06413_04 - D 2,4-D 2,4-dichlorophenoxyacetic acid

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D 2,4-D 2,4-dichlorophenoxyacetic acid (2,4-dichlorophenoxyethanoic acid): a syn- thetic * auxin used as a weedkiller of broad-leaved weeds. See pesticide . dalton See atomic mass unit . Dalton, John (1766–1844) British chemist and physicist. In 1801 he formu- lated his law of partial pressures ( see dal- ton’s law ), but he is best remembered for * Dalton’s atomic theory, which he an- nounced in 1803. Dalton also studied col- our blindness (a condition, once called Daltonism, that he shared with his brother). Dalton’s atomic theory A theory of * chemical combination, Ü rst stated by John Dalton in 1803. It involves the fol- lowing postulates: (1) Elements consist of indivisible small particles (atoms). (2) All atoms of the same element are identical; different elements have differ- ent types of atom. (3) Atoms can neither be created nor de- stroyed. (4) ‘Compound elements’ (i.e. compounds) are formed when atoms of different el- ements join in simple ratios to form ‘compound atoms’ (i.e. molecules). Dalton also proposed symbols for atoms of different elements (later re- placed by the present notation using let- ters). Dalton’s law The total pressure of a mixture of gases or vapours is equal to the sum of the partial pressures of its components, i.e. the sum of the pressures that each component would exert if it were present alone and occupied the same volume as the mixture of gases. Strictly speaking, the principle is true only for ideal gases. dam A structure built across a river to impound or divert the Û ow of water. A dam may be constructed for one or a number of purposes. It may raise the water level to increase the depth for navi- gation purposes; divert water; provide a head of water for the generation of hydroelectric power; or store water for industrial or domestic use, irrigation, Û ood control, or power production. In ad- dition, water stored by dams is often used for recreational purposes. Most dams are constructed of either concrete or of earth and rock. Gravity dams depend on the weight of their bulk for stability and usu- ally have a Û at vertical face upstream. Arch dams are curved concrete structures with a convex curve upstream. Pressure is transmitted to the sides of the dam. damping A decrease in the amplitude of an oscillation as a result of energy being drained from the oscillating system to overcome frictional or other resistive forces. For example, a pendulum soon comes to rest unless it is supplied with energy from an outside source; in a pen- dulum clock, energy is supplied through an * escapement from a wound spring or a falling mass to compensate for the en- ergy lost through friction. Damping is introduced intentionally in measuring in- struments of various kinds to overcome the problem of taking a reading from an oscillating needle. A measuring instru- ment is said to be critically damped if the system just fails to oscillate and the sys- tem comes to rest in the shortest possible time. If it is underdamped it will oscillate repeatedly before coming to rest; if it is overdamped it will not oscillate but it
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06413_04 - D 2,4-D 2,4-dichlorophenoxyacetic acid

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