- 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 first chemical element in the periodic system. Under ordinary conditions it is a colorless, odorless, tasteless gas composed of diatomic molecules, H<sub>2</sub>. The hydrogen atom, symbol H, consists of a nucleus of unit positive charge and a single electron. It has atomic number 1 and an atomic weight of 1.00797. The element is a major constituent of water and all organic matter, and is widely distributed not only on the Earth but throughout the universe. There are three isotopes of hydrogen: protium, mass 1, makes up 99.98% of the natural element; deuterium, mass 2, makes up about 0.02%; and tritium, mass 3, occurs in extremely small amounts in nature but may be produced artificially by various nuclear reactions.
Industry:Science
The actions taken to preserve the operation of devices, particularly of electromechanical equipment, to ensure that the devices can perform their intended functions when needed. The field of maintenance science is an interdisciplinary research area that employs techniques from physics, engineering, and decision analysis to develop methods for the safe and reliable performance of devices and equipment. Traditionally, the focus of maintenance has been on equipment availability—the ratio of operating time less downtime to total available time. Modern maintenance practices focus on increasing equipment effectiveness, that is, making sure that the equipment is both available and capable of producing superior-quality products.
Industry:Science
The <i>Burkholderia cepacia</i> complex is a group of bacteria prevalent in the natural environment that can cause human infections (so-called opportunistic infections) in vulnerable individuals. These microorganisms can colonize a variety of different habitats, including soil (especially at the rhizosphere—the interface between the soil and plant roots), water, polluted environments, and the lungs of patients with cystic fibrosis. Their success as microbial colonizers appears to be dependent on two innate properties: (1) <i>B. cepacia</i> complex bacteria can metabolize an enormous range of organic compounds, and (2) they possess considerable resistance to antimicrobial agents and may synthesize antibiotics of their own.
Industry:Science
The collective name for a proton or a neutron. These subatomic particles are the principal constituents of atomic nuclei and therefore of most matter in the universe. The proton and neutron share many characteristics. They have the same intrinsic spin, nearly the same mass, and similar interactions with other subatomic particles, and they can transform into one another by means of the weak interactions. Hence it is often useful to view them as two different states or configurations of the same particle, the nucleon. Nucleons are small compared to atomic dimensions and relatively heavy. Their characteristic size is of order 1/10,000 the size of a typical atom, and their mass is of order 2000 times the mass of the electron.
Industry:Science
Storage rings are usually associated with kilometer-long accelerators and particle colliders, but there is one miniature storage ring. The Nevatron, the first neutral-atom storage ring, is slightly larger than a quarter (25 mm or 1 in. in diameter) and weighs only 200 g (7 oz). While it cannot create antiprotons with teraelectronvolts (TeV) of energy, the Nevatron does bring together laser cooling and magnetic trapping to produce a storage ring for neutral rubidium-87 (<sup>87</sup>Rb) atoms. This ring has allowed observation of multiple atomic orbits, dual cloud loading, and atomic cloud shaping. It also provides the basis for an atomic interferometer, a device that may one day provide the world's most sensitive gyroscope.
Industry:Science
The effect of asymmetric compounds on polarized light. To exhibit this effect, a molecule must be nonsuperimposable on its mirror image, that is, must be related to its mirror image as the right hand is to the left hand. An optically active compound and its mirror image are called enantiomers or optical isomers. Enantiomers differ only in their geometric arrangements; they have identical chemical and physical properties. The right-handed and left-handed forms of a molecule can be distinguished only by their optical activity or by their interactions with other asymmetric molecules. Optical activity can be used to probe other aspects of molecular geometry, as well as to identify which enantiomer is present and its purity.
Industry:Science
The ability of the immune system to protect the body from various insults declines with age. Changes in both innate and acquired immunity appear to be involved in this phenomenon, and they can be described at the level of the entire organism or the cell. For example, the innate response of mounting a fever with infection occurs less frequently in aged persons; 20% of adults over age 65 with serious bacterial infections do not have fevers. This may be due to decreased sensitivity of the central nervous system to immune signals, rather than actual changes in the ability to generate such stimuli. Most changes in immune function associated with advancing age have been observed at the cellular level and involve acquired immunity.
Industry:Science
The fusion of a female gamete (egg) with a male gamete (sperm) to form a single cell, the zygote. Gametes are haploid because their nuclei contain only one set of chromosomes. In contrast, most animal cells are diploid since their nuclei contain two sets of chromosomes, one set derived from the egg and the other from the sperm. The reduction from the diploid to the haploid number occurs in a specialized set of two cell divisions, known as meiosis, during which gametes are formed in the gonads of an adult. Fertilization restores the diploid chromosome number. In the human, normal eggs and sperm contain 23 chromosomes each, whereas the zygote and most of its daughter cells contain 23 pairs, or 46, chromosomes per cell nucleus.
Industry:Science
The conventional process for the production and use of materials has been to extract natural resources such as fossil fuel or minerals, refine them, modify them further through manufacturing, and then distribute them as products. At the end of their useful lives, the products are discarded, usually to a landfill or an incinerator. Based on current material practices, society faces a number of problems, including the dispersion of persistent toxics and other contaminants throughout the Earth, depletion of nonrenewable resources such as petroleum, destruction of biodiversity, and the depletion and contamination of aquifers and drinking water sources. There is a growing perception that these material practices are unsustainable.
Industry:Science
The isotope of the element hydrogen with atomic weight 2.0144 and symbols <sup>2</sup>H or D. Considerations of nuclear stability and a discrepancy between the chemical and physical atomic weights of hydrogen led to the prediction of a stable isotope of hydrogen of mass 2. A successful search for this isotope, deuterium, was made by H. C. Urey, F. G. Brickwedde, and G. M. Murphy in 1931. The terrestrial natural abundance of deuterium is 1 in 6700 parts of ordinary hydrogen (protium), of atomic weight 1.0078. Small variations in natural sources are found as a result of fractionation by geological processes. Industrial hydrogen, particularly that generated by electrolysis of water, may contain significantly less deuterium.
Industry:Science
