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McGraw Hill Financial, Inc. is an American publicly traded corporation headquartered in Rockefeller Center in New York City. Its primary areas of business are financial, publishing, and business services.
A mineral, thorium silicate, in which the element thorium was discovered in 1828. Thorite is tetragonal in crystallization and has a crystal structure identical with that of the nesosilicate zircon, ZrSiO<sub>4</sub>. The idealized chemical formula of thorite is ThSiO<sub>4</sub>. All natural material departs widely from this composition owing to the partial substitution of uranium, rare earths, calcium, and iron for thorium. Structurally, thorite usually has completely lost its crystallinity because of radiation damage from the contained uranium and thorium (metamict state). The specific gravity ranges between about 4.3 and 5.4. The hardness on Mohs scale is about 4½. The color commonly is brownish yellow to brownish black and black. Thorogummite is a chemical variant of thorite with the same crystal structure and very similar properties. It is deficient in silica and contains small amounts of OH in substitution for oxygen.
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A mineral, β-ZnS, also called blende. It is the low-temperature form and more common polymorph of ZnS. Pure β-ZnS on heating inverts to wurtzite, α-ZnS, at 1020°C (1868°F), but this temperature can be lowered substantially by impurity-atom solid solution (especially Cd<sup>2+</sup> and Fe<sup>2+</sup>) and sulfur fugacity. Sphalerite crystallizes in the hextetrahedral class of the isometric system with a structure similar to that of diamond. The space group is F<span style="text-decoration:overline">4</span>3m, and the cubic unit cell has an edge <i>a</i> = 0.543 nanometer, which contains four ZnS molecules. Zinc atoms occupy the positions of half the carbon atoms of diamond, and sulfur atoms occupy the other half. Each zinc atom is bonded to four sulfur atoms, and each sulfur atom is bonded to four zinc atoms. The common crystal forms of sphalerite are the tetrahedron, dodecahedron, and cube, but crystals are frequently complex and twinned (see <b>illus.</b>). The mineral is most commonly in coarse to fine, granular, cleavable masses. The luster is resinous to submetallic; the color is white when pure, but is commonly yellow, brown, or black, darkening with increased percentage of iron. It has been shown that excess sulfur can also contribute to the darkening of the color. There is perfect dodecahedral cleavage; the hardness is 3½ on Mohs scale; specific gravity is 4.1 for pure sphalerite but decreases with increasing iron content.
Industry:Science
A mining method used to obtain valuable minerals from the earth by first removing the overlying soil and rock (overburden) and subsequently recovering the valuable mineral. Surface mining is done until the accumulated overburden removed becomes too thick (and expensive to remove) for the economic recovery of the mineral. For mineral deposits too deep to be surface-mined, underground mining methods are used.
Industry:Science
A minor accessory mineral, formula CaTiO<sub>3</sub>, occurring in basic rocks. Perovskite has given its name to a large family of materials, synthetic and natural, crystallizing in similar structures. The crystal structure is ideally cubic, with a framework of corner-sharing octahedra, containing titanium (Ti) or other relatively small cations surrounded by six oxygen (O) or fluorine (F) anions (see <b>illus.</b>). Within this framework are placed calcium (Ca) or other large cations, surrounded by twelve anions. Tilting of the octahedra and other distortions often lower the symmetry from cubic, giving the materials important ferroelectric properties and decreasing the coordination of the central cation. This flexibility gives the structure the ability to incorporate ions of different sizes and charges. Substitution of niobium (Nb), cerium (Ce), and other rare-earth elements in natural calcium titanate (CaTiO<sub>3</sub>) is common and can make perovskite an ore for these elements.
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A mixture of hydrocarbons whose boiling point is below 200°C (390°F), obtained in the fractional distillation of petroleum. Gasoline is a liquid at ambient temperature, but it volatilizes readily in air to form a flammable mixture. The hydrocarbon fuel is used to power the internal combustion engine. Gasoline is composed primarily of the alkanes (paraffins) hexane, heptane, and octane, plus smaller amounts of higher-boiling alkanes.
Industry:Science
A mode of electrical conduction in gases. Glow discharge commonly occurs under conditions of relatively low pressure and generally in the pressure range of 1–10 mm of mercury (100–1000 pascals). The discharge typically gives off light, so that the region of the discharge appears to glow with considerable intensity. This glow is quite diffuse as contrasted to a higher-pressure discharge, such as a high-pressure arc. Typical currents may be of the order of tens or hundreds of milliamperes, whereas the potential drop may be of the order of 100 V.
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A mode of thought, feeling, and behavior characterized centrally by false persecutory beliefs, more specifically referred to as paranoidness. The paranoid individual falsely believes he or she is being intentionally harassed, plotted against, threatened, demeaned, interfered with, wronged, exploited, or otherwise persecuted by malevolent others—either a specific person or a group. Commonly associated with these core persecutory beliefs are properties of suspiciousness, fearfulness, hostility, hypersensitivity, rigidity of conviction, and an exaggerated sense of self-reference. These properties are evident with varying degrees of intensity and duration.
Industry:Science
A model for the functionings of an agricultural system, with all inputs and outputs. An ecosystem may be as small as a set of microbial interactions that take place on the surface of roots, or as large as the globe. An agroecosystem may be at the level of the individual plant–soil–microorganism system, at the level of crops or herds of domesticated animals, at the level of farms or agricultural landscapes, or at the level of entire agricultural economies.
Industry:Science
A model which consists of a lattice of “spin” variables with two characteristic properties: (1) each of the spin variables independently takes on either the value +1 or the value −1; and (2) only pairs of nearest neighboring spins can interact. The study of this model (introduced by Ernst Ising in 1925) in two dimensions, where many exact calculations have been carried out explicitly, forms the basis of the modern theory of phase transitions and, more generally, of cooperative phenomena.
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A moderately coherent group of primitive reptiles constituting an order of the subclass Anapsida. Most members are characterized by a closed cheek (temporal) region. The order is divided into four suborders: Captorhinomorpha, Millerosauria, Procolophonia, and Pareiasauria. Except for the Procolophonia, which continue into the Late Triassic, Captorhinida are confined to the Permo-Carboniferous. They lived in lowlands, where they were associated with amphibians and synapsid reptiles. Some of the smaller animals of the Captorhinomorpha and the Millerosauria fed primarily on insects and small vertebrates, but the larger genera were exclusively herbivorous. Along with the caseid pelycosaurs, they were the dominant consumers of vegetation in the middle Permian ecosystems.
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