desalination or desalting Removal of dissolved salts from seawa¬ ter and from the salty waters of inland seas, highly mineralized ground- waters, and municipal wastewaters. Desalination makes such otherwise unusable waters fit for human consumption, irrigation, industrial applica¬ tions, and other purposes. Distillation is the most widely used desalination process; freezing and thawing, electrodialysis, and reverse osmosis are
© 2006 Encyclopaedia Britannica, Inc.
536 I Descartes ► Desmond
also used. All are energy-intensive and therefore expensive. Currently, more than 2 billion gallons (8 million cu m) of fresh water are produced each day by several thousand desalination plants throughout the world, the largest plants being in the Arabian Peninsula.
Descartes \da-'kart\, Rene (b. March 31, 1596, La Haye, Touraine, France—d. Feb. 11, 1650, Stockholm, Swed.) French mathematician, sci¬ entist, and philosopher, considered the father of modern philosophy. Edu¬ cated at a Jesuit college, he joined the military in 1618 and traveled widely for the next 10 years. In 1628 he settled in Holland, where he would remain until 1649. Descartes’s ambition was to introduce into philosophy the rigour and clarity of mathematics. In his Meditations on First Philoso¬ phy (1641), he undertook the methodical doubt of all knowledge about which it is possible to be deceived, including knowledge based on author¬ ity, the senses, and reason, in order to arrive at something about which he can be absolutely certain; using this point as a foundation, he then sought to construct new and more secure justifications of his belief in the exist¬ ence and immortality of the soul, the existence of God, and the reality of an external world. This indubitable point is expressed in the dictum Cogito ergo sum (“I think, therefore I am”). His metaphysical dualism distin¬ guished radically between mind, the essence of which is thinking, and matter, the essence of which is extension in three dimensions. Though his metaphysics is rationalistic (see rationalism), his physics and physiology are empiricistic (see empiricism) and mechanistic (see mechanism). In math¬ ematics, he founded analytic geometry and reformed algebraic notation.
descent System of acknowledged social parentage whereby a person may claim kinship ties with another. Descent systems vary widely. The practical importance of descent comes from its use as a means for indi¬ viduals to assert rights, duties, privileges, or status. Descent has special influence when rights to succession, inheritance, or residence follow kin¬ ship lines. One method of limiting the recognition of kinship is to empha¬ size the relationship through one parent only. Such unilineal kinship systems are of two main types—patrilineal systems, in which the rela¬ tionships through the father are emphasized; and matrilineal systems, in which maternal relationships are stressed. These systems differ radically from cognatic systems, in which everyone has similar obligations to, and expectations from, both paternal and maternal kin. The cognatic system is somewhat vague and tends to characterize the more industrialized coun¬ tries, in which individual rights and duties are increasingly defined insti¬ tutionally or legally.
desensitization or hyposensitization Treatment to eliminate allergic reactions (see allergy) by injecting increasing strengths of puri¬ fied extracts of the substance that causes the reaction. This creates spe¬ cial antibodies (blocking antibodies) in the patient’s serum that combine with the allergen, blocking its reaction with allergic antibodies. Desensi¬ tization can also be required when a penicillin-sensitive person needs to be treated with penicillin. See also anaphylaxis, antigen.
desert Large, extremely dry area of land with fairly sparse vegetation. It is one of the Earth’s major types of ecosystems. Areas with a mean annual precipitation of 10 in. (250 mm) or less are generally considered deserts. They include the high-latitude circumpolar areas as well as the
Agave shawii growing in a desert in North America.
© ROBERT AND UNDA MITCHELL
more familiar hot, arid regions of the low and mid-latitudes. Desert ter¬ rain may consist of rugged mountains, high plateaus, or plains; many occupy broad mountain-rimmed basins. Surface materials include bare bedrock, plains of gravel and boulders, and vast tracts of shifting sand. Wind-blown sands, commonly thought to be typical of deserts, make up only about 2% of North American deserts, 10% of the Sahara, and 30% of the Arabian desert.
desert varnish or patina Thin, dark red to black mineral coating (generally iron and manganese oxides and silica) deposited on pebbles and rocks on the surface of desert regions. As dew and soil moisture brought to the surface by capillary action evaporate, their dissolved min¬ erals are deposited on the surface. Wind abrasion removes the softer salts and polishes the surface to a glossy finish. Both high evaporation rates and sufficient precipitation are necessary for desert varnish to form.
desertification Spread of a desert environment into arid or semiarid regions, caused by climatic changes, human influence, or both. Climatic factors include periods of temporary but severe drought and long-term climatic changes toward dryness. Human factors include artificial climatic alteration, as through the removal of vegetation (which can lead to unnatu¬ rally high erosion), excessive cultivation, and the exhaustion of water supplies. Desertification drains an arid or semiarid land of its life¬ supporting capabilities. It is characterized by a declining groundwater table, salt accumulation in topsoil and water, a decrease in surface water, increasing erosion, and the disappearance of native vegetation.
Desiderio da Settignano Vda-se-'der-yo-da-.sat-ten-'ya-noN (b. c. 1430, Settignano, republic of Florence—d. January 1464, Florence) Ital¬ ian sculptor. Born into a family of stonemasons, he entered the Stone and Wood Carvers’ Guild of Florence in 1453. He based his style on Dona¬ tello’s work of the 1430s, and his skill as a marble cutter established him as a master of bas-relief. His delicate, sensitive, original technique was best expressed in portrait busts of women and children. His most impor¬ tant public work was the tomb of Carlo Marsuppini in the church of Santa Croce; the tomb’s rich architectural detail makes it one of the most out¬ standing of all Florentine wall monuments.
designer drug Synthetic version of a controlled narcotic substance. Designer drugs usually are synthesized for the first time in an attempt to create a chemical whose molecular structure differs only slightly from that of some well-known controlled substance but whose effects are essentially the same. Because of the difference in molecular structure, the designer drug, unlike the controlled substance, ordinarily will not be specifically listed as illicit by law-enforcement organizations. Many designer drugs are manufactured in clandestine laboratories, often by amateurs; for this reason they are sometimes more dangerous than the drugs they are intended to replace. One of the best-known is MDMA (3,4- methylene¬ dioxymethamphetamine), a variation of methamphetamine, popularly called Ecstasy. Nonnarcotic synthetic chemical compounds designed to interact with specific proteins and enzymes in order to combat disease also have been called designer drugs.
desktop publishing (DTP) Use of a personal computer to perform publishing tasks. DTP allows an individual to combine text, numerical data, and graphic elements in a document that can be output on a printer or a phototypesetter. A typical DTP system includes a personal computer, a high-resolution printer, and input devices such as an optical scanner. Text and graphic elements are commonly created or manipulated with several separate software programs and then combined with a page- makeup program. Powerful DTP programs offer full-featured graphics capabilities.