NIST See Bureau of Standards
nitrate Any salt or ester of nitric acid (HN0 3 ). The salts are inorganic compounds with ionic bonds, containing the nitrate ion (N0 3 ) and any cation. Many, particularly ammonium nitrate, are used as agricultural fer¬ tilizers (see saltpeter). Their runoff in surface water and groundwater can cause serious illness in humans. The esters are organic compounds with covalent bonds, having the structure R—O—N0 2 , in which R represents an organic combining group such as methyl, ethyl, or phenyl.
Nippur \ni-'pur\ Ancient Mesopotamian city southeast of Babylon.
Located in what is now southeastern Iraq, it was originally on the Eu¬ phrates River, whose course later changed. By 2500 bc it was the cen¬ tre of worship of the Sumerian storm god Enlil (see Sumer). Parthian con¬ struction (see Parthia) later buried Enlil’s sanctuary, and the city fell into decay in the 3rd century ad. It was abandoned in the 12th or 13th century. Excavations have revealed temples, a ziggurat, and thousands of clay tablets that are a primary source of information on ancient Sumerian civilization. Also uncovered were an Akkadian tomb (see Akkad) and a large temple devoted to the Meso¬ potamian goddess of healing.
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Female figure, made of gypsum, with a gold mask that stood at a temple altar in Nippur, c. 2700 bc; in the Iraq Museum, Baghdad
Nirenberg Vnir-on-.borgV, Mar¬ shall Warren (b. April 10, 1927,
New York, N.Y., U.S.) U.S. bio¬ chemist. He received his Ph.D. from the University of Michigan. He dem¬ onstrated that each possible triplet (codon) of the four different kinds of nitrogen-containing bases found in DNA and (in some viruses) in RNA (with three exceptions) ultimately causes the incorporation of a specific amino acid into a cell protein. His research earned him a Nobel Prize in 1968, which he shared with Robert William Holley and Har Gobind Khorana, whose work, like Nirenberg’s, helped show how genetic instructions in the cell nucleus control the com¬ position of proteins.
nirvana \nir-'va-n3\ (Sanskrit: “Extinction”) In Indian religious thought, the transcendent state of freedom achieved by the extinction of desire and of individual consciousness. Nirvana is the supreme goal of the disciplines of meditation, particularly in Buddhism. Release from desire (and consequent suffering) and the continuous round of rebirths consti¬ tutes enlightenment, or the experience of nirvana. Theravada Buddhism conceives of nirvana as tranquillity and peace; Mahayana Buddhism equates it with sunyata (emptiness), dharma-kaya (the essence of the Buddha), and dharma-datu (ultimate reality).
Nis or Nish \'nesh\ City (pop., 2000 est.: 182,583), southeast of Bel¬ grade, Serbia, Serbia and Montenegro. The ancient Roman city was men¬ tioned by Ptolemy in the 2nd century ad. It was the birthplace of Constantine I (c. 280), who adorned it with many buildings. Bomb dam¬ age in World War II and postwar construction erased much of its Turko- Byzantine style. It was held at various periods by Bulgarians, Hungarians, and Turks. During the Turkish period it became an important station on the route from Istanbul to Hungary. It was ceded to Serbia by the Treaty of Berlin in 1878 and was its capital until 1901. It is a railroad junction and commercial centre.
Nishapur See Neyshabur
nitre See saltpetre
nitric Vnl-trik\ acid Inorganic compound, colourless, fuming, highly corrosive liquid, chemical formula HN0 3 . A common laboratory reagent, it is important in the manufacture of fertilizers and explosives (including nitroglycerin), as well as in organic syntheses, metallurgy, ore flotation, and reprocessing of spent nuclear fuel. A strong acid, it is toxic and can cause severe bums. It attacks most metals and is used for etching steel and photoengraving.
nitric oxide Colourless, toxic gas (NO), formed from nitrogen and oxygen by the action of electric sparks or high temperatures or, more con¬ veniently, by the action of dilute nitric acid on copper or mercury. First prepared c. 1620 by Jan B. Helmont, it was first studied in 1772 by Joseph Priestley, who called it “nitrous air.” An industrial procedure for the manu¬ facture of hydroxylamine is based on the reaction of nitric oxide with hydrogen in the presence of a catalyst. The formation of nitric oxide from nitric acid and mercury is applied in a volumetric method of analysis for nitric acid or its salts. The gas is synthesized via ENZYME-catalyzed reac¬ tions in humans and other animals, where it serves as a signaling mol¬ ecule. Among its numerous biological roles, it causes dilation of blood vessels and as such is an important regulator of blood pressure. Nitric oxide is one of the components of air pollution generated by internal-combustion ENGINES.
nitrifying bacteria Vnl-tro-.fi-iqN Small group of oxygen-requiring bacteria that use nitrogen as an energy source. These microorganisms are important in the nitrogen cycle as converters of soil ammonia to nitrates, compounds usable by plants. The nitrification process requires two dis¬ tinct groups: bacteria that convert ammonia to nitrites, and bacteria that convert nitrites to nitrates. In agriculture, irrigation with dilute solutions of ammonia results in an increase in soil nitrates through the action of nitrifying bacteria. See also denitrifying bacteria.
nitrite Vnl-.trfiA Any salt or ester of nitrous acid (HN0 2 ). The salts are inorganic compounds with ionic bonds, containing the nitrite ion (N0 2 ) and any cation. The esters are organic compounds with covalent bonds, having the structure R—O—N—O, in which R represents a carbon- containing combining group and the bonding is from carbon to oxygen. These covalent nitrites are constitutional isomers (see isomerism) of the nitro compounds, nitric acid derivatives (R—N0 2 ), in which the bonding is from carbon to nitrogen. Nitrites are used as food preservatives and color enhancers, though they are so toxic they have caused deaths and combine with amines to produce carcinogens. They are used in medicine to dilate blood vessels.
nitro compound Vnl-.trcA Any of a class of chemical compounds in which the nitro group (—N0 2 ) forms part of the molecular structure. The most common examples are organic compounds, isomers of nitrite esters in which a carbon atom is linked by a covalent bond to the nitro group’s nitrogen atom. Many nitro compounds are commercially used as explo¬ sives, solvents, or raw materials and chemical intermediates. They are generally made by a reaction between nitric acid and an organic compound.
nitrogen Gaseous chemical element, chemical symbol N, atomic num¬ ber 7. A colourless, odourless, tasteless gas, it makes up 78% of Earth’s atmosphere and is a constituent of all living matter. As the nearly unre-
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nitrogen cycle ► Nizhny Novgorod I 1367
active diatomic molecule N 2 , it is useful as an inert atmosphere or to dilute other gases. Nitrogen is commercially produced by distillation of liquefied air. Nitrogen fixation, achieved naturally by soil microbes and industrially by the Haber-Bosch process, converts it to water-soluble compounds (including ammonia and nitrates). Industrially, ammonia is the starting material for most other nitrogen compounds (especially nitrates and nitrites), whose main uses are in agricultural fertilizers and explosives. In compounds, nitrogen usually has valence 3 or 5. It forms several oxides including nitrous oxide (N 2 0; laughing gas), nitric oxide (NO), nitrogen dioxide (N0 2 ), and other forms (such as N 2 0 3 and N 2 0 5 ). Some of the nitrogen oxides, often referred to generically as NO A , are notorious as contributors to urban air pollution. Other compounds include the nitrides, exceptionally hard materials made from nitrogen and a metal; cyanides; azides, used in detonators and percussion caps; and thousands of organic compounds containing nitrogen in functional groups or in a linear or ring structure (see heterocyclic compound). See also nitrogen cycle.