isnad \is-‘nad\ In Islam, a list of authorities who have transmitted accounts of the teachings or actions of Muhammad, one of the Companions of the Prophet, or of a later authority. Each of these accounts, known as Hadith, includes an isnad that gives the chain of authorities by which it has been handed down, using the form, “It has been related to me by A on the authority of B on the authority of C on the authority of D that Muhammad said....” See also 'ilm al-hadith.
ISO See International Organization for Standardization
isobaric spin See isospin
Isocrates \I-'sa-kr9-,tez\ (b. 436, Athens—d. 338 bc, Athens) Athenian author, rhetorician, and teacher. His school, unlike Plato’s more philo¬ sophical Academy, provided an education for the practical needs of soci¬ ety; it was given over almost entirely to rhetoric. He promoted Greek political unity and cultural superiority based on monarchy and advocated a unified Greek attack on Persia under Philip II of Macedonia to secure unity and peace in Greece. When Greece lost its independence after the Battle of Chaeronea, Isocrates, in despair, starved himself to death.
isolationism National policy of avoiding political or economic entanglements with other countries. Isolationism has been a recurrent theme in U.S. history. It was given expression in the Farewell Address of Pres. George Washington and in the early 19th-century Monroe Doctrine. The term is most often applied to the political atmosphere in the U.S. in the 1930s. The failure of Pres. Woodrow Wilson’s internationalism, lib¬ eral opposition to war as an instrument of policy, and the rigours of the Great Depression were among the reasons for Americans’ reluctance to concern themselves with the growth of fascism in Europe. The Johnson Act (1934) and the Neutrality acts (1935) effectively prevented economic or military aid to any country involved in the European disputes that were to escalate into World War II. U.S. isolationism encouraged the British in their policy of appeasement and contributed to French paralysis in the face of the growing threat posed by Nazi Germany. See also neutrality.
Isole Eolie See Eolie Islands
isoleucine \ 1 I-so- , lii- l sen\ One of the essential amino acids, present in most common proteins. It was first isolated in 1904 from fibrin, a protein involved in coagulation. It is used in medicine and biochemical research and as a nutritional supplement.
isomer Vl-so-morX One of two or more substances with identical molecu¬ lar formulas but different configurations, differing only in the arrangement of their component atoms. It usually refers to stereoisomers (rather than constitutional isomers or tautomers; see isomerism, tautomerism), of which there are two types. Optical isomers, or enantiomers (see optical activity), occur in mirror-image pairs. Geometric isomers are often the result of rigidity in the molecular structure; in organic compounds, this is usually due to a double bond (see bonding) or a ring structure. In the case of a double bond between two carbon atoms, if each has two other groups bonded to it and all are rigidly in the same plane, the corresponding groups can be on the same side (cis) of the C=C bond or across the C=C bond (trans) from each other. An analogous distinction can be made for ring structures that are all in a plane, between isomers whose substituent groups are on the same side and isomers whose substituent groups are on both sides of the plane. Diastereomers that are not enantiomers also fall into this category. Most cis-trans isomers are organic compounds.
isomerism U-'sa-mo-.ri-zonA Existence of sets of two or more sub¬ stances with identical molecular formulas (see chemical formula) but dif¬ ferent configurations and hence different properties. Jons Jacob Berzelius was the first to recognize and name it (1830). In constitutional (structural) isomerism, the molecular formula and molecular weight of the substances are the same, but their bonding differs. For example, C 2 H 6 0 is the molecu¬ lar formula for both ethanol (CH 3 CH 2 OH) and methyl ether (CH 3 OCH 3 ). Constitutional isomers that can be readily converted from one to another
© 2006 Encyclopaedia Britannica, Inc.
Isonzo ► Israel I 965
are called tautomers (see tautomerism). In stereoisomerism, substances with the same atoms are bonded in the same ways but differ in their three- dimensional configurations. See also isomer.
Isonzo \e-'zont-s6\, Battles of the (1915-17) Twelve battles along the Isonzo River on the eastern sector of the Italian front in World War I. The Isonzo River, running just inside Austria, is flanked by rugged peaks that the Austrians had fortified before Italy’s entry into the war in 1915. Luigi Cadoma (1850-1928) led all the attacks against Austria, but the Ital¬ ians could not penetrate the formidable natural barriers. Finally they struck with 51 divisions and dislodged the Austrians, but the Germans sent reinforcements and took the offensive, ending in the Battle of Caporetto.
isoprenoid Vj-so-'pre-.noidV or terpene Class of organic compounds made up of two or more structural units derived from isoprene. Isoprene is a five-carbon hydrocarbon with a branched-chain structure, two double bonds (see bonding), and the molecular formula C 5 H 8 . In isoprenoids, isoprene units (with one or neither of their double bonds) are linked into larger molecules having from two to thousands of five-carbon units that can take the form of both linear structures and rings. Many of these mol¬ ecules play a wide variety of roles in plant and animal physiological pro¬ cesses and as intermediates in the biological synthesis of other important molecules. They contribute greatly to the flavours and fragrances of essen¬ tial oils and other plant-derived substances. Geraniol (a contributor to rose perfumes; from geranium oil), menthol (from peppermint oil), citral (from lemongrass oil), limonene (from lemon and orange oils), pinene (from turpentine), and camphor each have two isoprene units. Examples with more units include phytol, a precursor of chlorophyll; squalene, the pre¬ cursor of cholesterol and other steroids; lycopene, the red pigment in tomatoes and an important phytochemical; and carotene, the pigment in carrots and a precursor of vitamin A. Natural rubber and the related gutta¬ percha are polyisoprenes comprising many thousands of isoprene units.
isospin Vl-sd-,spin\ or isobaric spin or isotopic spin Property characteristic of families of related subatomic particles differing mainly in the values of their electric charge. The families are known as isospin mul- tiplets. The components of atomic nuclei, the neutron and the proton, form an isospin doublet since they differ only in electric charge and subsidiary properties. They are considered different versions of the same object, called a nucleon. The isospin of a nucleon has a value of Vi.
isostasy \I-'sas-t3-se\ Theory describing the mass balance in the Earth’s crust, which treats all large portions of the crust as though they were floating on a denser underlying layer, about 70 mi (110 km) below the surface. In this theory, a mass above sea level is supported below sea level, so high mountains must be regions where the crust is very thick, with deep roots extending into the mantle. This is analogous to an iceberg floating on water, in which the greater part of the iceberg is under water.
isotope Vl-s3-,top\ One of two or more species of atoms of a chemical element having nuclei with the same number of protons but different num¬ bers of neutrons. They have the same atomic number and hence nearly identical chemical behaviour but different atomic MASSes. Most elements found in nature are mixtures of several isotopes; tin, for example, has 10 isotopes. In most cases, only stable isotopes of elements are found in nature. The radioactive forms break down spontaneously into different elements (see radioactivity). Isotopes of all elements heavier than bismuth are radioactive; some occur naturally because they have long half-lives.