06413_07.pdf

GABA See gamma-aminobutyric acid.

gabbro A coarse-grained basic intrusive

igneous rock with a similar composition

to * basalt, i.e. mainly plagioclase

feldspar, and pyroxine, with some

olivine. Accessory minerals include apa-

tite, hornblende, ilmenite, and mag-

netite.

Gabor, Dennis (1900–79) Hungarian-

born British physicist, who worked as a

research engineer from 1927 until 1933,

when he joined the British Thomson-

Houston company. In 1948 he joined the

staff of Imperial College, London. In that

same year, while working on electron

microscopes, he invented * holography,

for which he was awarded the 1971

Nobel Prize.

Gabriel reaction A method of making

a primary * amine (free from any sec-

ondary or tertiary amine impurities) from

a haloalkane (alkyl halide) using potas-

sium phthalimide. It is named after Sieg-

mund Gabriel (1851–1924).

gadolinium Symbol Gd. A soft silvery

metallic element belonging to the * lan-

thanoids; a.n. 64; r.a.m. 157.25; r.d. 7.901

(20 ° C); m.p. 1313 ° C; b.p. 3266 ° C. It occurs

in gadolinite, xenotime, monazite, and

residues from uranium ores. There are

seven stable natural isotopes and eleven

arti

biotic (living) and abiotic (nonliving)

components, functions as a single self-

regulating system. Named after the Greek

earth goddess, it proposes that the re-

sponses of living organisms to environ-

mental conditions ultimately bring about

changes that make the earth better

adapted to support life; the system would

rid itself of any species that adversely af-

fects the environment. The theory has

found favour with many conservationists.

gain See amplifier.

galactic centre The region at the cen-

tre of a galaxy. In the Milky Way (our

Galaxy) it corresponds to a radio source

in the direction of Sagittarius. This

source, called Sagittarius A, may corre-

spond to a massive * black hole.

galactic merger The joining of two

galaxies that approach within each

other’s gravitational Ü elds. They spiral to-

gether, forming one galaxy and produc-

ing a starburst, in which many new stars

are created by the collapse of interstellar

clouds. Streams of stars, some large

enough to be dwarf galaxies, form tails

behind the new galaxy.

galactic nucleus A bulge of older stars

(population II, see population type) that

surrounds the centre of a galaxy. The spi-

ral arms of a spiral galaxy (such as the

Milky Way) originate at the galactic nu-

cleus.

galactose A simple sugar, C 6 H 12 O 6 ,

stereoisomeric with glucose, that occurs

naturally as one of the products of the en-

zymic digestion of milk sugar (lactose)

and as a constituent of gum arabic.

galactosidase See lactase.

galaxy A vast collection of stars, dust,

and gas held together by the gravitational

attraction between its components.

Galaxies are usually classi

ed as elliptical,

spiral, or irregular in shape. Elliptical

galaxies appear like ellipsoidal clouds of

cial isotopes are known. Two of the

natural isotopes, gadolinium–155 and

gadolinium–157, are the best neutron ab-

sorbers of all the elements. The metal has

found limited applications in nuclear

technology and in ferromagnetic alloys

(with cobalt, copper, iron, and cerium).

Gadolinium compounds are used in elec-

tronic components. The element was dis-

covered by Jean de Marignac (1817–94) in

1880.

Gaia hypothesis The theory, based on

an idea put forward by the British scien-

tist James Ephraim Lovelock (1919– ),

that the whole earth, including both its

343

gall

at disc-shaped

collections of stars with prominent spiral

arms. Irregular galaxies have no apparent

structure or shape.

The sun belongs to a spiral galaxy

known as the Galaxy (with a capital G) or

the Milky Way System . There are some

10 11 stars in the system, which is about

30 000 parsecs across with a maximum

thickness at the centre of about 4000 par-

secs. The sun is about 10 000 parsecs from

the centre of the Galaxy.

The galaxies are separated from each

other by enormous distances, the nearest

large galaxy to our own (the Andromeda

galaxy) being about 6.7

many, the USA (especially in Missouri,

Kansas, and Oklahoma), and the UK.

Galilean satellites The four largest

satellites (moons) of Jupiter, so-called be-

cause they were discovered and described

by Galileo in 1610. They are, in order of

increasing distance from the planet, Io,

Europa, Ganymede and Callisto.

Galilean telescope See telescope.

Galilean transformations A set of

equations for transforming the position

and motion parameters from a frame of

reference with origin at O and coordinates

( x , y , z ) to a frame with origin at O

′

and co-

ordinates at ( x ′ , y ′ , z ′ ). They are:

10 5 parsecs

′

= x – vt

away.

galaxy cluster A group of * galaxies

containing many hundreds of members

extending over a radius of up to a few

megaparsecs (there also exist small groups

of galaxies, such as the * Local Group,

with a few tens of members). The richest

and most regular clusters, such as the

Coma cluster , with thousands of mem-

bers, are gravitationally bound systems; it

is not certain whether other less regular

and less concentrated clusters are also

bound. As well as galaxies, the clusters

contain hot intracluster gas , at tempera-

tures between 10 7 and 10 8 K; this can be

detected by its X-ray emission. On a scale

larger than clusters there are also super-

clusters , with extents of the order of a

hundred megaparsecs, containing about

a hundred galaxies. It is not known

whether superclusters are gravitationally

bound. See also missing mass.

Galen ( c. 130– c. 200) Greek physician,

who studied in Pergamum, Corinth, and

Alexandria. He practised in Pergamum be-

fore moving to Rome. Galen’s writings

from this time became the basis of medi-

cal teaching and practice for 1500 years.

galena A mineral form of lead(II) sul-

phide, PbS, crystallizing in the cubic sys-

tem; the chief ore of lead. It usually

occurs as grey metallic cubes, frequently

in association with silver, arsenic, copper,

zinc, and antimony. Important deposits

occur in Australia (at Broken Hill), Ger-

= y

z ′ = z

′

= t

The equations conform to Newtonian me-

chanics. Compare lorentz transforma-

tions.

Galileo Galilei (1564–1642) Italian

astronomer and physicist. In 1583 he

noticed that the time of swing of a * pen-

dulum is independent of its amplitude,

and three years later invented a hydro-

static balance for measuring * relative den-

sities. He became a professor in Padua in

1592 and it was there (in 1610) that he

made his Ü rst astronomical telescope.

With it he discovered four satellites of

Jupiter, mountains on the moon, and

sunspots. Returning to Pisa, his birth-

place, he studied motion, demonstrating

that the speed of a falling body is inde-

pendent of its weight. He also gave open

support to the sun-centred theory of the

universe advocated by * Copernicus, a

stand that brought him into con Û ict with

the church. He was summoned to Rome,

forced to retract before the Inquisition,

and banished under house arrest.

gall An abnormal growth of a plant tis-

sue or organ elicited by a foreign organ-

ism. Galls most frequently occur as

swellings or pits in stems, roots, leaves,

and buds. Organisms responsible for their

formation include bacteria, viruses, fungi,

nematodes, mites, and insects. The gall

structure is typically very distinct from

′

stars, with very little internal structure

apart from (in some cases) a denser nu-

cleus. Spiral galaxies are

gall bladder

344

surrounding normal tissue and often is

characteristic of the eliciting organism.

The mechanisms underlying gall forma-

tion are known in only a few cases. The

bacterium * Agrobacterium tumefaciens ,

which is responsible for crown galls, in-

duces a genetic change in infected host

tissue by transfer of a plasmid bearing

tumour-forming genes.

gall bladder A small pouch attached to

the * bile duct, present in most verte-

brates. * Bile, produced in the * liver, is

stored in the gall bladder and released

when food (especially fatty substances) en-

ters the duodenum.

gallium Symbol Ga. A soft silvery metal-

lic element belonging to group 13 (for-

merly IIIB) of the periodic table; a.n. 31;

r.a.m. 69.72; r.d. 5.90 (20

uids formed a battery). Galvani invented

* galvanized iron and the * galvanometer.

galvanic cell See voltaic cell.

galvanized iron Iron or steel that has

been coated with a layer of zinc to protect

it from corrosion. Corrugated mild-steel

sheets for roo Ü ng and mild-steel sheets

for dustbins, etc., are usually galvanized

by dipping them in molten zinc. The for-

mation of a brittle zinc–iron alloy is pre-

vented by the addition of small quantities

of aluminium or magnesium. Wire is

often galvanized by a cold electrolytic

process as no alloy forms in this process.

Galvanizing is an effective method of pro-

tecting steel because even if the surface is

scratched, the zinc still protects the un-

derlying metal. See sacrificial protec-

tion.

galvanometer An instrument for de-

tecting and measuring small electric cur-

rents. In the moving-coil instrument a

pivoted coil of Ü ne insulated copper wire

surrounds a

b.p. 2403 ° C. It occurs in zinc blende, baux-

ite, and kaolin, from which it can be ex-

tracted by fractional electrolysis. It also

occurs in gallite, CuGaS 2 , to an extent of

1%; although bauxite only contains 0.01%

this is the only commercial source. The

two stable isotopes are gallium–69 and

gallium–71; there are eight radioactive

isotopes, all with short half-lives. The

metal has only a few minor uses (e.g. as

an activator in luminous paints), but gal-

lium arsenide is extensively used as a

semiconductor in many applications. Gal-

lium corrodes most other metals because

it rapidly diffuses into their lattices. Most

gallium(I) and some gallium(II) com-

pounds are unstable. The element was

C); m.p. 29.78

xed soft-iron core between

the poles of a permanent magnet. The

interaction between the

eld of the per-

manent magnet and the sides of the coil,

produced when a current

ows through

it, causes a torque on the coil. The moving

coil carries either a pointer or a mirror

that de Û ects a light beam when it moves;

the extent of the de

ection is a measure

of the strength of the current. The gal-

vanometer can be converted into an * am-

meter or a * voltmeter. Digital electronic

instruments are increasingly replacing the

moving-coil type. See also ballistic gal-

vanometer.

gametangium An organ that produces

gametes. The term is usually restricted to

the sex organs of algae, fungi, mosses,

and ferns. See antheridium; archego-

nium; oogonium.

gamete A reproductive cell that fuses

with another gamete to form a zygote.

Examples of gametes are ova and sper-

matozoa. Gametes are * haploid, i.e. they

contain half the normal (diploid) number

of chromosomes; thus when two fuse, the

diploid number is restored ( see fertiliza-

ed by Paul Lecoq de Bois-

baudran (1838–1912) in 1875.

gallon 1. (Imperial gallon) The volume

occupied by exactly ten pounds of dis-

tilled water of density 0.998 859 gram per

millilitre in air of density 0.001 217 gram

per millilitre. 1 gallon = 4.546 09 litres

(cubic decimetres). 2. A unit of volume in

the US Customary system equal to

0.832 68 Imperial gallon, i.e. 3.785 44

litres.

GALP See glyceraldehyde 3-phosphate.

Galvani, Luigi (1737–98) Italian physiol-

ogist. In the late 1770s he observed that

the muscles of a dead frog twitched when

touched by two different metals. He con-

cluded that the muscle was producing

electricity, later disproved by * Volta (who

showed that the two metals and body

rst identi

345

gas

tion). Gametes are formed by * meiosis.

See also sexual reproduction.

gametogenesis The processes involved

in the formation of gametes. Gametes are

normally formed by * meiosis but some-

times by * mitosis (as in the gametophyte

generation of the ferns). In mammals ga-

metogenesis in the female is known as

* oogenesis and occurs in the ovaries; in

the male it is known as * spermatogenesis

and occurs in the testes.

gametophyte The generation in the

life cycle of a plant that bears the gamete-

producing sex organs. The gametophyte is

* haploid. It is the dominant phase in the

life cycle of mosses and liverworts, the

* sporophyte generation depending on it

either partially or completely. In club-

mosses, horsetails, and ferns it is the

* prothallus. In seed plants it is very much

reduced. For example, in angiosperms the

pollen grain is the male gametophyte and

the embryo sac is the female gameto-

phyte. See also alternation of genera-

tions.

gamma-aminobutyric acid (GABA)

An inhibitory * neurotransmitter in the

central nervous system (principally the

brain) that is capable of increasing the

permeability of * postsynaptic mem-

branes. GABA is synthesized by * decar-

boxylation of the amino acid glutamate.

gamma globulin See globulin.

gamma-iron See iron.

gamma radiation Electromagnetic ra-

diation emitted by excited atomic nuclei

during the process of passing to a lower

excitation state. Gamma radiation ranges

in energy from about 10 –15 to 10 –10 joule

(10 keV to 10 MeV) corresponding to a

wavelength range of about 10 –10 to 10 –14

metre. A common source of gamma radia-

tion is cobalt–60, the decay process of

which is:

27 Co → 28 Ni → 28 Ni

The de-excitation of nickel–60 is accompa-

nied by the emission of gamma-ray pho-

tons having energies 1.17 MeV and 1.33

MeV.

gamma-ray astronomy * Astronomy

involving gamma ray photons (with ener-

gies in excess of 100 MeV). The cosmic ra-

diation with the highest energy can be

detected by electron–photon cascades,

which take place in the atmosphere.

Gamma rays having lower energies can

only be detected above the atmosphere.

Many high-energy processes in * astro-

physics are responsible for the production

of gamma rays; one example is the decay

of neutral * pions.

An interesting phenomenon is the

gamma-ray burst . These events last for a

few seconds, during which they are the

strongest source of gamma rays in the

sky. It is thought that they may be the re-

sult of the formation of a * black hole, ei-

ther when a large star collapses or when

two neutron stars collide.

ganglion A mass of nervous tissue con-

taining many * cell bodies and * synapses,

usually enclosed in a connective-tissue

sheath. In vertebrates most ganglia occur

outside the central nervous system; excep-

tions are the * basal ganglia in the brain.

In invertebrates ganglia occur along the

nerve cords and the most anterior pair

( cerebral ganglia ) are analogous to the ver-

tebrate brain; invertebrate ganglia consti-

tute a part of the central nervous system.

gangue Rock and other waste material

present in an ore.

ganoid scale See scales.

garnet Any of a group of silicate miner-

als that conform to the general formula

A 3 B 2 (SiO 4 ) 3 . The elements representing A

may include magnesium, calcium, man-

ganese, and iron(II); those representing B

may include aluminium, iron(III), chro-

mium, or titanium. Six varieties of garnet

are generally recognized:

pyrope, Mg 3 Al 2 Si 3 O 12 ;

almandine, Fe 3 2+ Al 2 Si 3 O 12 ;

spessartite, Mn 3 Al 2 Si 3 O 12 ;

grossularite, Ca 3 Al 2 Si 3 O 12 ;

andradite, Ca 3 (Fe 3+ ,Ti) 2 Si 3 O 12 ;

uvarovite, Ca 3 Cr 2 Si 3 O 12 .

Varieties of garnet are used as gemstones

and abrasives.

gas A state of matter in which the mat-

ter concerned occupies the whole of its

container irrespective of its quantity. In

an * ideal gas, which obeys the * gas laws

exactly, the molecules themselves would

have a negligible volume and negligible

gas chromatography

346

nite vol-

ume, there are small forces between

molecules, and in polyatomic gases colli-

sions are to a certain extent inelastic ( see

equation of state).

gas chromatography A technique for

separating or analysing mixtures of gases

by * chromatography. The apparatus con-

sists of a very long tube containing the

stationary phase. This may be a solid,

such as kieselguhr ( gas–solid chromatog-

raphy , or GSC ), or a nonvolatile liquid,

such as a hydrocarbon oil coated on a

solid support ( gas–liquid chromatography ,

or GLC ). The sample is often a volatile liq-

uid mixture, which is vaporized and

swept through the column by a carrier

gas (e.g. hydrogen). The components of

the mixture pass through the column at

different rates because they adsorb to dif-

ferent extents on the stationary phase.

They are detected as they leave, either by

measuring the thermal conductivity of

the gas or by a

is provided by the alveoli of the lungs and

the leaves of plants.

gas equation See gas laws.

gas laws Laws relating the temperature,

pressure, and volume of an * ideal gas.

* Boyle’s law states that the pressure ( p ) of

a specimen is inversely proportional to

the volume ( V ) at constant temperature

( pV = constant). The modern equivalent of

* Charles’ law states that the volume is di-

rectly proportional to the thermodynamic

temperature ( T ) at constant pressure ( V / T =

constant); originally this law stated the

constant expansivity of a gas kept at con-

stant pressure. The pressure law states

that the pressure is directly proportional

to the thermodynamic temperature for a

specimen kept at constant volume. The

three laws can be combined in the univer-

sal gas equation , pV = nRT , where n is the

amount of gas in the specimen and R is

the * gas constant. The gas laws were Ü rst

established experimentally for real gases,

although they are obeyed by real gases to

only a limited extent; they are obeyed

best at high temperatures and low pres-

sures. See also equation of state.

gasohol A mixture of petrol (gasoline)

and alcohol (i.e. typically ethanol at 10%,

or methanol at 3%), used as an alternative

fuel for cars and other vehicles in many

countries. The ethanol is obtained as a

* biofuel by fermentation of agricultural

crops or crop residues, for example sugar

cane waste. Many cars can also use a mix-

ture of 85% ethanol and 15% petrol, called

E85. Ethanol-based gasohol has a higher

octane rating and burns more completely

than conventional petrol, thus lowering

some emissions. However, the ethanol

can damage certain engine components,

such as rubber seals. Methanol-based gaso-

hol is more toxic and corrosive, and its

emissions include formaldehyde, a known

carcinogen.

gas oil A high-density petroleum frac-

tion (between kerosene and lubricating

oil), whose molecules have up to 25 car-

bon atoms. It is used as a domestic and in-

dustrial heating fuel.

gasoline See petroleum.

gas thermometer A device for measur-

ing temperature in which the working

ame detector.

Gas chromatography is usually used for

analysis; components can be identi

ed by

the time they take to pass through the

column. It is sometimes also used for sep-

arating mixtures.

Gas chromatography is often used to

separate a mixture into its components,

which are then directly injected into a

mass spectrometer. This technique is

known as gas chromatography–mass spec-

troscopy or GCMS .

gas constant (universal molar gas con-

stant) Symbol R . The constant that ap-

pears in the universal gas equation ( see

gas laws). It has the value 8.314 510(70)

JK –1 mol –1 .

gas-cooled reactor See nuclear reac-

tor.

gaseous exchange The transfer of

gases between an organism and the exter-

nal environment in either direction. It oc-

curs by diffusion across a * concentration

gradient and includes the exchange of

oxygen and carbon dioxide in respiration

and photosynthesis. Successful gaseous

exchange requires a large surface area, as

forces between them, and collisions be-

tween molecules would be perfectly elas-

tic. In practice, however, the behaviour of

real gases deviates from the gas laws be-

cause their molecules occupy a

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