GSC(gas–solid chromatography) See
gas chromatography.
G-seriesSee nerve agents.
guanidine A crystalline basic com-
pound HN:C(NH 2 ) 2 , related to urea.
guanineA purine derivative. It is
one of the major component bases of
nucleotides and the nucleic acids
DNA and RNA.
gumAny of a variety of substances
obtained from plants. Typically they
are insoluble in organic solvents but
form gelatinous or sticky solutions
with water. Gum resins are mixtures
of gums and natural resins. Gums are
produced by the young xylem vessels
of some plants (mainly trees) in re-
sponse to wounding or pruning. The
exudate hardens when it reaches the
plant surface and thus provides a
temporary protective seal while the
cells below divide to form a perma-
nent repair. Excessive gum formation
is a symptom of some plant diseases.
guncottonSee cellulose nitrate.
gun metalA type of bronze usually
containing 88–90% copper, 8–10% tin,
and 2–4% zinc. Formerly used for can-
nons, it is still used for bearings and
other parts that require high resis-
tance to wear and corrosion.
gunpowderAn explosive consist-
ing of a mixture of potassium nitrate,
sulphur, and charcoal.
gypsumA monoclinic mineral
form of hydrated *calcium sulphate,
CaSO 4 .2H 2 O. It occurs inÜve vari-
eties: rock gypsum, which is often
red stained and granular; gypsite, an
impure earthy form occurring as a
surface deposit; alabaster, a pure
Üne-grained translucent form; satin
spar, which isÜbrous and silky; and
selenite, which occurs as transparent
crystals in muds and clays. It is used
in the building industry and in the
manufacture of cement, rubber,
paper, and plaster of Paris.
gyromagnetic ratio Symbol γ.
The ratio of the angular momentum
of an atomic system to its magnetic
moment. The inverse of the gyro-
magnetic ratio is called the magneto-
mechanical ratio.257 gyromagnetic ratio
g