dissociation Breaking of a chemical compound into simpler constitu¬ ents as a result of added energy, as in the case of gaseous molecules dis¬ sociated by heating; also, the effect of a solvent on a dissolved polar compound (electrolyte), as in the case of an inorganic salt, such as sodium chloride, dissolved in water. All electrolytes dissociate into ions to a greater or lesser extent in polar solvents (in which the molecules are elec¬ tric dipoles). The degree of dissociation can be used to determine the equi¬ librium constant. Dissociation is used to explain electrical conductivity and many other properties of electrolytic solutions.
dissociative identity disorder formerly multiple personality disorder Rare condition indicated by the absence of a clear and com¬ prehensive identity. In most cases two or more independent and distinct personality systems develop in the same individual. Each personality may alternately inhabit the person’s conscious awareness to the exclusion of the others, but one is usually dominant. The various personalities typi¬ cally differ from one another in outlook, temperament, and body language and might assume different first names. The condition is generally viewed as resulting from dissociative mental processes—that is, the splitting off from conscious awareness and control of thoughts, feelings, memories, and other mental components in response to situations that are painful, disturbing, or somehow unacceptable to the person experiencing them. Treatment is aimed at integrating the disparate personalities back into a single and unified personality.
dissonance See consonance and dissonance dissonance, cognitive See cognitive dissonance
distaff Device used in hand spinning in which individual fibres are drawn out of a mass of prepared fibres held on a stick (the distaff), twisted together to form a continuous strand, and wound on a second stick (the spindle). It is most often used for making linen; wool does not require a distaff (see carding). The first stage in mechanizing spinning was to mount the spindle horizontally in bearings to rotate with a large hand-driven wheel; the distaff, carrying the mass of fibre, was held in the left hand, and the spinning wheel slowly turned with the right. The Saxon, or Sax¬ ony, wheel incorporated a bobbin on which the yarn was wound continu¬ ously; the distaff holding the raw fibre became a stationary vertical rod, and the wheel was activated by a foot treadle, freeing both the operator’s
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distance formula ► diverticulum I 553
hands. From 17th-century England, the word distaff became a synonym for maternal as most spinning was done by women in their homes. See also DOMESTIC SYSTEM.
distance formula Algebraic expression that gives the distances between pairs of points in terms of their coordinates (see coordinate sys¬ tem). In two- and three-dimensional Euclidean space, the distance formu¬ las for points in rectangular coordinates are based on the Pythagorean theorem. The distance between the points (a,b) and ( c,d) is given by V(a - c ) 2 + (b - d) 2 . In three dimensional space, the distance between the points (a, b, c ) and ( d , e, f) is “n J(a - d) 2 + (b — e ) 2 + (c - f) 2 .
distemper Viral disease in two forms, canine and feline. Canine dis¬ temper is acute and highly contagious, affecting dogs, foxes, wolves, mink, raccoons, and ferrets. Most untreated cases are fatal. Infected ani¬ mals are best treated with prompt injections of serum globulins; second¬ ary infections are warded off by antibiotics. Immunity can be conferred by vaccination. Feline distemper causes a severe drop in the number of the infected cat’s white blood cells. It rarely lasts more than a week, but the mortality rate is high. Vaccines offer effective immunity.
disthene See kyanite
distillation Vaporization of a liquid and subsequent condensation of the resultant GAS back to liquid form. It is used to separate liquids from non¬ volatile solids or solutes (e.g., water from salt and other components of seawater) or to separate two or more liquids with different boiling points (e.g., alcohol from fermented beers and wines). Many variations have been devised for industrial applications. An important one is fractional distillation, in which vapour from a heated liquid mixture is contacted by a series of trays and condensed liquid as it rises through a vertical col¬ umn. The most volatile fraction of the mixture emerges from the top of the column, while less volatile fractions are withdrawn at lower points. In petroleum refining, this method is very efficient for removing naphtha, kerosene, and gas oils from crude oil.
A laboratory distillation apparatus demonstrating desalination of water. In the distil¬ lation flask, salt water is boiled to produce water vapour, or steam, while the salt remains in the liquid solution. The vapour rises through the top of the flask and passes into the condenser, which consists of a glass tube within a larger tube. Cold water flows through the space between the tubes, cooling the vapour in the inner tube so that it condenses and flows into the receiving flask.
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distilled liquor Alcoholic beverage obtained by distillation from wine or other fermented fruit juice or from various cereal grains that have first been brewed. The essential ingredient is usually a natural sugar or a starchy substance that may be easily converted into a sugar. The distil¬ lation process is based on the different boiling points of water (212 °F [100 °C]) and alcohol (173 °F [78.5 °C]). The alcohol vapours that arise while the fermented liquid boils are trapped and recondensed to create a liquid of much greater alcoholic strength. The resultant distillate is matured, often for several years, before it is packaged and sold. See also aquavit; brandy; gin; liqueur; rum; vodka; and whiskey.
distribution See frequency distribution, normal distribution
distributive law One of the laws relating to number operations. In sym¬ bols, it is stated; a{b + c) = ab + ac. The monomial factor a is distributed, or separately applied, to each term of the polynomial factor b + c, resulting in the product ab + ac. It can also be stated in words: The result of first adding several numbers and then multiplying the sum by some number is the same as first multiplying each separately by the number and then adding the products. See also associative law; commutative law.
District Court, U.S. See United States District Court
District of Columbia Federal district of the U.S. Coextensive with the city of Washington, it is bounded by Maryland and Virginia. Originally 100 sq mi (259 sq km), the territory was authorized by Congress in 1790 and granted by Maryland and Virginia; it now occupies 68 sq mi (176 sq km). The site was chosen by Pres. George Washington and became the seat of the federal government by 1800. Part of the district (Alexandria, Va.) was retroceded to Virginia in 1847. The slave trade was prohibited in the District in 1850 and slavery was abolished in 1862. The territorial government was abolished in 1874 in favour of government by a com¬ mission appointed by the president. Residents were granted suffrage in national elections in 1961 by the 23rd Amendment to the U.S. Constitu¬ tion. The mayor-council form of government was established in 1967. Originally appointees of the president, the mayor and councilors became elected officials in 1973 and received local legislative powers in 1974.