his constant companion. In the bhakti movement of Vaishnavism, Radha symbolizes the human soul and Krishna the divine. The allegorical love of Radha and Krishna has been celebrated in the poetry of many Indian languages, and Radha is often worshiped along with Krishna, especially in northern and eastern India.
radial engine Type of internal-combustion engine used mainly in small airplanes, in which the cylinders (ranging from five to as many as 28, depending on engine size) are mounted in a circle around the crankshaft, sometimes in banks of two or more. Once the dominant piston-engine type, radials are now in only limited production; most new requirements are met by remanufacturing existing stock.
radiant heating Heating system in which heat is transmitted by radia¬ tion from a heated surface. Radiant heating systems usually employ either electric-resistance wiring or hot-water heating pipes, which may embed¬ ded in the floor, ceiling, or walls. Panel heating is a form of radiant heat¬ ing characterized by very large surfaces (typically an entire ceiling or floor) containing electrical conductors, hot-water pipes, or hot-air ducts. With many such systems there is no visible heating equipment in the room.
radiation Process by which energy is emitted from a source and propa¬ gated through the surrounding medium, or the energy involved in this process. Radiation consists of a flow of atomic or subatomic particles or of waves. Familial' examples are light (a form of electromagnetic radia¬ tion) and sound (a form of acoustic radiation). Both electromagnetic and acoustic radiation can be described as waves with a range of frequencies and intensities. Electromagnetic radiation is also often treated as discrete packets of energy, called photons. All matter is constantly bombarded by radiation from cosmic and terrestrial sources, and radioactive elements emit several types of radiation (see radioactivity). See also Cherenkov radi¬ ation, Hawking radiation, infrared radiation, synchrotron radiation, thermal
RADIATION, ULTRAVIOLET RADIATION.
radiation injury Tissue damage caused by exposure to ionizing radia¬ tion. Structures with rapid cell turnover (e.g., skin, stomach or intestinal lining, and bone marrow) are most susceptible. High-dose irradiation of the last two causes radiation sickness. Nausea and vomiting subside in a few hours. They are followed in intestinal cases by abdominal pain, fever, and diarrhea leading to dehydration and a fatal shocklike state, and in bone-marrow cases (two to three weeks later) by fever, weakness, hair loss, infection, and hemorrhage. In severe cases, death occurs from infec¬ tion and uncontrollable bleeding. Lower radiation doses can cause can¬ cer (notably leukemia and breast cancer), sometimes years later. Radiation exposure in early pregnancy can produce abnormalities in the embryo, whose cells are multiplying rapidly.
radiation pressure Pressure on a surface resulting from electromag¬ netic radiation that impinges on it. The pressure is a result of the momen¬ tum carried by the radiation. When radiation is reflected rather than absorbed, the radiation pressure is doubled. Radiation pressure can some¬ times be great enough to produce a force that is useful.
radiation therapy or radiotherapy or therapeutic radiol¬ ogy Use of radiation sources to treat or relieve diseases, usually cancer (including leukemia). The ionizing radiation primarily used to destroy dis¬ eased cells works best on fast-growing cancers. However, radiation can also cause cancer (see radiation injury) and is no longer used for benign conditions. Other complications include nausea, hair loss, weight loss, and weakness. Radioactive substances may be implanted in tumours (see nuclear medicine). External radiation involves 10-20 sessions over several months, either after surgical removal of the growth or when surgery is impossible; it can deliver higher doses to deep tumours than implantation. Infrared radiation and ultraviolet radiation is applied with lamps to relieve INFLAMMATION.
radical Term used in chemistry with one predominant and two subsid¬ iary, looser meanings. It most often refers to a free radical. It can also mean an ion or a functional group.
radical In politics, one who desires extreme change of part or all of the social order. The term (which derives from the Latin word for “root,” and thus implies change beginning at a system’s roots) was given this sense by Charles James Fox in 1797 when he demanded “radical reform” con¬ sisting of universal manhood suffrage. In France before 1848, republicans and advocates of universal male suffrage were called radicals. The term was later applied to Marxists (see Marxism) who called for fundamental
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Radical Republican ► radish I 1583
social change to eradicate divisions among social classes. In popular usage, it is applied to political extremism, not necessarily violent, of both the left and the right.
Radical Republican Member of the Republican Party in the 1860s committed to the emancipation of slaves and the equal treatment and enfranchisment of blacks. Zealous antislavery advocates in the Congress pressed Pres. Abraham Lincoln to include emancipation as a war aim. They later opposed his policy of lenient Reconstruction of the South under presidential control and passed harsher measures in the Wade-Davis Bill. After Lincoln’s death the Radicals supported Pres. Andrew Johnson but soon demanded congressional control of Reconstruction. Johnson’s attempt to break the Radicals’ power led them to pass the Tenure of Office Act; his challenge of the act led to his impeachment. Radical Republican leaders included Henry Winter Davis (1817-65), Thaddeus Stevens, Charles Sumner, and Benjamin Butler. Their influence waned as white control over Southern governments gradually returned in the 1870s.
Radical-Socialist Party French political party. The oldest of France’s political parties, it was founded in 1901 but originated in the 1870s in the reformist wing of the French Republican Party, known as the Radicals, led by Georges Clemenceau. Traditionally a centrist party, it was most prominent in the Third Republic and Fourth Republic. In the 1920s and ’30s it joined coalition governments with the French Socialist Party. After 1945 it led other centrist groups to form politically important coalitions. By the late 20th century, however, it was only a minor party. In 1998 it renamed itself the Radical Party of the Left.
radio Electromagnetic radiation of lower frequency (hence longer wave¬ length) than visible light or infrared radiation, and consisting of the range of frequencies used for navigation signals, AM and FM broadcasting, tele¬ vision transmissions, cell-phone communications, and various forms of radar. For radio transmission, information is imparted to a carrier wave by varying (modulating) its amplitude, frequency, or duration. The tech¬ nology of radio arose from the work of Michael Faraday, James Clerk Max¬ well, Heinrich Hertz, Gugueimo Marconi, and others, and improvement followed the development of the vacuum tube, the electronic-tube oscilla¬ tor, the tuned circuit, and other components. Later innovations have included the replacement of tubes by transistors and of wires by printed circuits. See also radio and radar astronomy.
radio and radar astronomy Study of celestial bodies by measur¬ ing the energy they emit or reflect at radio wavelengths. It began in 1931 with Karl Jansky’s discovery of radio waves from an extraterrestrial source. After 1945, huge dish antennas, improved receivers and data- processing methods, and radio interferometers let astronomers study fainter sources and obtain greater detail. Radio waves penetrate much of the gas and dust in space, giving a much clearer picture of the centre and structure of the Milky Way Galaxy than optical observation can. This has allowed detailed studies of the interstellar medium in the Galaxy and the discovery of previously unknown cosmic objects (e.g., pulsars, quasars). In radar astronomy, radio signals are sent to near-Earth bodies or phe¬ nomena (e.g., meteor trails, the Moon, asteroids, nearby planets) and the reflections detected, providing precise measurement of the objects’ dis¬ tances and surface structure. Because radar waves can penetrate even dense clouds, they have provided astronomers’ only maps of the surface of Venus. Radio and radar studies of the Moon revealed its sandlike sur¬ face before landings were made. Radio observations have also contrib¬ uted greatly to knowledge about the Sun. See also radio telescope.