named after the US chemist Frank
Gooch (1852–1929).
Gouy balanceA method of meas-
uring magnetic susceptibility. The
sample is suspended from a balance,
with the bottom part of the sample
between the poles of an electromag-
net. When the magneticÜeld is
switched on, the sample experiences
aÜeld gradient which causes an ap-
parent change in weight. In particu-
lar, paramagnetic substances show
an increase in weight, which, after
correction for a smaller diamagnetic
contribution, can be used to calculate
the paramagnetic part of the suscep-
tibility. This can be used to calculate
the number of unimpaired electrons
in the sample. Magnetic measure-
ments of this type are widely used to
investigate the electronic structures
of metal complexes. The Evans bal-
anceis a portable version of the
Gouy balance using permanent mag-
nets and giving a direct readout.
Other methods of measuring mag-
netic susceptibility include magnetic
resonance techniques and the use of
a SQID (superconducting quantum
interference device).
Gouy–Chapman model A model
of the electrical double layer in
which thermal motion causing disor-
dering of the layer is taken into ac-
count. This is very similar to the
Debye–Hückel theory of the ionic
atmosphere around an ion, except
that the concept of a central single
ion is replaced by that of an inÜnite
plane electrode. The Gouy–Chapman
model understates the structure in a
double layer, but can be improved by
the Stern model, in which the ions
closest to the electrode are ordered
and the ions are described by the
Gouy–Chapman model outside the
Ürst layer.
graft copolymerSee polymer.
Graham, Thomas(1805–69) Scot-
tish chemist, who became professor
of chemistry at Glasgow University in
1830, moving to University College,
London, in 1837. His 1829 paper on
gaseous diffusion introduced *Gra-
ham’s law. He went on to study diffu-
sion in liquids, leading in 1861 to the
deÜnition of *colloids.Graham’s law The rates at which
gases diffuse is inversely propor-
tional to the square roots of their
densities. This principle is made use
of in the diffusion method of separat-
ing isotopes. The law was formulated
in 1829 by Thomas *Graham.gramSymbol g. One thousandth of
a kilogram. The gram is the funda-
mental unit of mass in *c.g.s. units
and was formerly used in such units
as the gram-atom, gram-molecule,
and gram-equivalent, which have
now been replaced by the *mole.grape sugarSee glucose.
graphiteSee carbon.graphitic compoundsSubstances
in which atoms or molecules are
trapped between the layers in
graphite. See lamellar solids.gravimetric analysisA type of
quantitative analysis that depends on
weighing. For instance, the amount
of silver in a solution of silver salts
could be measured by adding excess
hydrochloric acid to precipitate silver
chloride,Ültering the precipitate,
washing, drying, and weighing.
graySymbol Gy. The derived SI unit
of absorbed dose of ionizing radia-
tion (see radiation units). It is
named after the British radiobiologist
L. H. Gray (1905–65).greenhouse effect An effect oc-
curring in the atmosphere because of
the presence of certain gases (green-
house gases) that absorb infrared ra-251 greenhouse effect
g