Anthropology is the scientific study of humans and human behavior and societies in the past and present. Social anthropology and cultural anthropology study the values of societies. Linguistic anthropology studies. Biological or physical anthropology studies the biological development of humans. Archaeology, which studies past human cultures through investigation of physical evidence, is thought of as a branch of anthropology in the United States and Canada, while in Europe, it is viewed as a discipline in its own right or grouped under other related disciplines, such as history; the abstract noun anthropology is first attested in reference to history. Its present use first appeared in Renaissance Germany in the works of Otto Casmann, their New Latin anthropologia derived from the combining forms of the Greek words ánthrōpos and lógos. It began to be used in English via French Anthropologie, by the early 18th century. In 1647, the Bartholins, founders of the University of Copenhagen, defined l'anthropologie as follows: Anthropology, to say the science that treats of man, is divided ordinarily and with reason into Anatomy, which considers the body and the parts, Psychology, which speaks of the soul.
Sporadic use of the term for some of the subject matter occurred subsequently, such as the use by Étienne Serres in 1839 to describe the natural history, or paleontology, of man, based on comparative anatomy, the creation of a chair in anthropology and ethnography in 1850 at the National Museum of Natural History by Jean Louis Armand de Quatrefages de Bréau. Various short-lived organizations of anthropologists had been formed; the Société Ethnologique de Paris, the first to use Ethnology, was formed in 1839. Its members were anti-slavery activists; when slavery was abolished in France in 1848 the Société was abandoned. Meanwhile, the Ethnological Society of New York the American Ethnological Society, was founded on its model in 1842, as well as the Ethnological Society of London in 1843, a break-away group of the Aborigines' Protection Society; these anthropologists of the times were liberal, anti-slavery, pro-human-rights activists. They maintained international connections. Anthropology and many other current fields are the intellectual results of the comparative methods developed in the earlier 19th century.
Theorists in such diverse fields as anatomy and Ethnology, making feature-by-feature comparisons of their subject matters, were beginning to suspect that similarities between animals and folkways were the result of processes or laws unknown to them then. For them, the publication of Charles Darwin's On the Origin of Species was the epiphany of everything they had begun to suspect. Darwin himself arrived at his conclusions through comparison of species he had seen in agronomy and in the wild. Darwin and Wallace unveiled evolution in the late 1850s. There was an immediate rush to bring it into the social sciences. Paul Broca in Paris was in the process of breaking away from the Société de biologie to form the first of the explicitly anthropological societies, the Société d'Anthropologie de Paris, meeting for the first time in Paris in 1859; when he read Darwin, he became an immediate convert to Transformisme, as the French called evolutionism. His definition now became "the study of the human group, considered as a whole, in its details, in relation to the rest of nature".
Broca, being what today would be called a neurosurgeon, had taken an interest in the pathology of speech. He wanted to localize the difference between man and the other animals, which appeared to reside in speech, he discovered the speech center of the human brain, today called Broca's area after him. His interest was in Biological anthropology, but a German philosopher specializing in psychology, Theodor Waitz, took up the theme of general and social anthropology in his six-volume work, entitled Die Anthropologie der Naturvölker, 1859–1864; the title was soon translated as "The Anthropology of Primitive Peoples". The last two volumes were published posthumously. Waitz defined anthropology as "the science of the nature of man". By nature he meant matter animated by "the Divine breath". Following Broca's lead, Waitz points out that anthropology is a new field, which would gather material from other fields, but would differ from them in the use of comparative anatomy and psychology to differentiate man from "the animals nearest to him".
He stresses. The history of civilization, as well as ethnology, are to be brought into the comparison, it is to be presumed fundamentally that the species, man, is a unity, that "the same laws of thought are applicable to all men". Waitz was influential among the British ethnologists. In 1863 the explorer Richard Francis Burton and the speech therapist James Hunt broke away from the Ethnological Society of London to form the Anthropological Society of London, which henceforward would follow the path of the new anthropology rather than just ethnology, it was the 2nd society dedicated to general anthropology in existence. Representatives from the French Société were present. In his keynote address, printed in the first volume of its new publication, The Anthropological Review, Hunt stressed the work of Waitz, adopting his definitions as a standard. Among the first associates were the young Edward Burnett Tylor, inventor of cultural anthropology, his brother Alfred Tylor, a geologist. Edward had referred to himself as an ethnologist.
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The terms anno Domini and before Christ are used to label or number years in the Julian and Gregorian calendars. The term anno Domini is Medieval Latin and means "in the year of the Lord", but is presented using "our Lord" instead of "the Lord", taken from the full original phrase "anno Domini nostri Jesu Christi", which translates to "in the year of our Lord Jesus Christ"; this calendar era is based on the traditionally reckoned year of the conception or birth of Jesus of Nazareth, with AD counting years from the start of this epoch, BC denoting years before the start of the era. There is no year zero in this scheme, so the year AD 1 follows the year 1 BC; this dating system was devised in 525 by Dionysius Exiguus of Scythia Minor, but was not used until after 800. The Gregorian calendar is the most used calendar in the world today. For decades, it has been the unofficial global standard, adopted in the pragmatic interests of international communication and commercial integration, recognized by international institutions such as the United Nations.
Traditionally, English followed Latin usage by placing the "AD" abbreviation before the year number. However, BC is placed after the year number, which preserves syntactic order; the abbreviation is widely used after the number of a century or millennium, as in "fourth century AD" or "second millennium AD". Because BC is the English abbreviation for Before Christ, it is sometimes incorrectly concluded that AD means After Death, i.e. after the death of Jesus. However, this would mean that the approximate 33 years associated with the life of Jesus would neither be included in the BC nor the AD time scales. Terminology, viewed by some as being more neutral and inclusive of non-Christian people is to call this the Current or Common Era, with the preceding years referred to as Before the Common or Current Era. Astronomical year numbering and ISO 8601 avoid words or abbreviations related to Christianity, but use the same numbers for AD years; the Anno Domini dating system was devised in 525 by Dionysius Exiguus to enumerate the years in his Easter table.
His system was to replace the Diocletian era, used in an old Easter table because he did not wish to continue the memory of a tyrant who persecuted Christians. The last year of the old table, Diocletian 247, was followed by the first year of his table, AD 532; when he devised his table, Julian calendar years were identified by naming the consuls who held office that year—he himself stated that the "present year" was "the consulship of Probus Junior", 525 years "since the incarnation of our Lord Jesus Christ". Thus Dionysius implied that Jesus' incarnation occurred 525 years earlier, without stating the specific year during which his birth or conception occurred. "However, nowhere in his exposition of his table does Dionysius relate his epoch to any other dating system, whether consulate, year of the world, or regnal year of Augustus. Among the sources of confusion are: In modern times, incarnation is synonymous with the conception, but some ancient writers, such as Bede, considered incarnation to be synonymous with the Nativity.
The civil or consular year began on 1 January but the Diocletian year began on 29 August. There were inaccuracies in the lists of consuls. There were confused summations of emperors' regnal years, it is not known. Two major theories are that Dionysius based his calculation on the Gospel of Luke, which states that Jesus was "about thirty years old" shortly after "the fifteenth year of the reign of Tiberius Caesar", hence subtracted thirty years from that date, or that Dionysius counted back 532 years from the first year of his new table, it has been speculated by Georges Declercq that Dionysius' desire to replace Diocletian years with a calendar based on the incarnation of Christ was intended to prevent people from believing the imminent end of the world. At the time, it was believed by some that the resurrection of the dead and end of the world would occur 500 years after the birth of Jesus; the old Anno Mundi calendar theoretically commenced with the creation of the world based on information in the Old Testament.
It was believed that, based on the Anno Mundi calendar, Jesus was born in the year 5500 with the year 6000 of the Anno Mundi calendar marking the end of the world. Anno Mundi 6000 was thus equated with the resurrection and the end of the world but this date had passed in the time of Dionysius; the Anglo-Saxon historian the Venerable Bede, familiar with the work of Dionysius Exiguus, used Anno Domini dating in his Ecclesiastical History of the English People, completed in 731. In this same history, he used another Latin term, ante vero incarnationis dominicae tempus anno sexagesimo, equivalent to the English "before Christ", to identify years before the first year of this era. Both Dionysius and Bede regarded Anno Domini as beginning at the incarnation of Jesus, but "the distinction between Incarnation and Nativity was not drawn until the late 9th century, when in some places the Incarnation epoch was identified with Christ's conception, i.e. the Annunciation on March 25". On the continent of Europe, Anno
The Booch method is a method for object-oriented software development. It is composed of an object modeling language, an iterative object-oriented development process, a set of recommended practices; the method was authored by Grady Booch when he was working for Rational Software, published in 1992 and revised in 1994. It was used in software engineering for object-oriented analysis and design and benefited from ample documentation and support tools; the notation aspect of the Booch method was superseded by the Unified Modeling Language, which features graphical elements from the Booch method along with elements from the object-modeling technique and object-oriented software engineering. Methodological aspects of the Booch method have been incorporated into several methodologies and processes, the primary such methodology being the Rational Unified Process; the Booch notation is characterized by cloud shapes to represent classes and distinguishes the following diagrams: The process is organized around a macro and a micro process.
The macro process identifies the following activities cycle: Conceptualization: establish core requirements Analysis: develop a model of the desired behavior Design: create an architecture Evolution: for the implementation Maintenance: for evolution after the deliveryThe micro process is applied to new classes, structures or behaviors that emerge during the macro process. It is made of the following cycle: Identification of classes and objects Identification of their semantics Identification of their relationships Specification of their interfaces and implementation Class diagrams, Object diagrams, State Event diagrams and Module diagrams; the Booch Method of Object-Oriented Analysis & Design
Hasan Ibn al-Haytham was an Arab mathematician and physicist of the Islamic Golden Age. Sometimes called "the father of modern optics", he made significant contributions to the principles of optics and visual perception in particular, his most influential work being his Kitāb al-Manāẓir, written during 1011–1021, which survived in the Latin edition. A polymath, he wrote on philosophy and medicine. Ibn al-Haytham was the first to explain that vision occurs when light bounces on an object and is directed to one's eyes, and he was the first to point out. He was an early proponent of the concept that a hypothesis must be proved by experiments based on confirmable procedures or mathematical evidence—hence understanding the scientific method five centuries before Renaissance scientists. Born in Basra, he spent most of his productive period in the Fatimid capital of Cairo and earned his living authoring various treatises and tutoring members of the nobilities. Ibn al-Haytham is sometimes given the byname al-Baṣrī after his birthplace, or al-Miṣrī.
Ibn al-Haytham was nicknamed the "Second Ptolemy" by Abu'l-Hasan Bayhaqi, the "The Physicist" by John Peckham. Ibn al-Haytham paved the way for the modern science of physical optics. Ibn al-Haytham was born c. 965 to an Arab family in Basra, at the time part of the Buyid emirate. He held a position with the title vizier in his native Basra, made a name for himself for his knowledge of applied mathematics; as he claimed to be able to regulate the flooding of the Nile, he was invited to by Fatimid Caliph al-Hakim in order to realise a hydraulic project at Aswan. However, Ibn al-Haytham was forced to concede the impracticability of his project. Upon his return to Cairo, he was given an administrative post. After he proved unable to fulfill this task as well, he contracted the ire of the caliph Al-Hakim bi-Amr Allah, is said to have been forced into hiding until the caliph's death in 1021, after which his confiscated possessions were returned to him. Legend was kept under house arrest during this period.
During this time, he wrote his influential Book of Optics. Alhazen continued to live in Cairo, in the neighborhood of the famous University of al-Azhar, lived from the proceeds of his literary production until his death in c. 1040. Among his students were Sorkhab, a Persian from Semnan, Abu al-Wafa Mubashir ibn Fatek, an Egyptian prince. Alhazen's most famous work is his seven-volume treatise on optics Kitab al-Manazir, written from 1011 to 1021. Optics was translated into Latin by an unknown scholar at the end of the 12th century or the beginning of the 13th century, it was printed by Friedrich Risner in 1572, with the title Opticae thesaurus: Alhazeni Arabis libri septem, nuncprimum editi. Risner is the author of the name variant "Alhazen"; this work enjoyed a great reputation during the Middle Ages. Works by Alhazen on geometric subjects were discovered in the Bibliothèque nationale in Paris in 1834 by E. A. Sedillot. In all, A. Mark Smith has accounted for 18 full or near-complete manuscripts, five fragments, which are preserved in 14 locations, including one in the Bodleian Library at Oxford, one in the library of Bruges.
Two major theories on vision prevailed in classical antiquity. The first theory, the emission theory, was supported by such thinkers as Euclid and Ptolemy, who believed that sight worked by the eye emitting rays of light; the second theory, the intromission theory supported by Aristotle and his followers, had physical forms entering the eye from an object. Previous Islamic writers had argued on Euclidean, Galenist, or Aristotelian lines; the strongest influence on the Book of Optics was from Ptolemy's Optics, while the description of the anatomy and physiology of the eye was based on Galen's account. Alhazen's achievement was to come up with a theory that combined parts of the mathematical ray arguments of Euclid, the medical tradition of Galen, the intromission theories of Aristotle. Alhazen's intromission theory followed al-Kindi in asserting that "from each point of every colored body, illuminated by any light, issue light and color along every straight line that can be drawn from that point".
This however left him with the problem of explaining how a coherent image was formed from many independent sources of radiation. What Alhazen needed was for each point on an object to correspond to one point only on the eye, he attempted to resolve this by asserting that the eye would only perceive perpendicular rays from the object—for any one point on the eye, only the ray that reached it directly, without being refracted by any other part of the eye, would be perceived. He argued, using a physical analogy, that perpendicular rays were stronger than oblique rays: in the same way that a ball thrown directly at a board might break the board, whereas a ball thrown obliquely at the board would glance off, perpendicular rays were stronger than refracted rays, it was only perpendicular rays which were perceived
Structural analysis is the determination of the effects of loads on physical structures and their components. Structures subject to this type of analysis include all that must withstand loads, such as buildings, vehicles, attire, soil strata and biological tissue. Structural analysis employs the fields of applied mechanics, materials science and applied mathematics to compute a structure's deformations, internal forces, support reactions and stability; the results of the analysis are used to verify a structure's fitness for use precluding physical tests. Structural analysis is thus a key part of the engineering design of structures. A structure refers to a system of connected parts used to support a load. Important examples related to Civil Engineering include buildings and towers. To design a structure, an engineer must account for its safety and serviceability, while considering economic and environmental constraints. Other branches of engineering work on a wide variety of non-building structures.
A structural system is the combination of their materials. It is important for a structural engineer to be able to classify a structure by either its form or its function, by recognizing the various elements composing that structure; the structural elements guiding the systemic forces through the materials are not only such as a connecting rod, a truss, a beam, or a column, but a cable, an arch, a cavity or channel, an angle, a surface structure, or a frame. Once the dimensional requirement for a structure have been defined, it becomes necessary to determine the loads the structure must support. Structural design, therefore begins with specifying loads; the design loading for a structure is specified in building codes. There are two types of codes: general building codes and design codes, engineers must satisfy all of the code's requirements in order for the structure to remain reliable. There are two types of loads; the first type of loads are dead loads that consist of the weights of the various structural members and the weights of any objects that are permanently attached to the structure.
For example, beams, the floor slab, walls, plumbing, electrical fixtures, other miscellaneous attachments. The second type of loads are live loads which vary in their location. There are many different types of live loads like building loads, highway bridge loads, railroad bridge loads, impact loads, wind loads, snow loads, earthquake loads, other natural loads. To perform an accurate analysis a structural engineer must determine information such as structural loads, support conditions, material properties; the results of such an analysis include support reactions and displacements. This information is compared to criteria that indicate the conditions of failure. Advanced structural analysis may examine dynamic response and non-linear behavior. There are three approaches to the analysis: the mechanics of materials approach, the elasticity theory approach, the finite element approach; the first two make use of analytical formulations which apply simple linear elastic models, leading to closed-form solutions, can be solved by hand.
The finite element approach is a numerical method for solving differential equations generated by theories of mechanics such as elasticity theory and strength of materials. However, the finite-element method depends on the processing power of computers and is more applicable to structures of arbitrary size and complexity. Regardless of approach, the formulation is based on the same three fundamental relations: equilibrium and compatibility; the solutions are approximate when any of these relations are only satisfied, or only an approximation of reality. Each method has noteworthy limitations; the method of mechanics of materials is limited to simple structural elements under simple loading conditions. The structural elements and loading conditions allowed, are sufficient to solve many useful engineering problems; the theory of elasticity allows the solution of structural elements of general geometry under general loading conditions, in principle. Analytical solution, however, is limited to simple cases.
The solution of elasticity problems requires the solution of a system of partial differential equations, more mathematically demanding than the solution of mechanics of materials problems, which require at most the solution of an ordinary differential equation. The finite element method is the most restrictive and most useful at the same time; this method itself relies upon other structural theories for equations to solve. It does, make it possible to solve these equations with complex geometry and loading conditions, with the restriction that there is always some numerical error. Effective and reliable use of this method requires a solid understanding of its limitations; the simplest of the three methods here discussed, the mechanics of materials method is available for simple structural members subject to specific loadings such as axially loaded bars, prismatic beams in a state of pure bending, circular shafts subject to torsion. The solutions can under certain con
Isotope analysis is the identification of isotopic signature, the abundance of certain stable isotopes and chemical elements within organic and inorganic compounds. Isotopic analysis can be used to understand the flow of energy through a food web, to reconstruct past environmental and climatic conditions, to investigate human and animal diets in the past, for food authentification, a variety of other physical, geological and chemical processes. Stable isotope ratios are measured using mass spectrometry, which separates the different isotopes of an element on the basis of their mass-to-charge ratio. Isotopic oxygen is incorporated into the body through ingestion at which point it is used in the formation of, for archaeological purposes and teeth; the oxygen is incorporated into the hydroxylcarbonic apatite of tooth enamel. Bone is continually remodelled throughout the lifetime of an individual. Although the rate of turnover of isotopic oxygen in hydroxyapatite is not known, it is assumed to be similar to that of collagen.
Should an individual remain in a region for 10 years or longer, the isotopic oxygen ratios in the bone hydroxyapatite would reflect the oxygen ratios present in that region. Teeth are not subject to continual remodelling and so their isotopic oxygen ratios remain constant from the time of formation; the isotopic oxygen ratios of teeth represent the ratios of the region in which the individual was born and raised. Where deciduous teeth are present, it is possible to determine the age at which a child was weaned. Breast milk production draws upon the body water of the mother, which has higher levels of 18O due to the preferential loss of 16O through sweat and expired water vapour. While teeth are more resistant to chemical and physical changes over time, both are subject to post-depositional diagenesis; as such, isotopic analysis makes use of the more resistant phosphate groups, rather than the less abundant hydroxyl group or the more diagenetic carbonate groups present. Isotope analysis has widespread applicability in the natural sciences.
These include numerous applications in the biological and environmental sciences. Archaeological materials, such as bone, organic residues, hair, or sea shells, can serve as substrates for isotopic analysis. Carbon and zinc isotope ratios are used to investigate the diets of past people. Carbon isotopes are analysed in archaeology to determine the source of carbon at the base of the foodchain. Examining the 12C/13C isotope ratio, it is possible to determine whether animals and humans ate predominantly C3 or C4 plants. Potential C3 food sources include wheat, tubers, fruits and many vegetables, while C4 food sources include millet and sugar cane. Carbon isotope ratios can be used to distinguish between marine and terrestrial food sources. Carbon isotope ratios can be measured in bone collagen or bone mineral, each of these fractions of bone can be analysed to shed light on different components of diet; the carbon in bone collagen is predominantly sourced from dietary protein, while the carbon found in bone mineral is sourced from all consumed dietary carbon, included carbohydrates and protein.
To obtain an accurate picture of palaeodiets, it is important to understand processes of diagenesis that may affect the original isotopic signal. It is important for the researcher to know the variations of isotopes within individuals, between individuals, over time. Isotope analysis has been useful in archaeology as a means of characterization. Characterization of artifacts involves determining the isotopic composition of possible source materials such as metal ore bodies and comparing these data to the isotopic composition of analyzed artifacts. A wide range of archaeological materials such as metals and lead-based pigments have been sourced using isotopic characterization. In the Bronze Age Mediterranean, lead isotope analysis has been a useful tool for determining the sources of metals and an important indicator of trade patterns. Interpretation of lead isotope data is, however contentious and faces numerous instrumental and methodological challenges. Problems such as the mixing and re-using of metals from different sources, limited reliable data and contamination of samples can be difficult problems in interpretation.
All biologically active elements exist in a number of different isotopic forms, of which two or more are stable. For example, most carbon is present as 12C, with 1% being 13C; the ratio of the two isotopes may be altered by biological and geophysical processes, these differences can be utilized in a number of ways by ecologists. The main elements used in isotope ecology are carbon, oxygen and sulfur, but include silicon and strontium. Stable isotopes have become a popular method for understanding aquatic ecosystems because they can help scientists in understanding source links and process information in marine food webs; these analyses can be used to a certain degree in terrestrial systems. Certain isotopes can signify distinct primary producers forming the bases of food webs and trophic level positioning; the stable isotope compositions are expressed in terms of delta values in permil, i.e. parts per thousand differences from a standard. They express the proportion of an isotope, in a sample.
The values are expressed as: δX = × 103where X represents the isotope of interest and R represents the ratio of the isotope of
A chemical reaction is a process that leads to the chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the positions of electrons in the forming and breaking of chemical bonds between atoms, with no change to the nuclei, can be described by a chemical equation. Nuclear chemistry is a sub-discipline of chemistry that involves the chemical reactions of unstable and radioactive elements where both electronic and nuclear changes can occur; the substance involved in a chemical reaction are called reactants or reagents. Chemical reactions are characterized by a chemical change, they yield one or more products, which have properties different from the reactants. Reactions consist of a sequence of individual sub-steps, the so-called elementary reactions, the information on the precise course of action is part of the reaction mechanism. Chemical reactions are described with chemical equations, which symbolically present the starting materials, end products, sometimes intermediate products and reaction conditions.
Chemical reactions happen at a characteristic reaction rate at a given temperature and chemical concentration. Reaction rates increase with increasing temperature because there is more thermal energy available to reach the activation energy necessary for breaking bonds between atoms. Reactions may proceed in the forward or reverse direction until they go to completion or reach equilibrium. Reactions that proceed in the forward direction to approach equilibrium are described as spontaneous, requiring no input of free energy to go forward. Non-spontaneous reactions require input of free energy to go forward. Different chemical reactions are used in combinations during chemical synthesis in order to obtain a desired product. In biochemistry, a consecutive series of chemical reactions form metabolic pathways; these reactions are catalyzed by protein enzymes. Enzymes increase the rates of biochemical reactions, so that metabolic syntheses and decompositions impossible under ordinary conditions can occur at the temperatures and concentrations present within a cell.
The general concept of a chemical reaction has been extended to reactions between entities smaller than atoms, including nuclear reactions, radioactive decays, reactions between elementary particles, as described by quantum field theory. Chemical reactions such as combustion in fire and the reduction of ores to metals were known since antiquity. Initial theories of transformation of materials were developed by Greek philosophers, such as the Four-Element Theory of Empedocles stating that any substance is composed of the four basic elements – fire, water and earth. In the Middle Ages, chemical transformations were studied by Alchemists, they attempted, in particular, to convert lead into gold, for which purpose they used reactions of lead and lead-copper alloys with sulfur. The production of chemical substances that do not occur in nature has long been tried, such as the synthesis of sulfuric and nitric acids attributed to the controversial alchemist Jābir ibn Hayyān; the process involved heating of sulfate and nitrate minerals such as copper sulfate and saltpeter.
In the 17th century, Johann Rudolph Glauber produced hydrochloric acid and sodium sulfate by reacting sulfuric acid and sodium chloride. With the development of the lead chamber process in 1746 and the Leblanc process, allowing large-scale production of sulfuric acid and sodium carbonate chemical reactions became implemented into the industry. Further optimization of sulfuric acid technology resulted in the contact process in the 1880s, the Haber process was developed in 1909–1910 for ammonia synthesis. From the 16th century, researchers including Jan Baptist van Helmont, Robert Boyle, Isaac Newton tried to establish theories of the experimentally observed chemical transformations; the phlogiston theory was proposed in 1667 by Johann Joachim Becher. It postulated the existence of a fire-like element called "phlogiston", contained within combustible bodies and released during combustion; this proved to be false in 1785 by Antoine Lavoisier who found the correct explanation of the combustion as reaction with oxygen from the air.
Joseph Louis Gay-Lussac recognized in 1808 that gases always react in a certain relationship with each other. Based on this idea and the atomic theory of John Dalton, Joseph Proust had developed the law of definite proportions, which resulted in the concepts of stoichiometry and chemical equations. Regarding the organic chemistry, it was long believed that compounds obtained from living organisms were too complex to be obtained synthetically. According to the concept of vitalism, organic matter was endowed with a "vital force" and distinguished from inorganic materials; this separation was ended however by the synthesis of urea from inorganic precursors by Friedrich Wöhler in 1828. Other chemists who brought major contributions to organic chemistry include Alexander William Williamson with his synthesis of ethers and Christopher Kelk Ingold, among many discoveries, established the mechanisms of substitution reactions. Chemical equations are used to graphically illustrate chemical reactions, they consist of chemical or structural formulas of the reactants on the left and those of the products on the right.
They are separated by an arrow which indicates the type of the reaction.