gauge that reads zero at atmospheric
pressure.
pressure gauge Any device used
to measure *pressure. Three basic
types are in use: the liquid-column
gauge (e.g. the mercury barometer
and the manometer), the expanding-
element gauge (e.g. the Bourdon
gauge and the aneroid barometer),
and the electrical transducer. In the
last category the strain gauge is an
example. Capacitor pressure gauges
also come into this category. In these
devices, the pressure to be measured
displaces one plate of a capacitor and
thus alters its capacitance.
presumptive test A simple test
for a given substance using a reagent
that changes colour when mixed
with the substance under investiga-
tion. Presumptive tests are not deÜni-
tive and further conÜrmatory tests
are always required. They are used
extensively in forensic science. Ex-
amples are the Duquenois–Levine
test for marijuana and Scott’s test for
cocaine. In general analytical chem-
istry, presumptive tests are often
called spot tests.
Priestley, Joseph (1733–1804)
British chemist, who in 1755 became
a Presbyterian minister. In Leeds, in
1767, he experimented with carbon
dioxide (‘Üxed air’) from a nearby
brewery; with it he invented soda
water. He moved to a ministry in
Birmingham in 1780, and in 1791 his
revolutionary views caused a mob to
burn his house, as a result of which
he emigrated to the USA in 1794. In
the early 1770s he experimented
with combustion and produced the
gases hydrogen chloride, sulphur
dioxide, and dinitrogen oxide (ni-
trous oxide). In 1774 he isolated oxy-
gen (see also lavoisier, antoine).
primary alcoholSee alcohols.primary amineSee amines.primary cellA *voltaic cell in
which the chemical reaction produc-
ing the e.m.f. is not satisfactorily re-
versible and the cell cannot therefore
be recharged by the application of a
charging current. See daniell cell;
leclanché cell; weston cell; mer-
cury cell. Compare secondary cell.
principal quantum numberSee
atom.prismaneA saturated hydrocarbon,
C 6 H 6 , in which the six carbon atoms
are arranged at the corners of a trian-
gular prism. The structure was sug-
gested in 1869 by Albert Ladenburg
as a possible structure for benzene
(since referred to as Ladenburg ben-
zene). The actual compound was syn-
thesized by T.J. Katz and N. Acton in- Hexamethylprismane, in which
the carbon atoms are linked to
methyl groups rather than hydrogen
atoms, was synthesized in 1966.
pressure gauge 438p
C
HC
HCHCHCHCHPrismaneprochiral Denoting an organic mol-
ecule lacking a chiral centre but con-
taining one or more carbon atoms
attached to two identical ligands and
two other different ligands (Caabc).
The carbon atom is said to be a
prochiral centre. The name prochiral
is used because if a is replaced by a
ligand d, different from either a, b, or
c, there is a chiral centre in the mol-
ecule Cabcd.
producer gas (air gas)A mixture
of carbon monoxide and nitrogen
made by passing air over very hot
carbon. Usually some steam is added