- 行业: Printing & publishing
- Number of terms: 178089
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- Company Profile:
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 metapopulation is a set of geographically distinct local populations occupying discrete habitat patches. In some patches, the local population can become extinct—for example, if there are not enough reproductive females, or if there is a drought or other environmental hazard that depletes the resources used by the local population. Alternatively, individuals migrating from other patches can colonize an empty patch. Thus, the dynamics of a metapopulation depends on the relation between extinction and colonization events.
Industry:Science
A method for preparing a large number of chemical compounds, commonly known as a combinatorial library, which are then screened to identify compounds having a desired function, such as a particular biological or catalytic activity. Combinatorial synthesis is an aspect of combinatorial chemistry, which allows for the simultaneous generation and rapid testing for a desired property of large numbers of chemically related compounds. One could regard combinatorial chemistry as the scientist's attempt to mimic the natural principles of random mutation and selection of the fittest. Combinatorial chemistry has already become an invaluable tool in the areas of molecular recognition, materials science, drug discovery and optimization, and catalyst development.
Industry:Science
A method for separating homogeneous mixtures based upon equilibration of liquid and vapor phases. Substances that differ in volatility appear in different proportions in vapor and liquid phases at equilibrium with one another. Thus, vaporizing part of a volatile liquid produces vapor and liquid products that differ in composition. This outcome constitutes a separation among the components in the original liquid. Through appropriate configurations of repeated vapor-liquid contactings, the degree of separation among components differing in volatility can be increased manyfold.
Industry:Science
A method for the quantitation of the effects on a biological system by its exposure to a substance, as well as the quantitation of the concentration of a substance by some observable effect on a biological system. The biological material in which the effect is measured can range from subcellular components and microorganisms to groups of animals. The substance can be stimulatory, such as an ion increasing taxis behavior in certain protozoans, or inhibitory, such as an antibiotic for bacterial growth. Bioassays are most frequently used when there is a number of steps, usually poorly understood, between the substance and the behavior observed, or when the substance is a complex mixture of materials and it is not clear what the active components are. Bioassays can be replaced, in time, by either a more direct measure of concentration of the active principle, such as an analytical method (for example, mass spectrometry, high-pressure liquid chromatography, radioimmunoassay), or a more direct measurement of the effect, such as binding to a surface receptor in the case of many drugs, as the substance or its mechanism of action is better characterized.
Industry:Science
A method in which very large numbers of chemical entities are synthesized by condensing a small number of reagents togther in all combinations defined by a small set of reactions. The main objective of combinatorial chemistry is synthesis of arrays of chemical or biological compounds called libraries. These libraries are screened to identify useful components, such as drug candidates. Synthesis and screening are often treated as separate tasks because they require different conditions, instrumentation, and scientific expertise. Synthesis involves the development of new chemical reactions to produce the compounds, while screening aims to identify the biological effect of these compounds, such as strong binding to proteins and other biomolecular targets.
Industry:Science
A method of burning fuel in which the fuel is continually fed into a bed of reactive or inert particles supported by upflowing air, which causes the bed to behave like a turbulent fluid. Fluidized beds have long been used for the combustion of low-quality, difficult-to-burn fuels and more recently have been developed for the clean burning of coal.
Industry:Science
A method of chemical analysis based on the optical activity of the substance being determined. Optically active materials are asymmetric; that is, their molecules or crystals have no plane or center of symmetry. These asymmetric molecules can occur in either of two forms, <i>d</i>- and <i>l</i>-, called optical isomers. Asymmetric substances possess the power of rotating the plane of polarization of plane-polarized light. Measurement of the extent of this rotation is called polarimetry. Polarimetry is applied to both organic and inorganic materials.
Industry:Science
A method of chemical separation that involves passage of a liquid phase through a solid phase and relies on subtle chemical interactions to resolve complex mixtures into pure compounds. A small amount of the sample to be separated is injected onto the top of a column that is densely packed with spherical particles of small diameter, that is, the stationary phase. A liquid solvent, the mobile phase, flows through the column continuously to carry the sample from the top to the bottom of the column. During passage through the column, the components of the sample are transferred back and forth continuously between the two phases, and small thermodynamic differences in the chemical interactions of the various sample components with the mobile and stationary phases slow the passage of some solutes more than others and lead to their separation. The technique can be performed on very small scales for chemical analysis, dealing with micrograms or even nanograms of sample, or it can be performed on an industrial scale for purification of commercial products. The technique has great resolving power (see <b>illus.</b>).
Industry:Science
A method of communication employing electrical signaling impulses produced and received manually or by machines. Telegraph signals are transmitted over open wire or cable land lines, submarine cables, or radio. Telegraphy as a communication technique uses essentially a narrow frequency band and a transmission rate adapted to machine operations.
Industry:Science
A method of dating geological and archeological specimens by counting the radiation-damage tracks produced by spontaneous fission of uranium impurities in minerals and glasses. During fission two fragments of the uranium nucleus fly apart with high energy, traveling a total distance of about 25 micrometers (0.001 in.) and creating a single, narrow but continuous, submicroscopic trail of altered material, where atoms have been ejected from their normal positions. Such a trail, or track, can be revealed by using a chemical reagent to dissolve the altered material, and the trail can then be seen in an ordinary microscope. The holes produced in this way can be enlarged by continued chemical attack until they are visible to the unaided eye.
Industry:Science
