checking of blood sugar, may manage diabetes. If not, injected or oral insulin is necessary. Complications, including heart disease, diabetic ret¬ inopathy (a leading cause of blindness), kidney disease, and nerve disor¬ ders, especially in the legs and feet, account for most deaths. Degree of blood-sugar control does not always correlate with progression of com¬ plications. Gestational diabetes may occur as a complication of pregnancy.
diagenesis Sum of all processes, chiefly chemical, that produce changes in a sediment after its deposition but before its final lithification. Usually, not all the minerals in a sediment are in chemical equilibrium, so changes in interstitial water composition or in temperature or in both will lead to chemical alteration of one or more of the minerals present. Diagenesis is considered a relatively low-pressure, low-temperature alter¬ ation process that involves such processes as cementation, reworking, replacement, crystallization, and leaching.
Diaghilev Vde-'a-go-.lefV, Sergey (Pavlovich) (b. March 31, 1872, Novgorod province, Russia—d.
Aug. 19, 1929, Venice, Italy) Rus¬ sian impresario, founder-director of the Ballets Russes. After studying law at the University of St. Petersburg (1890-96), he cofounded and edited (1899-1904) the avant-garde maga¬ zine Mir Iskusstva (“World of Art”).
He then left Russia for Paris to present productions of Russian bal¬ let and opera, to wide acclaim. In 1909 he established the Ballets Russes, in which he achieved a stun¬ ning synthesis of dance, art, and music by bringing together superb choreographers, dancers, composers, and artists and set designers. He led the company until his death.
diagnosis Identification of a dis¬ ease or disorder. Diagnosis requires a medical history (including family history), a physical examination, and usually tests and diagnostic proce¬ dures (e.g., BLOOD ANALYSIS, DIAGNOSTIC imaging). A list of possible causes— the differential diagnosis—is devel¬ oped and then narrowed down by further tests that eliminate or support specific possibilities.
diagnostic imaging or medical imaging Use of electromagnetic radiation to produce images of internal body structures for diagnosis. X-rays have been used since 1895. Denser tissues, such as bones, absorb more X-rays and show as lighter areas on X-ray film. A contrast medium can be used to highlight soft tissues in still X-ray pictures or can be fol¬ lowed on X-ray motion-picture films as it moves through the body or part of the body to record body processes. In computed axial tomography, X-rays are focused on specific tissue planes, and a series of such parallel “slices” of the body are processed by computer to produce a 3-D image. The risks of X-ray exposure are reduced by more precise techniques using lower doses and by use of other imaging techniques. See also angiocardiography; angiography; magnetic resonance imaging; nuclear medicine; positron emission tomography; ultrasound.
dial gauge Any of a number of deviation-type gauges that indicate the amount by which an object being gauged deviates from the standard. This deviation is usually shown in units of measurement, but some gauges show only whether the deviation is within a certain range. They include dial indicators, in which movement of a gauging spindle deflects a pointer on a graduated dial; wiggler indicators, used by machinists to centre or align work in machine tools; comparators, or visual gauges; and air gauges, used to gauge holes of various types.
dialect Variety of a language spoken by a group of people and having features of vocabulary, grammar, and/or pronunciation that distinguish it from other varieties of the same language. Dialects usually develop as a result of geographic, social, political, or economic barriers between groups of people who speak the same language. When dialects diverge to the point that they are mutually incomprehensible, they become languages
Bayt ul-Mukarram mosque and shop¬ ping mall, Dhaka, Bangladesh.
FREDERIC OHRINGER-NANCY PALMER AGENCY/EB INC.
Sergey Diaghilev, c. 1916.
DANCE COLLECTION, THE NEW YORK PUBLIC LIBRARY AT LINCOLN CENTER, ASTER, LENOX AND TILDEN FOUNDATIONS
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dialectical materialism ► diary I 541
in their own right. This was the case with Latin, various dialects of which evolved into the different Romance languages. See also koine.
dialectical materialism Philosophical approach expressed through the writings of Karl Marx and Friedrich Engels, and later by Georgy Ple- khanov, Vladimir Ilich Lenin, and Joseph Stalin, the official philosophy of communism. Its central tenet, borrowed from Hegelianism, is that all his¬ torical growth, change, and development results from the struggle of opposites. (In philosophical terms, a thesis is opposed by its antithesis, which results in a synthesis.) Specifically, it is the class struggle—the struggle between the capitalist and landowning classes, on the one hand, and the proletariat and peasantry, on the other—that creates the dynamic of history. The laws of historical dialectics are seen to be so powerful that individual leaders are of little historical consequence. Originally con¬ ceived as operating primarily in the social, economic, and political realm, the principle was extended in the 20th century to the scientific realm as well, with major effects on Soviet science. Marx and Engels stated their philosophical views mainly in the course of polemics and brief historical studies; there is no systematic exposition of dialectical materialism.
dialysis \dl-'a-b-s9s\ In chemistry, separation of suspended colloidal (see colloid) particles from dissolved ions or small molecules via their unequal rates of diffusion through pores of semipermeable membranes (e.g., parchment, collodion, cellophane). A slow process, dialysis may be accel¬ erated by heating or by applying an electric field if the particles are charged.
dialysis \dl-'a-b-s3s\ or hemodialysis \ I he-mo-dI-'a-b-s9s\ Process of removing blood from a patient with kidney failure, purifying it with a hemodialyzer (artificial kidney), and returning it to the bloodstream. Many substances (including urea and inorganic salts) in the blood pass through a porous membrane in the machine into a sterile solution; particles such as blood cells and proteins are too large to pass. This process controls the acid-base balance of the blood and its content of water and dissolved materials.
diamagnetism Kind of magnetism characteristic of materials that line up at right angles to a nonuniform magnetic field and that partly expel from their interior the magnetic field in which they are placed. In most mate¬ rials, the magnetic fields of the electrons balance each other and add up to zero. However, when placed in an external magnetic field, the inter¬ action of this field with the electrons induces an internal field in the oppo¬ site direction. The substance can then be weakly repelled by magnetic poles. Examples of diamagnetic substances include bismuth, antimony, SODIUM CHLORIDE, GOLD, and MERCURY.
diamond Mineral composed of pure carbon, the hardest naturally occurring substance known and a valuable gemstone. Diamonds are formed deep in the Earth by tremendous pressures and temperatures over long periods of time. In the crystal structure of diamond, each carbon atom is linked to four other, equidistant, carbon atoms. This tight crystal struc¬ ture results in properties that are very different from those of graphite, the other common form of pure carbon. Diamonds vary from colourless to black and may be transparent, translucent, or opaque. Most gem diamonds are transparent and colourless or nearly so. Colourless or pale blue stones are most valued, but most gem diamonds are tinged with yellow. Because of their extreme hardness, diamonds have important industrial applica¬ tions. Most industrial diamonds are gray or brown and are translucent or opaque. In the symbolism of gemstones, the diamond represents steadfast love and is the birthstone for April.