X-ray crystallography is a technique used for determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to diffract into many specific directions. By measuring the angles and intensities of these diffracted beams, a crystallographer can produce a three-dimensional picture of the density of electrons within the crystal. From this electron density, the mean positions of the atoms in the crystal can be determined, as well as their chemical bonds, their crystallographic disorder, various other information. Since many materials can form crystals—such as salts, minerals, semiconductors, as well as various inorganic and biological molecules—X-ray crystallography has been fundamental in the development of many scientific fields. In its first decades of use, this method determined the size of atoms, the lengths and types of chemical bonds, the atomic-scale differences among various materials minerals and alloys; the method revealed the structure and function of many biological molecules, including vitamins, drugs and nucleic acids such as DNA.
X-ray crystallography is still the primary method for characterizing the atomic structure of new materials and in discerning materials that appear similar by other experiments. X-ray crystal structures can account for unusual electronic or elastic properties of a material, shed light on chemical interactions and processes, or serve as the basis for designing pharmaceuticals against diseases. In a single-crystal X-ray diffraction measurement, a crystal is mounted on a goniometer; the goniometer is used to position the crystal at selected orientations. The crystal is illuminated with a finely focused monochromatic beam of X-rays, producing a diffraction pattern of spaced spots known as reflections; the two-dimensional images taken at different orientations are converted into a three-dimensional model of the density of electrons within the crystal using the mathematical method of Fourier transforms, combined with chemical data known for the sample. Poor resolution or errors may result if the crystals are too small, or not uniform enough in their internal makeup.
X-ray crystallography is related to several other methods for determining atomic structures. Similar diffraction patterns can be produced by scattering electrons or neutrons, which are interpreted by Fourier transformation. If single crystals of sufficient size cannot be obtained, various other X-ray methods can be applied to obtain less detailed information. If the material under investigation is only available in the form of nanocrystalline powders or suffers from poor crystallinity, the methods of electron crystallography can be applied for determining the atomic structure. For all above mentioned X-ray diffraction methods, the scattering is elastic. By contrast, inelastic X-ray scattering methods are useful in studying excitations of the sample such as plasmons, crystal-field and orbital excitations and phonons, rather than the distribution of its atoms. Crystals, though long admired for their regularity and symmetry, were not investigated scientifically until the 17th century. Johannes Kepler hypothesized in his work Strena seu de Nive Sexangula that the hexagonal symmetry of snowflake crystals was due to a regular packing of spherical water particles.
The Danish scientist Nicolas Steno pioneered experimental investigations of crystal symmetry. Steno showed that the angles between the faces are the same in every exemplar of a particular type of crystal, René Just Haüy discovered that every face of a crystal can be described by simple stacking patterns of blocks of the same shape and size. Hence, William Hallowes Miller in 1839 was able to give each face a unique label of three small integers, the Miller indices which remain in use today for identifying crystal faces. Haüy's study led to the correct idea that crystals are a regular three-dimensional array of atoms and molecules. In the 19th century, a complete catalog of the possible symmetries of a crystal was worked out by Johan Hessel, Auguste Bravais, Evgraf Fedorov, Arthur Schönflies and William Barlow. From the available data and physical reasoning, Barlow proposed several crystal structures in the 1880s that were validated by X-ray crystallography. Wilhelm Röntgen discovered X-rays in 1895, just as the studies of crystal symmetry were being concluded.
Physicists were uncertain of the nature of X-rays, but soon suspected that they were waves of electromagnetic radiation—in other words, another form of light. At that time, the wave model of light—specifically, the Maxwell theory of electromagnetic radiation—was well accepted among scientists, experiments by Charles Glover Barkla showed that X-rays exhibited phenomena associated with electromagnetic waves, including transverse polarization and spectral lines akin to those observed in the visible wavelengths. Single-slit experiments in the laboratory of Arnold Sommerfeld suggested that X-rays had a wavelength of about 1 angstrom. However, X-rays are composed of photons, thus are not only waves of electromagnetic radiation but exhibit particle-like properties. Albert Einstein introduced the photon concept in 1905, but it was not broadly accepted until 1922, when Arthur
Crystallography is the experimental science of determining the arrangement of atoms in crystalline solids. 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, graphein "to write". In July 2012, the United Nations recognised the importance of the science of crystallography by proclaiming that 2014 would be the International Year of Crystallography. X-ray crystallography is used to determine the structure of large biomolecules such as proteins. Before the development of X-ray diffraction crystallography, the study of crystals was based on physical measurements of their geometry; this involved measuring the angles of crystal faces relative to each other and to theoretical reference axes, establishing the symmetry of the crystal in question. This physical measurement is carried out using a goniometer; the position in 3D space of each crystal face is plotted on a stereographic net such as a Wulff net or Lambert net.
The pole to each face is plotted on the net. Each point is labelled with its Miller index; the final plot allows the symmetry of the crystal to be established. Crystallographic methods now depend on analysis of the diffraction patterns of a sample targeted by a beam of some type. X-rays are most used; this is facilitated by the wave properties of the particles. Crystallographers explicitly state the type of beam used, as in the terms X-ray crystallography, neutron diffraction and electron diffraction; these three types of radiation interact with the specimen in different ways. X-rays interact with the spatial distribution of electrons in the sample. Electrons are charged particles and therefore interact with the total charge distribution of both the atomic nuclei and the electrons of the sample. Neutrons are scattered by the atomic nuclei through the strong nuclear forces, but in addition, the magnetic moment of neutrons is non-zero, they are therefore scattered by magnetic fields. When neutrons are scattered from hydrogen-containing materials, they produce diffraction patterns with high noise levels.
However, the material can sometimes be treated to substitute deuterium for hydrogen. Because of these different forms of interaction, the three types of radiation are suitable for different crystallographic studies. An image of a small object is made using a lens to focus the beam, similar to a lens in a microscope. However, the wavelength of visible light is three orders of magnitude longer than the length of typical atomic bonds and atoms themselves. 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, because there exists no material from which a lens capable of focusing this type of radiation can be created. Scientists have had some success focusing X-rays with microscopic Fresnel zone plates made from gold, by critical-angle reflection inside long tapered capillaries. Diffracted X-ray or neutron beams cannot be focused to produce images, so the sample structure must be reconstructed from the diffraction pattern.
Sharp features in the diffraction pattern arise from periodic, repeating structure in the sample, which are very strong due to coherent reflection of many photons from many spaced instances of similar structure, while non-periodic components of the structure result in diffuse 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 ordered and repetitive structure, crystals give diffraction patterns of sharp Bragg reflection spots, are ideal for analyzing the structure of solids. Coordinates in square brackets such as denote a direction vector. Coordinates in angle brackets or chevrons such as <100> denote a family of directions which are related by symmetry operations. In the cubic crystal system for example, <100> would mean, or the negative of any of those directions. Miller indices in parentheses such as denote a plane of the crystal structure, regular repetitions of that plane with a particular spacing.
In the cubic system, the normal to the plane is the direction, but in lower-symmetry cases, the normal to is not parallel to. Indices in curly brackets or braces such as denote a family of planes and their normals which are equivalent in cubic materials due to symmetry operations, much the way angle brackets denote a family of directions. In non-cubic materials, <hkl> is not perpendicular to. Some materials that have been analyzed crystallographically, such as proteins, do not occur as crystals; such molecules are placed in solution and allowed to crystallize through vapor diffusion. A drop of solution containing the molecule and precipitants is sealed in a container with a reservoir containing a hygroscopic solution. Water in the drop diffuses to the reservoir increasing the concentration and allowing a crystal to form. If the concentration were to rise more the molecule would 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 used as X-ray sources, bec
Middlesex is an ancient county in southeast England. It is now within the wider urbanised area of London, its area is now mostly within the ceremonial county of Greater London, with small sections in other neighbouring ceremonial counties. It was established in the Anglo-Saxon system from the territory of the Middle Saxons, existed as an official unit until 1965; the historic county includes land stretching north of the River Thames from 17 miles west to 3 miles east of the City of London with the rivers Colne and Lea and a ridge of hills as the other boundaries. The low-lying county, dominated by clay in its north and alluvium on gravel in its south, was the second smallest county by area in 1831; the City of London was a county in its own right from the 12th century and was able to exert political control over Middlesex. Westminster Abbey dominated most of the early financial and ecclesiastical aspects of the county; as London grew into Middlesex, the Corporation of London resisted attempts to expand the city boundaries into the county, which posed problems for the administration of local government and justice.
In the 18th and 19th centuries the population density was high in the southeast of the county, including the East End and West End of London. From 1855 the southeast was administered, with sections of Kent and Surrey, as part of the area of the Metropolitan Board of Works; when county councils were introduced in England in 1889 about 20% of the area of Middlesex, along with a third of its population, was transferred to the new County of London and the remainder became an administrative county governed by the Middlesex County Council that met at the Middlesex Guildhall in Westminster, in the County of London. The City of London, Middlesex, became separate counties for other purposes and Middlesex regained the right to appoint its own sheriff, lost in 1199. In the interwar years suburban London expanded further, with improvement and expansion of public transport, the setting up of new industries. After the Second World War, the population of the County of London and inner Middlesex was in steady decline, with high population growth continuing in the outer parts.
After a Royal Commission on Local Government in Greater London all of the original area was incorporated into an enlarged Greater London in 1965, with the rest transferred to neighbouring counties. Since 1965 various areas called. Middlesex was the former postal county of 25 post towns; the name refers to the tribal origin of its inhabitants. The word is formed from the Old English,'middel' and'Seaxe'. In 704, it is recorded as Middleseaxon in an Anglo-Saxon chronicle, written in Latin, about land at Twickenham; the Latin text reads: "in prouincia quæ nuncupatur Middelseaxan Haec". The Saxons derived their name from a kind of knife for which they were known; the seax has a lasting symbolic impact in the English counties of Essex and Middlesex, both of which feature three seaxes in their ceremonial emblem. Their names, along with those of Sussex and Wessex, contain a remnant of the word "Saxon". There were settlements in the area of Middlesex that can be traced back thousands of years before the creation of a county.
Middlesex was part of the Kingdom of Essex It was recorded in the Domesday Book as being divided into the six hundreds of Edmonton, Gore, Hounslow and Spelthorne. The City of London has been self-governing since the thirteenth century and became a county in its own right, a county corporate. Middlesex included Westminster, which had a high degree of autonomy. Of the six hundreds, Ossulstone contained the districts closest to the City of London. During the 17th century it was divided into four divisions, along with the Liberty of Westminster took over the administrative functions of the hundred; the divisions were named Finsbury, Holborn and Tower. The county had parliamentary representation from the 13th century; the title Earl of Middlesex was created twice, in 1622 and 1677, but became extinct in 1843. The economy of the county was dependent on the City of London from early times and was agricultural. A variety of goods were provided for the City, including crops such as grain and hay and building materials.
Recreation at day trip destinations such as Hackney, Islington and Twickenham, as well as coaching, inn-keeping and sale of goods and services at daily shops and stalls to the considerable passing trade provided much local employment and formed part of the early economy. However, during the 18th century the inner parishes of Middlesex became suburbs of the City and were urbanised; the Middlesex volume of John Norden's Speculum Britanniae of 1593 summarises: This is plentifully stored, as it seemeth beautiful, with many fair and comely buildings of the merchants of London, who have planted their houses of recreation not in the meanest places, which they have cunningly contrived, curiously beautified with divers devices, neatly decked with rare inventions, environed with orchards of sundry, delicate fruits, gardens with delectable walks, alleys and a great variety of pleasing dainties: all of which seem to be beautiful ornaments unto this country. Thomas Cox wrote in 1794: We may call it all London, being chiefly inhabited by the citizens, who fill the towns in it with their country houses, to which they resort that they may breathe a little sweet air, free from the fogs and smoke of the City.
In 1803 Sir John Sinclair, president of the Board of Agr
Integrated Authority File
The Integrated Authority File or GND is an international authority file for the organisation of personal names, subject headings and corporate bodies from catalogues. It is used for documentation in libraries and also by archives and museums; the GND is managed by the German National Library in cooperation with various regional library networks in German-speaking Europe and other partners. The GND falls under the Creative Commons Zero licence; the GND specification provides a hierarchy of high-level entities and sub-classes, useful in library classification, an approach to unambiguous identification of single elements. It comprises an ontology intended for knowledge representation in the semantic web, available in the RDF format; the Integrated Authority File became operational in April 2012 and integrates the content of the following authority files, which have since been discontinued: Name Authority File Corporate Bodies Authority File Subject Headings Authority File Uniform Title File of the Deutsches Musikarchiv At the time of its introduction on 5 April 2012, the GND held 9,493,860 files, including 2,650,000 personalised names.
There are seven main types of GND entities: LIBRIS Virtual International Authority File Information pages about the GND from the German National Library Search via OGND Bereitstellung des ersten GND-Grundbestandes DNB, 19 April 2012 From Authority Control to Linked Authority Data Presentation given by Reinhold Heuvelmann to the ALA MARC Formats Interest Group, June 2012
Dorsa Barlow is a wrinkle ridge system on the Moon, in Mare Tranquilitatis near the border with Mare Serenitatis, centered at 14.0°N 30.6°E / 14.0. It is about 110 km long and was named after British crystallographer William Barlow in 1976. Map of the region Dorsa Barlow at The Moon Wiki
Virtual International Authority File
The Virtual International Authority File is an international authority file. It is a joint project of several national libraries and operated by the Online Computer Library Center. Discussion about having a common international authority started in the late 1990s. After a series of failed attempts to come up with a unique common authority file, the new idea was to link existing national authorities; this would present all the benefits of a common file without requiring a large investment of time and expense in the process. The project was initiated by the US Library of Congress, the German National Library and the OCLC on August 6, 2003; the Bibliothèque nationale de France joined the project on October 5, 2007. The project transitioned to being a service of the OCLC on April 4, 2012; the aim is to link the national authority files to a single virtual authority file. In this file, identical records from the different data sets are linked together. A VIAF record receives a standard data number, contains the primary "see" and "see also" records from the original records, refers to the original authority records.
The data are available for research and data exchange and sharing. Reciprocal updating uses the Open Archives Initiative Protocol for Metadata Harvesting protocol; the file numbers are being added to Wikipedia biographical articles and are incorporated into Wikidata. VIAF's clustering algorithm is run every month; as more data are added from participating libraries, clusters of authority records may coalesce or split, leading to some fluctuation in the VIAF identifier of certain authority records. Authority control Faceted Application of Subject Terminology Integrated Authority File International Standard Authority Data Number International Standard Name Identifier Wikipedia's authority control template for articles Official website VIAF at OCLC
Islington is a district in Greater London and part of the London Borough of Islington. It is a residential district of Inner London, extending from Islington's High Street to Highbury Fields, encompassing the area around the busy High Street, Upper Street, Essex Road, Southgate Road to the east. Islington grew as a sprawling Middlesex village along the line of the Great North Road, has provided the name of the modern borough; this gave rise to some confusion, as neighbouring districts may be said to be in Islington. This district is bounded by Liverpool Road to the west and City Road and Southgate Road to the south-east, its northernmost point is in the area of Canonbury. The main north-south high street, Upper Street splits at Highbury Corner to Holloway Road to the west and St. Paul's Road to the east; the Angel business improvement district, an area centered around the Angel tube station, exists within southern Islington district and northern portions of two other districts in the London Borough of Islington – Finsbury and Pentonville.
Islington was named by the Saxons Giseldone Gislandune. The name means "Gīsla's hill" from the Old English personal name dun; the name mutated to Isledon, which remained in use well into the 17th century when the modern form arose. In medieval times, Islington was just one of many small manors thereabouts, along with Bernersbury, Neweton Berewe or Hey-bury and Canonesbury; some roads on the edge of the area, including Essex Road, were known as streets by the medieval period indicating a Roman origin, but little physical evidence remains. What is known is that the Great North Road from Aldersgate came into use in the 14th century, connecting with a new turnpike up Highgate Hill; this was along the line of modern Upper Street, with a toll gate at The Angel defining the extent of the village. The Back Road, the modern Liverpool Road, was a drovers' road where cattle would be rested before the final leg of their journey to Smithfield. Pens and sheds were erected along this road to accommodate the animals.
The first recorded church, St Mary's, was erected in the twelfth century and was replaced in the fifteenth century. Islington lay on the estates of the Dean and Chapter of St Pauls. There were substantial medieval moated manor houses in the area, principally at Canonbury and Highbury. In 1548, there were 440 communicants listed and the rural atmosphere, with access to the City and Westminster, made it a popular residence for the rich and eminent; the local inns harboured sheltered recusants. The Royal Agricultural Hall was built in 1862 on the Liverpool Road site of William Dixon's Cattle Layers; the hall was 75 ft high and the arched glass roof spanned 125 ft. It was built for the annual Smithfield Show in December of that year but was popular for other purposes, including recitals and the Royal Tournament, it was the primary exhibition site for London until the 20th century and the largest building of its kind, holding up to 50,000 people. It was requisitioned for use by the Mount Pleasant sorting office during World War II and never re-opened.
The main hall has now been incorporated into the Business Design Centre. The hill on which Islington stands has long supplied the City of London with water, the first projects drawing water through wooden pipes from the many springs that lay at its foot, in Finsbury; these included Sadler's London Spa and Clerkenwell. By the 17th century these traditional sources were inadequate to supply the growing population and plans were laid to construct a waterway, the New River, to bring fresh water from the source of the River Lea, in Hertfordshire to New River Head, below Islington in Finsbury; the river was opened on 29 September 1613 by the constructor of the project. His statue still stands; the course of the river ran to the east of Upper Street, much of its course is now covered and forms a linear park through the area. The Regent's Canal passes through Islington, for much of which in an 886-metre tunnel that runs from Colebrook Row east of the Angel, to emerge at Muriel Street near Caledonian Road.
The stretch is marked above with a series of pavement plaques so walkers may find their way from one entrance to the other. The area of the canal east of the tunnel and north of the City Road was once dominated by much warehousing and industry surrounding the large City Road Basin and Wenlock Basin; those old buildings that survive here are now residential or small creative work units. This stretch has one side accessed from the towpath; the canal was constructed in 1820 to carry cargo from Limehouse into the canal system. There is no tow-path in the tunnel so bargees had to walk their barges through, braced against the roof. Commercial use of the canal has declined since the 1960s. In the 17th and 18th centuries the availability of water made Islington a good place for growing vegetables to feed London; the manor became a popular excursion destination for Londoners, attracted to the area by its rural feel. Many public houses were therefore built to serve the needs of both the excursionists and travellers on the turnpike.
By 1716, there were 56 ale-house keepers in Upper Street offering pleasure and tea gardens, activities such as archery, skittle alleys and bowling. By the 18th century and dancing were offered, together with billiards, firework displays and balloon ascents; the King's Head Tavern, now a Victorian building with a theatre, has remained on the same site, opposite the parish c