06413_07 - G GABA See gamma-aminobutyric acid. gabbro A...

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G 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 Ü 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 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.
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06413_07 - G GABA See gamma-aminobutyric acid. gabbro A...

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