To make sense of the lectionary text, we have to back up to the beginning of Chapter First and foremost, this is a story about power—desire, too, I suppose—but ultimately power. The Ammonite threat persists, but David is now getting so settled and comfortable in his role as king that he no longer feels the need to go out and personally engage in the fighting. So he stays behind, on his couch.
Therefore, obtaining information about the spatial arrangement of atoms requires the use of radiation with shorter wavelengths, such as X-ray or neutron beams. Employing shorter wavelengths implied abandoning microscopy and true imaging, however, because there exists no material from which a lens capable of focusing this type of radiation can be created.
Sharp features in the diffraction pattern arise from periodic, repeating structure in the sample, which are often very strong due to coherent reflection of many photons from many regularly spaced instances of similar structure, while non-periodic components of the structure result in diffuse and usually weak diffraction features - areas with a higher density and repetition of atom order tend to reflect more light toward one point in space when compared to those areas with fewer atoms and less repetition.
Because of their highly ordered and repetitive structure, crystals give diffraction patterns of sharp Bragg reflection spots, and are ideal for analyzing the structure of solids.
Miller index Coordinates in square brackets such as  denote a direction vector in real space. Miller indices in parentheses such as denote a plane of the crystal structure, and regular repetitions of that plane with a particular spacing.
In the cubic system, the normal to the hkl plane is the direction [hkl], but in lower-symmetry cases, the normal to hkl is not parallel to [hkl]. Techniques[ edit ] Some materials that have been analyzed crystallographically, such as proteinsdo not occur naturally as crystals.
Typically, such molecules are placed in solution and allowed to slowly crystallize through vapor diffusion. A drop of solution containing the molecule, buffer, and precipitants is sealed in a container with a reservoir containing a hygroscopic solution.
Water in the drop diffuses to the reservoir, slowly increasing the concentration and allowing a crystal to form. If the concentration were to rise more quickly, the molecule would simply precipitate out of solution, resulting in disorderly granules rather than an orderly and hence usable crystal.
Once a crystal is obtained, data can be collected using a beam of radiation. Although many universities that engage in crystallographic research have their own X-ray producing equipment, synchrotrons are often used as X-ray sources, because of the purer and more complete patterns such sources can generate.
Synchrotron sources also have a much higher intensity of X-ray beams, so data collection takes a fraction of the time normally necessary at weaker sources. Complementary neutron crystallography techniques are used to identify the positions of hydrogen atoms, since X-rays only interact very weakly with light elements such as hydrogen.
Producing an image from a diffraction pattern requires sophisticated mathematics and often an iterative process of modelling and refinement. In this process, the mathematically predicted diffraction patterns of an hypothesized or "model" structure are compared to the actual pattern generated by the crystalline sample.
Ideally, researchers make several initial guesses, which through refinement all converge on the same answer. Models are refined until their predicted patterns match to as great a degree as can be achieved without radical revision of the model.
This is a painstaking process, made much easier today by computers. The mathematical methods for the analysis of diffraction data only apply to patterns, which in turn result only when waves diffract from orderly arrays.
Hence crystallography applies for the most part only to crystals, or to molecules which can be coaxed to crystallize for the sake of measurement.
In spite of this, a certain amount of molecular information can be deduced from patterns that are generated by fibers and powderswhich while not as perfect as a solid crystal, may exhibit a degree of order.
This level of order can be sufficient to deduce the structure of simple molecules, or to determine the coarse features of more complicated molecules. For example, the double-helical structure of DNA was deduced from an X-ray diffraction pattern that had been generated by a fibrous sample.
In materials science[ edit ] Crystallography is used by materials scientists to characterize different materials. In single crystals, the effects of the crystalline arrangement of atoms is often easy to see macroscopically, because the natural shapes of crystals reflect the atomic structure.
In addition, physical properties are often controlled by crystalline defects. The understanding of crystal structures is an important prerequisite for understanding crystallographic defects. Mostly, materials do not occur as a single crystal, but in poly-crystalline form i.
Because of this, the powder diffraction method, which takes diffraction patterns of polycrystalline samples with a large number of crystals, plays an important role in structural determination.
Other physical properties are also linked to crystallography. For example, the minerals in clay form small, flat, platelike structures. Clay can be easily deformed because the platelike particles can slip along each other in the plane of the plates, yet remain strongly connected in the direction perpendicular to the plates.
Such mechanisms can be studied by crystallographic texture measurements. In another example, iron transforms from a body-centered cubic bcc structure to a face-centered cubic fcc structure called austenite when it is heated. The fcc structure is a close-packed structure unlike the bcc structure; thus the volume of the iron decreases when this transformation occurs.
Crystallography is useful in phase identification. When manufacturing or using a material, it is generally desirable to know what compounds and what phases are present in the material, as their composition, structure and proportions will influence the material's properties.
Each phase has a characteristic arrangement of atoms. X-ray or neutron diffraction can be used to identify which patterns are present in the material, and thus which compounds are present.
Crystallography covers the enumeration of the symmetry patterns which can be formed by atoms in a crystal and for this reason is related to group theory and geometry.
Biology[ edit ] X-ray crystallography is the primary method for determining the molecular conformations of biological macromoleculesparticularly protein and nucleic acids such as DNA and RNA.Search Help This function enables you to search for a Keystone Symposia meeting by any word in the meeting title, location, organizer names, meeting summary or sessions (including session names, speaker names and talk titles).
Fifth, Davidson recognizes the acceleration of the narrative in 2 Samuel , which represents David’s impulsive behavior. Sixth, the sentence structure in 2 Samuel states that David “took her” and David “lay with her” emphasizing the psychological pressure applied by the subject of the sentence.
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CHEST Annual Meeting Abstracts. Find abstracts of original investigations from slides and posters presented at CHEST , held October , in San Antonio, Texas, featuring essential updates in lung diseases, improving patient care, and trends in morbidity and mortality..
Browse the CHEST Annual Meeting abstracts. Crystallography is the experimental science of determining the arrangement of atoms in crystalline solids (see crystal structure).The word "crystallography" derives from the Greek words crystallon "cold drop, frozen drop", with its meaning extending to all solids with some degree of transparency, and graphein "to write".
In July , the United Nations recognised the importance of the science. The linguistic description and analysis of 2 Samuel from the perspective of functional grammar in the tradition of Simon Dik.
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