action is easier to perform than the
Sandmeyer reaction and takes place
at lower temperature, but generally
gives lower yields. It was discovered
in 1890 by the German chemist Lud-
wig Gattermann (1860–1920). See also
gattermann–koch reaction.gaucheSee conformation; tor-
sion angle.Gay-Lussac, Joseph(1778–1850)
French chemist and physicist whose
discovery of the laws of chemical
combination in gases helped to estab-
lish the atomic theory. It also led to
*Avogadro’s law. See also charles’
law.Gay Lussac’s law 1.When gases
combine chemically the volumes of
the reactants and the volume of the
product, if it is gaseous, bear simple
relationships to each other when
measured under the same conditions
of temperature and pressure. The law
wasÜrst stated in 1808 by J. L. Gay-
Lussac and led to *Avogadro’s law.
2.See charles’ law.
gaylussiteA mineral consisting
of a hydrated mixed carbonate of
sodium and calcium, Na 2 CO 3.
CaCO 3 .5H 2 O.GC-IRSee gas chromatography in-
frared.
GCMSSee gas chromatography.Geiger counter (Geiger–Müller
counter)A device used to detect and
measure ionizing radiation. It con-
sists of a tube containing a low-
pressure gas (usually argon or neon
with methane) and a cylindrical hol-
low cathode through the centre of
which runs aÜne-wire anode. A po-
tential difference of about 1000 volts
is maintained between the elec-
trodes. An ionizing particle or pho-
ton passing through a window into
the tube will cause an ion to be pro-
duced and the high p.d. will acceler-ate it towards its appropriate elec-
trode, causing an avalanche of fur-
ther ionizations by collision. The
consequent current pulses can be
counted in electronic circuits or sim-
ply ampliÜed to work a small loud-
speaker in the instrument. It was
Ürst devised in 1908 by the German
physicist Hans Geiger (1882–1945).
Geiger and W. Müller produced an
improved design in 1928.gelA lyophilic *colloid that has co-
agulated to a rigid or jelly-like solid.
In a gel, the disperse medium has
formed a loosely-held network of
linked molecules through the disper-
sion medium. Examples of gels are
silica gel and gelatin.gelatin(e)A colourless or pale yel-
low water-soluble protein obtained
by boiling collagen with water and
evaporating the solution. It swells
when water is added and dissolves in
hot water to form a solution that sets
to a gel on cooling. It is used in pho-
tographic emulsions and adhesives,
and in jellies and other foodstuffs.gel electrophoresis See elec-
trophoresis.
gelÜltrationA type of column
*chromatography in which a mix-
ture of liquids is passed down a col-
umn containing a gel. Small
molecules in the mixture can enter
pores in the gel and move slowly
down the column; large molecules,
which cannot enter the pores, move
more quickly. Thus, mixtures of mol-
ecules can be separated on the basis
of their size. The technique is used
particularly for separating proteins
but it can also be applied to other
polymers and to cell nuclei, viruses,
etc.geligniteA high explosive made
from nitroglycerin, cellulose nitrate,
sodium nitrate, and wood pulp.gauche 244g