- 行业: Printing & publishing
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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.
The angiosperms, or flowering plants, are geologically a very young group. They first appeared in the fossil record in the earliest Cretaceous about 140 million years ago, almost 300 million years after the first occurrence of terrestrial plants. Despite this late appearance, angiosperms have developed a bewildering diversity exceeding by far that of any other plant group. In the present flora, angiosperms comprise between 250,000 and 300,000 living species. Timing of angiosperm origin and early diversification is still controversial, but new findings of exquisitely preserved Cretaceous flowers have greatly extended our knowledge of early angiosperms. It is clear from the fossil record that extensive extinctions have taken place in several angiosperm lineages, and that the total angiosperm diversity is much higher than what is seen today.
Industry:Science
The branch of chemistry that deals with the interpretation of chemical phenomena and properties in terms of the underlying physical processes, and with the development of techniques for their investigation. Chemical physics refers to a branch of physical chemistry in which the emphasis is on the interpretation and analysis of the physical properties of individual molecules and bulk systems, instead of their reactions. Theoretical chemistry is another major branch, in which the emphasis is on the calculation of the properties of molecules and systems, and which uses the techniques of quantum mechanics and statistical thermodynamics. For the present purpose it is convenient to regard physical chemistry as dealing with three aspects of matter: its equilibrium properties, the structure of molecules and bulk matter, and matter's ability to change.
Industry:Science
The Global Positioning System (GPS) consists of a constellation of 24 or more satellites that transmit navigation signals in two frequency bands called L1 (centered at 1575.42 MHz) and L2 (centered at 1227.6 MHz). The signals are synchronized to within about 10 nanoseconds of a composite time frame known as GPS System Time. The satellites also transmit messages that accurately describe their orbital path in space. GPS users with unobstructed visibility can receive up to 12 of these signals, each one coming from a different direction. The GPS receiver measures the arrival time of each signal. With a minimum of four signals, the receiver can calculate its three physical position coordinates (for example, latitude, longitude, and altitude) plus a precise correction for its internal clock. In general, the more signals used, the better the result.
Industry:Science
The cooling of a space or substance below the environmental temperature. The art was known to the ancient Egyptians and people of India, who used evaporation to cool liquids in porous earthen jars exposed to dry night air; and to the early Chinese, Greeks, and Romans, who used natural ice or snow stored in underground pits for cooling wine and other delicacies. In the eighteenth and early nineteenth centuries natural ice cut from lakes and ponds in winter was stored underground for use in summer. The technique of mechanical refrigeration began with the invention of machines for making artificial ice. Great strides have been made in the twentieth century in the application of mechanical refrigeration to fields other than ice making, including the direct cooling and freezing of perishable foods and air conditioning for industry and human comfort.
Industry:Science
The ejection on one or more electrons from an atom, molecule, or positive ion following the absorption of one or more photons. The process of electron ejection from matter following the absorption of electromagnetic radiation has been under investigation for over a century. The earliest measurements involved the irradiation of metal surfaces by ultraviolet radiation. The theoretical interpretation of this phenomenon, known as the photoelectric effect, played an important role in establishing quantum mechanics. It was shown that, contrary to classical ideas, energy exchanges between radiation and matter are mediated by integral numbers of photons. The ejection of electrons from a metal requires that the photon energy exceed the binding energy of electrons in the metal (that the wavelength of light be sufficiently small), whatever the photon flux.
Industry:Science
The generalization that the transition from one energy state of a molecular system to another occurs so nearly instantaneously that the nuclei of the atoms involved can be considered as stationary during the process. The Franck-Condon principle is closely related to the Born-Oppenheimer approximation, in which the various motions (electronic, nuclear vibrations and rotations) are considered to be separable, and in which the electrons respond to the instantaneous vibrations of the system whereas the system responds only to the average position of the electrons. The principle, proposed by J. Franck in 1925 and developed quantum-mechanically by E. U. Condon in 1928, is important in discussing systems of more than one atom. It is therefore valuable in molecular spectroscopy and in the interpretation of the optical properties of liquids and solids.
Industry:Science
The direct conversion of heat into electrical energy, or the reverse, in solid or liquid conductors by means of three interrelated phenomena—the Seebeck effect, the Peltier effect, and the Thomson effect—including the influence of magnetic fields upon each. The Seebeck effect concerns the electromotive force (emf) generated in a circuit composed of two different conductors whose junctions are maintained at different temperatures. The Peltier effect refers to the reversible heat generated at the junction between two different conductors when a current passes through the junction. The Thomson effect involves the reversible generation of heat in a single current-carrying conductor along which a temperature gradient is maintained. Specifically excluded from the definition of thermoelectricity are the phenomena of Joule heating and thermionic emission.
Industry:Science
The geometry of riemannian manifolds. Riemannian geometry was initiated by B. Riemann in 1854, following the pioneering work of C. F. Gauss on surface theory in 1827. Riemann introduced a coordinate space in which the infinitesimal distance between two neighboring points is specified by a quadratic differential form, given below. Such a space is a natural generalization of euclidean geometry and Gauss's geometry of surfaces in three-dimensional euclidean space, as well as the noneuclidean geometries: hyperbolic geometry (previously discovered by J. Bolyai and N. I. Lobachevsky) and elliptic geometry. A riemannian manifold is a topological space that further generalizes this notion. Riemannian geometry derives great importance from its application in the general theory of relativity, where the universe is considered to be a riemannian manifold.
Industry:Science
The functional significance of carbohydrates in biological systems is quite diverse. For example, carbohydrates cover the surface of some bacteria, such as <i>Haemophilus influenzae</i>. Carbohydrates in the outer lipid coat may aid in disguising these bacteria by mimicking substances naturally present in humans. They are also key components of cartilage, where they form polymeric chains of negatively charged monosaccharides. The negative charges provide electrostatic repulsion when the cartilage undergoes compression. Carbohydrates also are present on the surface of many proteins. Their presence is sometimes necessary to assist in protein folding or receptor binding. The protein that covers the surface of the human immunodeficiency virus (HIV), gp120, is heavily glycosylated; these carbohydrates are believed to shield the protein from immune attack.
Industry:Science
Substances showing negative thermal expansion (NTE) over small temperature ranges have long been known. For example, water exhibits negative thermal expansion over a range of several degrees close to its freezing point. The world we live in would be a very different place if it were not for that NTE, which causes ice to float on water. NTE is also known to occur in many materials, such as silicon (Si), at very low temperatures, but this NTE disappears well before approaching ambient temperature. NTE also occurs well above ambient temperature over limited temperature ranges in materials such as some crystalline forms of silicon dioxide (SiO<sub>2</sub>). The discovery in 1995 that cubic zirconium tungstate (ZrW<sub>2</sub>O<sub>8</sub>) exhibits strong NTE over a temperature range exceeding 1000°C led to a surge of interest in materials exhibiting NTE.
Industry:Science
