DACITE.The name of a somewhat variable group of extrusive igneous rocks similar to the rhyolites but richer in plagioclase feldspar. Typical dacites are felsitic to porphyritic in texture. Dacites are the extrusive equivalents of the quartz-rich varieties of diorites and are sometimes classified as quartz-bearing andesites. The porphyritic types usually occur toward the center of the thicker dacite flows, dikes and sills, as well as the marginal zones of laccoliths. Dacites are common in the Cordilleran province of North, Central and South America. The term, dacite, was proposed by G. Stache of Austria for lavas in the old Roman province of Dacia.
DACRYOCYSTITIS.Infection of the lacrimal sac of the eye. The lacrimal glands produce tears for lubrication and protection of the eye. When the naso-lacrimal duct becomes obstructed, infection of the tearproducing sac is likely. Dacryocystitis appears most often in infants and in adults over 40 years of age. Secretions which cannot drain through the obstructed duct spill back out through the eye. Medical attention is required to avoid complications, such as infection of the cornea. See also Vision and the Eye.D' ALEMBERT PRINCIPLE. The principle, first pointed out by d' Alembert in 1742, that Newton's third law (see Newton's Laws of Dynamics) holds for forces acting upon bodies entirely free to move as well as upon fixed bodies in stationary equilibrium. In the former case the "reactions" concerned are due solely to inertia. Thus, in the act of throwing a ball, one pushes upon the ball with a certain force, and the inertia of the ball causes it to push back on the hand with an equal force. The condition of the system during such a process is said to be one of kinetic equilibrium. From this point of view, obviously, any system of bodies must always be in equilibrium, either kinetic or static.
DALTONIDE COMPOUNDS. See Chemical Composition.DALTON LAW. The law of partial pressures in mixed gases and vapors. If several gases not reacting chemically with each other are introduced into the same container, the pressure of the resulting mixture is equal to the sum of the pressures which would be observed if each gas were separately enclosed in that container. We may, for example, regard the atmospheric pressure as the sum of a nitrogen pressure, an oxygen pressure, an argon pressure, a carbon dioxide pressure, a water-vapor pressure, etc. The same principle holds for mixtures of the saturated vapors of two or more liquids evaporating in the same closed space, provided one liquid does not dissolve the vapor from the other (as water dissolves ammonia). Like other gas laws, this law is approximately valid only within limits. See also Combustion.
Dam Classification and ConstructionDams may be classified as follows: 1) Timber dams.2) Rock-fill dams.3) Earth dams. Plain. Core wall. Hydraulic fill. 4) Masonry dams.Gravity, solid and hollow. Arch, single and multiple.Timber Dams. The timber dam is rarely used because of its short life and the limitation in height to which it may be carri...
