine An organic nitrog-
enous base (see formula), sparingly
soluble in water, that gives rise to a
group of biologically important deriv-
atives, notably uracil, thymine,
and cytosine, which occur in nu-
cleotides and nucleic acids (DNA and
RNA).
445 pyrometry
p
OHOHOHPyrogallolNN
Pyrimidine
pyrolusite See manganese(iv)
oxide.pyrolysis Chemical decomposition
occurring as a result of high tempera-
ture.pyrometric cones See seger
cones.pyrometry The measurement of
high temperatures from the amount
of radiation emitted, using a pyrom-
eter. Modern narrow-band or spectral
pyrometers use infrared-sensitive
photoelectric cells behindÜlters that
exclude visible light. In the optical
pyrometer (or disappearingÜlament
pyrometer) the image of the incan-
descent source is focused in the
plane of a tungstenÜlament that is
heated electrically. A variable resistor
is used to adjust the current through
theÜlament until it blends into the
image of the source, when viewed
through a redÜlter and an eyepiece.
The temperature is then read from a
calibrated ammeter or a calibrated
dial on the variable resistor. In the
total-radiation pyrometer radiation
emitted by the source is focused by aproduces a polarization of some 10–5
Cm–2.
pyrogallol 1,2,3-trihydroxyben-
zene, C 6 H 3 (OH) 3 , a white crystalline
solid, m.p. 132°C. Alkaline solutions
turn dark brown on exposure to air
through reaction with oxygen. It is a
powerful reducing agent, employed
in photographic developers. It is also
used in volumetric gas analysis as an
absorber of oxygen.pyrite(iron pyrites) A mineral form
of iron(II) sulphide, FeS 2. SuperÜcially
it resembles gold in appearance,
hence it is also known as fool’s gold,
but it is harder and more brittle than
gold (which may be cut with a knife).
Pyrite crystallizes in the cubic sys-
tem, is brass yellow in colour, has a
metallic lustre, and a hardness of
6–6.5 on the Mohs’ scale. It is the
most common and widespread of the
sulphide minerals and is used as a
source of sulphur for the production
of sulphuric acid. Sources include the
Rio Tinto mines in Spain.
pyro- PreÜx denoting an oxo acid
that could be obtained from a lower
acid by dehydration of two mol-
ecules. For example, pyrosulphuric
acid is H 2 S 2 O 7 (i.e. 2H 2 SO 4 minus
H 2 O).
pyroboric acid See boric acid.
pyroelectricity The property of
certain crystals, such as tourmaline,
of acquiring opposite electrical
charges on opposite faces when
heated. In tourmaline a rise in tem-
perature of 1 K at room temperature