title: History of chemistry

text: Dalton also proposed a modern atomic theory in 1803 which stated that all matter was composed of small indivisible particles termed atoms, atoms of a given element possess unique characteristics and weight, and three types of atoms exist: simple (elements), compound (simple molecules), and complex (complex molecules). In 1808, Dalton first published "New System of Chemical Philosophy" (1808–1827), in which he outlined the first modern scientific description of the atomic theory. This work identified chemical elements as a specific type of atom, therefore rejecting Newton's theory of chemical affinities.


title: History of chemistry

text: Instead, Dalton inferred proportions of elements in compounds by taking ratios of the weights of reactants, setting the atomic weight of hydrogen to be identically one. Following Jeremias Benjamin Richter (known for introducing the term "stoichiometry"), he proposed that chemical elements combine in integral ratios. This is known as the law of multiple proportions or Dalton's law, and Dalton included a clear description of the law in his "New System of Chemical Philosophy". The law of multiple proportions is one of the basic laws of stoichiometry used to establish the atomic theory. Despite the importance of the work as the first view of atoms as physically real entities and the introduction of a system of chemical symbols, "New System of Chemical Philosophy" devoted almost as much space to the caloric theory as to atomism.


title: History of chemistry

text: French chemist Joseph Proust proposed the law of definite proportions, which states that elements always combine in small, whole number ratios to form compounds, based on several experiments conducted between 1797 and 1804 Along with the law of multiple proportions, the law of definite proportions forms the basis of stoichiometry. The law of definite proportions and constant composition do not prove that atoms exist, but they are difficult to explain without assuming that chemical compounds are formed when atoms combine in constant proportions.


title: History of chemistry

text: A Swedish chemist and disciple of Dalton, Jöns Jacob Berzelius embarked on a systematic program to try to make accurate and precise quantitative measurements and to ensure the purity of chemicals. Along with Lavoisier, Boyle, and Dalton, Berzelius is known as the father of modern chemistry. In 1828 he compiled a table of relative atomic weights, where oxygen was used as a standard, with its weight set at 100, and which included all of the elements known at the time. This work provided evidence in favor of Dalton's atomic theory – that inorganic chemical compounds are composed of atoms combined in whole number amounts. He determined the exact elementary constituents of a large number of compounds; the results strongly supported Proust's Law of Definite Proportions. In discovering that atomic weights are not integer multiples of the weight of hydrogen, Berzelius also disproved Prout's hypothesis that elements are built up from atoms of hydrogen.


title: History of chemistry

text: Motivated by his extensive atomic weight determinations and in a desire to aid his experiments, he introduced the classical system of chemical symbols and notation with his 1808 publication "Lärbok i Kemien", in which elements are abbreviated to one or two letters to make a distinct symbol from their Latin name. This system of chemical notation—in which the elements were given simple written labels, such as O for oxygen, or Fe for iron, with proportions denoted by numbers—is the same basic system used today. The only difference is that instead of the subscript number used today (e.g., HO), Berzelius used a superscript (HO). Berzelius is credited with identifying the chemical elements silicon, selenium, thorium, and cerium. Students working in Berzelius's laboratory also discovered lithium and vanadium.


title: History of chemistry

text: Berzelius developed the radical theory of chemical combination, which holds that reactions occur as stable groups of atoms called radicals are exchanged between molecules. He believed that salts are compounds formed of acids and bases, and discovered that the anions in acids were attracted to a positive electrode (the anode), whereas the cations in a base were attracted to a negative electrode (the cathode). Berzelius did not believe in the Vitalism Theory, but instead in a regulative force which produced organization of tissues in an organism. Berzelius is also credited with originating the chemical terms "catalysis", "polymer", "isomer", and "allotrope", although his original definitions differ dramatically from modern usage. For example, he coined the term "polymer" in 1833 to describe organic compounds which shared identical empirical formulas but which differed in overall molecular weight, the larger of the compounds being described as "polymers" of the smallest. By this long-superseded, pre-structural definition, glucose (CHO) was viewed as a polymer of formaldehyde (CHO).


title: History of chemistry

text: English chemist Humphry Davy was a pioneer in the field of electrolysis, using Alessandro Volta's voltaic pile to split up common compounds and thus isolate a series of new elements. He went on to electrolyse molten salts and discovered several new metals, especially sodium and potassium, highly reactive elements known as the alkali metals. Potassium, the first metal that was isolated by electrolysis, was discovered in 1807 by Davy, who derived it from caustic potash (KOH). Before the 19th century, no distinction was made between potassium and sodium. Sodium was first isolated by Davy in the same year by passing an electric current through molten sodium hydroxide (NaOH). When Davy heard that Berzelius and Pontin prepared calcium amalgam by electrolyzing lime in mercury, he tried it himself. Davy was successful, and discovered calcium in 1808 by electrolyzing a mixture of lime and mercuric oxide. He worked with electrolysis throughout his life and, in 1808, he isolated magnesium, strontium and barium.


title: History of chemistry

text: Davy also experimented with gases by inhaling them. This experimental procedure nearly proved fatal on several occasions, but led to the discovery of the unusual effects of nitrous oxide, which came to be known as laughing gas. Chlorine was discovered in 1774 by Swedish chemist Carl Wilhelm Scheele, who called it ""dephlogisticated marine acid"" (see phlogiston theory) and mistakenly thought it contained oxygen. Scheele observed several properties of chlorine gas, such as its bleaching effect on litmus, its deadly effect on insects, its yellow-green colour, and the similarity of its smell to that of aqua regia. However, Scheele was unable to publish his findings at the time. In 1810, chlorine was given its current name by Humphry Davy (derived from the Greek word for green), who insisted that chlorine was in fact an element. He also showed that oxygen could not be obtained from the substance known as oxymuriatic acid (HCl solution). This discovery overturned Lavoisier's definition of acids as compounds of oxygen. Davy was a popular lecturer and able experimenter.


title: History of chemistry

text: French chemist Joseph Louis Gay-Lussac shared the interest of Lavoisier and others in the quantitative study of the properties of gases. From his first major program of research in 1801–1802, he concluded that equal volumes of all gases expand equally with the same increase in temperature: this conclusion is usually called "Charles's law", as Gay-Lussac gave credit to Jacques Charles, who had arrived at nearly the same conclusion in the 1780s but had not published it. The law was independently discovered by British natural philosopher John Dalton by 1801, although Dalton's description was less thorough than Gay-Lussac's. In 1804 Gay-Lussac made several daring ascents of over 7,000 meters above sea level in hydrogen-filled balloons—a feat not equaled for another 50 years—that allowed him to investigate other aspects of gases. Not only did he gather magnetic measurements at various altitudes, but he also took pressure, temperature, and humidity measurements and samples of air, which he later analyzed chemically.


title: History of chemistry

text: In 1808 Gay-Lussac announced what was probably his single greatest achievement: from his own and others' experiments he deduced that gases at constant temperature and pressure combine in simple numerical proportions by volume, and the resulting product or products—if gases—also bear a simple proportion by volume to the volumes of the reactants. In other words, gases under equal conditions of temperature and pressure react with one another in volume ratios of small whole numbers. This conclusion subsequently became known as "Gay-Lussac's law" or the "Law of Combining Volumes". With his fellow professor at the École Polytechnique, Louis Jacques Thénard, Gay-Lussac also participated in early electrochemical research, investigating the elements discovered by its means. Among other achievements, they decomposed boric acid by using fused potassium, thus discovering the element boron. The two also took part in contemporary debates that modified Lavoisier's definition of acids and furthered his program of analyzing organic compounds for their oxygen and hydrogen content.


title: History of chemistry

text: The element iodine was discovered by French chemist Bernard Courtois in 1811. Courtois gave samples to his friends, Charles Bernard Desormes (1777–1862) and Nicolas Clément (1779–1841), to continue research. He also gave some of the substance to Gay-Lussac and to physicist André-Marie Ampère. On December 6, 1813, Gay-Lussac announced that the new substance was either an element or a compound of oxygen. It was Gay-Lussac who suggested the name ""iode"", from the Greek word ιώδες (iodes) for violet (because of the color of iodine vapor). Ampère had given some of his sample to Humphry Davy. Davy did some experiments on the substance and noted its similarity to chlorine. Davy sent a letter dated December 10 to the Royal Society of London stating that he had identified a new element. Arguments erupted between Davy and Gay-Lussac over who identified iodine first, but both scientists acknowledged Courtois as the first to isolate the element.


title: History of chemistry

text: In 1815, Humphry Davy invented the Davy lamp, which allowed miners within coal mines to work safely in the presence of flammable gases. There had been many mining explosions caused by firedamp or methane often ignited by open flames of the lamps then used by miners. Davy conceived of using an iron gauze to enclose a lamp's flame, and so prevent the methane burning inside the lamp from passing out to the general atmosphere. Although the idea of the safety lamp had already been demonstrated by William Reid Clanny and by the then unknown (but later very famous) engineer George Stephenson, Davy's use of wire gauze to prevent the spread of flame was used by many other inventors in their later designs. There was some discussion as to whether Davy had discovered the principles behind his lamp without the help of the work of Smithson Tennant, but it was generally agreed that the work of both men had been independent. Davy refused to patent the lamp, and its invention led to him being awarded the Rumford medal in 1816.


title: History of chemistry

text: After Dalton published his atomic theory in 1808, certain of his central ideas were soon adopted by most chemists. However, uncertainty persisted for half a century about how atomic theory was to be configured and applied to concrete situations; chemists in different countries developed several different incompatible atomistic systems. A paper that suggested a way out of this difficult situation was published as early as 1811 by the Italian physicist Amedeo Avogadro (1776–1856), who hypothesized that equal volumes of gases at the same temperature and pressure contain equal numbers of molecules, from which it followed that relative molecular weights of any two gases are the same as the ratio of the densities of the two gases under the same conditions of temperature and pressure. Avogadro also reasoned that simple gases were not formed of solitary atoms but were instead compound molecules of two or more atoms. Thus Avogadro was able to overcome the difficulty that Dalton and others had encountered when Gay-Lussac reported that above 100 °C the volume of water vapor was twice the volume of the oxygen used to form it. According to Avogadro, the molecule of oxygen had split into two atoms in the course of forming water vapor.


title: History of chemistry

text: Avogadro's hypothesis was neglected for half a century after it was first published. Many reasons for this neglect have been cited, including some theoretical problems, such as Jöns Jacob Berzelius's "dualism", which asserted that compounds are held together by the attraction of positive and negative electrical charges, making it inconceivable that a molecule composed of two electrically similar atoms—as in oxygen—could exist. An additional barrier to acceptance was the fact that many chemists were reluctant to adopt physical methods (such as vapour-density determinations) to solve their problems. By mid-century, however, some leading figures had begun to view the chaotic multiplicity of competing systems of atomic weights and molecular formulas as intolerable. Moreover, purely chemical evidence began to mount that suggested Avogadro's approach might be right after all. During the 1850s, younger chemists, such as Alexander Williamson in England, Charles Gerhardt and Charles-Adolphe Wurtz in France, and August Kekulé in Germany, began to advocate reforming theoretical chemistry to make it consistent with Avogadrian theory.


title: History of chemistry

text: In 1825, Friedrich Wöhler and Justus von Liebig performed the first confirmed discovery and explanation of isomers, earlier named by Berzelius. Working with cyanic acid and fulminic acid, they correctly deduced that isomerism was caused by differing arrangements of atoms within a molecular structure. In 1827, William Prout classified biomolecules into their modern groupings: carbohydrates, proteins and lipids. After the nature of combustion was settled, a dispute about vitalism and the essential distinction between organic and inorganic substances began. The vitalism question was revolutionized in 1828 when Friedrich Wöhler synthesized urea, thereby establishing that organic compounds could be produced from inorganic starting materials and disproving the theory of vitalism.


title: History of chemistry

text: This opened a new research field in chemistry, and by the end of the 19th century, scientists were able to synthesize hundreds of organic compounds. The most important among them are mauve, magenta, and other synthetic dyes, as well as the widely used drug aspirin. The discovery of the artificial synthesis of urea contributed greatly to the theory of isomerism, as the empirical chemical formulas for urea and ammonium cyanate are identical (see Wöhler synthesis). In 1832, Friedrich Wöhler and Justus von Liebig discovered and explained functional groups and radicals in relation to organic chemistry, as well as first synthesizing benzaldehyde. Liebig, a German chemist, made major contributions to agricultural and biological chemistry, and worked on the organization of organic chemistry. Liebig is considered the "father of the fertilizer industry" for his discovery of nitrogen as an essential plant nutrient, and his formulation of the Law of the Minimum which described the effect of individual nutrients on crops.


title: History of chemistry

text: In 1840, Germain Hess proposed Hess's law, an early statement of the law of conservation of energy, which establishes that energy changes in a chemical process depend only on the states of the starting and product materials and not on the specific pathway taken between the two states. In 1847, Hermann Kolbe obtained acetic acid from completely inorganic sources, further disproving vitalism. In 1848, William Thomson, 1st Baron Kelvin (commonly known as Lord Kelvin) established the concept of absolute zero, the temperature at which all molecular motion ceases. In 1849, Louis Pasteur discovered that the racemic form of tartaric acid is a mixture of the levorotatory and dextrotatory forms, thus clarifying the nature of optical rotation and advancing the field of stereochemistry. In 1852, August Beer proposed Beer's law, which explains the relationship between the composition of a mixture and the amount of light it will absorb. Based partly on earlier work by Pierre Bouguer and Johann Heinrich Lambert, it established the analytical technique known as spectrophotometry. In 1855, Benjamin Silliman, Jr. pioneered methods of petroleum cracking, which made the entire modern petrochemical industry possible.


title: History of chemistry

text: Avogadro's hypothesis began to gain broad appeal among chemists only after his compatriot and fellow scientist Stanislao Cannizzaro demonstrated its value in 1858, two years after Avogadro's death. Cannizzaro's chemical interests had originally centered on natural products and on reactions of aromatic compounds; in 1853 he discovered that when benzaldehyde is treated with concentrated base, both benzoic acid and benzyl alcohol are produced—a phenomenon known today as the Cannizzaro reaction. In his 1858 pamphlet, Cannizzaro showed that a complete return to the ideas of Avogadro could be used to construct a consistent and robust theoretical structure that fit nearly all of the available empirical evidence. For instance, he pointed to evidence that suggested that not all elementary gases consist of two atoms per molecule—some were monatomic, most were diatomic, and a few were even more complex.


title: History of chemistry

text: Another point of contention had been the formulas for compounds of the alkali metals (such as sodium) and the alkaline earth metals (such as calcium), which, in view of their striking chemical analogies, most chemists had wanted to assign to the same formula type. Cannizzaro argued that placing these metals in different categories had the beneficial result of eliminating certain anomalies when using their physical properties to deduce atomic weights. Unfortunately, Cannizzaro's pamphlet was published initially only in Italian and had little immediate impact. The real breakthrough came with an international chemical congress held in the German town of Karlsruhe in September 1860, at which most of the leading European chemists were present. The Karlsruhe Congress had been arranged by Kekulé, Wurtz, and a few others who shared Cannizzaro's sense of the direction chemistry should go. Speaking in French (as everyone there did), Cannizzaro's eloquence and logic made an indelible impression on the assembled body. Moreover, his friend Angelo Pavesi distributed Cannizzaro's pamphlet to attendees at the end of the meeting; more than one chemist later wrote of the decisive impression the reading of this document provided. For instance, Lothar Meyer later wrote that on reading Cannizzaro's paper, "The scales seemed to fall from my eyes." Cannizzaro thus played a crucial role in winning the battle for reform. The system advocated by him, and soon thereafter adopted by most leading chemists, is substantially identical to what is still used today.


title: History of chemistry

text: In 1856, Sir William Henry Perkin, age 18, given a challenge by his professor, August Wilhelm von Hofmann, sought to synthesize quinine, the anti-malaria drug, from coal tar. In one attempt, Perkin oxidized aniline using potassium dichromate, whose toluidine impurities reacted with the aniline and yielded a black solid—suggesting a "failed" organic synthesis. Cleaning the flask with alcohol, Perkin noticed purple portions of the solution: a byproduct of the attempt was the first synthetic dye, known as mauveine or Perkin's mauve. Perkin's discovery is the foundation of the dye synthesis industry, one of the earliest successful chemical industries.


title: History of chemistry

text: German chemist August Kekulé von Stradonitz's most important single contribution was his structural theory of organic composition, outlined in two articles published in 1857 and 1858 and treated in great detail in the pages of his extraordinarily popular "Lehrbuch der organischen Chemie" ("Textbook of Organic Chemistry"), the first installment of which appeared in 1859 and gradually extended to four volumes. Kekulé argued that tetravalent carbon atoms – that is, carbon forming exactly four chemical bonds – could link together to form what he called a "carbon chain" or a "carbon skeleton," to which other atoms with other valences (such as hydrogen, oxygen, nitrogen, and chlorine) could join. He was convinced that it was possible for the chemist to specify this detailed molecular architecture for at least the simpler organic compounds known in his day. Kekulé was not the only chemist to make such claims in this era. The Scottish chemist Archibald Scott Couper published a substantially similar theory nearly simultaneously, and the Russian chemist Aleksandr Butlerov did much to clarify and expand structure theory. However, it was predominantly Kekulé's ideas that prevailed in the chemical community.


title: History of chemistry

text: British chemist and physicist William Crookes is noted for his cathode ray studies, fundamental in the development of atomic physics. His researches on electrical discharges through a rarefied gas led him to observe the dark space around the cathode, now called the Crookes dark space. He demonstrated that cathode rays travel in straight lines and produce phosphorescence and heat when they strike certain materials. A pioneer of vacuum tubes, Crookes invented the Crookes tube – an early experimental discharge tube, with partial vacuum with which he studied the behavior of cathode rays. With the introduction of spectrum analysis by Robert Bunsen and Gustav Kirchhoff (1859–1860), Crookes applied the new technique to the study of selenium compounds. Bunsen and Kirchhoff had previously used spectroscopy as a means of chemical analysis to discover caesium and rubidium. In 1861, Crookes used this process to discover thallium in some seleniferous deposits. He continued work on that new element, isolated it, studied its properties, and in 1873 determined its atomic weight. During his studies of thallium, Crookes discovered the principle of the Crookes radiometer, a device that converts light radiation into rotary motion. The principle of this radiometer has found numerous applications in the development of sensitive measuring instruments.


title: History of chemistry

text: In 1862, Alexander Parkes exhibited Parkesine, one of the earliest synthetic polymers, at the International Exhibition in London. This discovery formed the foundation of the modern plastics industry. In 1864, Cato Maximilian Guldberg and Peter Waage, building on Claude Louis Berthollet's ideas, proposed the law of mass action. In 1865, Johann Josef Loschmidt determined the number of molecules in a mole, later named Avogadro's number.


title: History of chemistry

text: In 1865, August Kekulé, based partially on the work of Loschmidt and others, established the structure of benzene as a six carbon ring with alternating single and double bonds. Kekulé's novel proposal for benzene's cyclic structure was much contested but was never replaced by a superior theory. This theory provided the scientific basis for the dramatic expansion of the German chemical industry in the last third of the 19th century. Kekulé is also famous for having clarified the nature of aromatic compounds, which are compounds based on the benzene molecule. In 1865, Adolf von Baeyer began work on indigo dye, a milestone in modern industrial organic chemistry which revolutionized the dye industry.


title: History of chemistry

text: Swedish chemist and inventor Alfred Nobel found that when nitroglycerin was incorporated in an absorbent inert substance like "kieselguhr" (diatomaceous earth) it became safer and more convenient to handle, and this mixture he patented in 1867 as dynamite. Nobel later on combined nitroglycerin with various nitrocellulose compounds, similar to collodion, but settled on a more efficient recipe combining another nitrate explosive, and obtained a transparent, jelly-like substance, which was a more powerful explosive than dynamite. Gelignite, or blasting gelatin, as it was named, was patented in 1876; and was followed by a host of similar combinations, modified by the addition of potassium nitrate and various other substances.


title: History of chemistry

text: An important breakthrough in making sense of the list of known chemical elements (as well as in understanding the internal structure of atoms) was Dmitri Mendeleev's development of the first modern periodic table, or the periodic classification of the elements. Mendeleev, a Russian chemist, felt that there was some type of order to the elements and he spent more than thirteen years of his life collecting data and assembling the concept, initially with the idea of resolving some of the disorder in the field for his students. Mendeleev found that, when all the known chemical elements were arranged in order of increasing atomic weight, the resulting table displayed a recurring pattern, or periodicity, of properties within groups of elements. Mendeleev's law allowed him to build up a systematic periodic table of all the 66 elements then known based on atomic mass, which he published in "Principles of Chemistry" in 1869. His first Periodic Table was compiled on the basis of arranging the elements in ascending order of atomic weight and grouping them by similarity of properties.


title: History of chemistry

text: Mendeleev had such faith in the validity of the periodic law that he proposed changes to the generally accepted values for the atomic weight of a few elements and, in his version of the periodic table of 1871, predicted the locations within the table of unknown elements together with their properties. He even predicted the likely properties of three yet-to-be-discovered elements, which he called ekaboron (Eb), ekaaluminium (Ea), and ekasilicon (Es), which proved to be good predictors of the properties of scandium, gallium, and germanium, respectively, which each fill the spot in the periodic table assigned by Mendeleev.


title: History of chemistry

text: At first the periodic system did not raise interest among chemists. However, with the discovery of the predicted elements, notably gallium in 1875, scandium in 1879, and germanium in 1886, it began to win wide acceptance. The subsequent proof of many of his predictions within his lifetime brought fame to Mendeleev as the founder of the periodic law. This organization surpassed earlier attempts at classification by Alexandre-Émile Béguyer de Chancourtois, who published the telluric helix, an early, three-dimensional version of the periodic table of the elements in 1862, John Newlands, who proposed the law of octaves (a precursor to the periodic law) in 1864, and Lothar Meyer, who developed an early version of the periodic table with 28 elements organized by valence in 1864. Mendeleev's table did not include any of the noble gases, however, which had not yet been discovered. Gradually the periodic law and table became the framework for a great part of chemical theory. By the time Mendeleev died in 1907, he enjoyed international recognition and had received distinctions and awards from many countries.


title: History of chemistry

text: In 1873, Jacobus Henricus van 't Hoff and Joseph Achille Le Bel, working independently, developed a model of chemical bonding that explained the chirality experiments of Pasteur and provided a physical cause for optical activity in chiral compounds. van 't Hoff's publication, called V"oorstel tot Uitbreiding der Tegenwoordige in de Scheikunde gebruikte Structuurformules in de Ruimte", etc. (Proposal for the development of 3-dimensional chemical structural formulae) and consisting of twelve pages of text and one page of diagrams, gave the impetus to the development of stereochemistry. The concept of the "asymmetrical carbon atom", dealt with in this publication, supplied an explanation of the occurrence of numerous isomers, inexplicable by means of the then current structural formulae. At the same time he pointed out the existence of relationship between optical activity and the presence of an asymmetrical carbon atom.


title: History of chemistry

text: American mathematical physicist J. Willard Gibbs's work on the applications of thermodynamics was instrumental in transforming physical chemistry into a rigorous deductive science. During the years from 1876 to 1878, Gibbs worked on the principles of thermodynamics, applying them to the complex processes involved in chemical reactions. He discovered the concept of chemical potential, or the "fuel" that makes chemical reactions work. In 1876 he published his most famous contribution, "On the Equilibrium of Heterogeneous Substances", a compilation of his work on thermodynamics and physical chemistry which laid out the concept of free energy to explain the physical basis of chemical equilibria. In these essays were the beginnings of Gibbs’ theories of phases of matter: he considered each state of matter a phase, and each substance a component. Gibbs took all of the variables involved in a chemical reaction – temperature, pressure, energy, volume, and entropy – and included them in one simple equation known as Gibbs' phase rule.


title: History of chemistry

text: Within this paper was perhaps his most outstanding contribution, the introduction of the concept of free energy, now universally called Gibbs free energy in his honor. The Gibbs free energy relates the tendency of a physical or chemical system to simultaneously lower its energy and increase its disorder, or entropy, in a spontaneous natural process. Gibbs's approach allows a researcher to calculate the change in free energy in the process, such as in a chemical reaction, and how fast it will happen. Since virtually all chemical processes and many physical ones involve such changes, his work has significantly impacted both the theoretical and experiential aspects of these sciences. In 1877, Ludwig Boltzmann established statistical derivations of many important physical and chemical concepts, including entropy, and distributions of molecular velocities in the gas phase. Together with Boltzmann and James Clerk Maxwell, Gibbs created a new branch of theoretical physics called statistical mechanics (a term that he coined), explaining the laws of thermodynamics as consequences of the statistical properties of large ensembles of particles. Gibbs also worked on the application of Maxwell's equations to problems in physical optics. Gibbs's derivation of the phenomenological laws of thermodynamics from the statistical properties of systems with many particles was presented in his highly influential textbook "Elementary Principles in Statistical Mechanics", published in 1902, a year before his death. In that work, Gibbs reviewed the relationship between the laws of thermodynamics and the statistical theory of molecular motions. The overshooting of the original function by partial sums of Fourier series at points of discontinuity is known as the Gibbs phenomenon.


title: History of chemistry

text: German engineer Carl von Linde's invention of a continuous process of liquefying gases in large quantities formed a basis for the modern technology of refrigeration and provided both impetus and means for conducting scientific research at low temperatures and very high vacuums. He developed a dimethyl ether refrigerator (1874) and an ammonia refrigerator (1876). Though other refrigeration units had been developed earlier, Linde's were the first to be designed with the aim of precise calculations of efficiency. In 1895 he set up a large-scale plant for the production of liquid air. Six years later he developed a method for separating pure liquid oxygen from liquid air that resulted in widespread industrial conversion to processes utilizing oxygen (e.g., in steel manufacture).


title: History of chemistry

text: In 1883, Svante Arrhenius developed an ion theory to explain conductivity in electrolytes. In 1884, Jacobus Henricus van 't Hoff published "Études de Dynamique chimique" (Studies in Dynamic Chemistry), a seminal study on chemical kinetics. In this work, van 't Hoff entered for the first time the field of physical chemistry. Of great importance was his development of the general thermodynamic relationship between the heat of conversion and the displacement of the equilibrium as a result of temperature variation. At constant volume, the equilibrium in a system will tend to shift in such a direction as to oppose the temperature change which is imposed upon the system. Thus, lowering the temperature results in heat development while increasing the temperature results in heat absorption. This principle of mobile equilibrium was subsequently (1885) put in a general form by Henry Louis Le Chatelier, who extended the principle to include compensation, by change of volume, for imposed pressure changes. The van 't Hoff-Le Chatelier principle, or simply Le Chatelier's principle, explains the response of dynamic chemical equilibria to external stresses.


title: History of chemistry

text: In 1884, Hermann Emil Fischer proposed the structure of purine, a key structure in many biomolecules, which he later synthesized in 1898. He also began work on the chemistry of glucose and related sugars. In 1885, Eugen Goldstein named the cathode ray, later discovered to be composed of electrons, and the canal ray, later discovered to be positive hydrogen ions that had been stripped of their electrons in a cathode ray tube; these would later be named protons. The year 1885 also saw the publishing of J. H. van 't Hoff's "L'Équilibre chimique dans les Systèmes gazeux ou dissous à I'État dilué" (Chemical equilibria in gaseous systems or strongly diluted solutions), which dealt with this theory of dilute solutions. Here he demonstrated that the "osmotic pressure" in solutions which are sufficiently dilute is proportionate to the concentration and the absolute temperature so that this pressure can be represented by a formula that only deviates from the formula for gas pressure by a coefficient i. He also determined the value of "i" by various methods, for example by means of the vapor pressure and François-Marie Raoult's results on the lowering of the freezing point. Thus van 't Hoff was able to prove that thermodynamic laws are not only valid for gases, but also for dilute solutions. His pressure laws, given general validity by the electrolytic dissociation theory of Arrhenius (1884–1887) – the first foreigner who came to work with him in Amsterdam (1888) – are considered the most comprehensive and important in the realm of natural sciences. In 1893, Alfred Werner discovered the octahedral structure of cobalt complexes, thus establishing the field of coordination chemistry.


title: History of chemistry

text: The most celebrated discoveries of Scottish chemist William Ramsay were made in inorganic chemistry. Ramsay was intrigued by the British physicist John Strutt, 3rd Baron Rayleigh's 1892 discovery that the atomic weight of nitrogen found in chemical compounds was lower than that of nitrogen found in the atmosphere. He ascribed this discrepancy to a light gas included in chemical compounds of nitrogen, while Ramsay suspected a hitherto undiscovered heavy gas in atmospheric nitrogen. Using two different methods to remove all known gases from air, Ramsay and Lord Rayleigh were able to announce in 1894 that they had found a monatomic, chemically inert gaseous element that constituted nearly 1 percent of the atmosphere; they named it argon.


title: History of chemistry

text: The following year, Ramsay liberated another inert gas from a mineral called cleveite; this proved to be helium, previously known only in the solar spectrum. In his book "The Gases of the Atmosphere" (1896), Ramsay showed that the positions of helium and argon in the periodic table of elements indicated that at least three more noble gases might exist. In 1898 Ramsay and the British chemist Morris W. Travers isolated these elements—called neon, krypton, and xenon—from air and brought them to a liquid state at low temperature and high pressure. Sir William Ramsay worked with Frederick Soddy to demonstrate, in 1903, that alpha particles (helium nuclei) were continually produced during the radioactive decay of a sample of radium. Ramsay was awarded the 1904 Nobel Prize for Chemistry in recognition of "services in the discovery of the inert gaseous elements in the air, and his determination of their place in the periodic system."


title: History of chemistry

text: In 1897, J. J. Thomson discovered the electron using the cathode ray tube. In 1898, Wilhelm Wien demonstrated that canal rays (streams of positive ions) can be deflected by magnetic fields and that the amount of deflection is proportional to the mass-to-charge ratio. This discovery would lead to the analytical technique known as mass spectrometry in 1912.


title: History of chemistry

text: Marie Skłodowska-Curie was a Polish-born French physicist and chemist who is famous for her pioneering research on radioactivity. She and her husband are considered to have laid the cornerstone of the nuclear age with their research on radioactivity. Marie was fascinated with the work of Henri Becquerel, a French physicist who discovered in 1896 that uranium casts off rays similar to the X-rays discovered by Wilhelm Röntgen. Marie Curie began studying uranium in late 1897 and theorized, according to a 1904 article she wrote for Century magazine, "that the emission of rays by the compounds of uranium is a property of the metal itself—that it is an atomic property of the element uranium independent of its chemical or physical state." Curie took Becquerel's work a few steps further, conducting her own experiments on uranium rays. She discovered that the rays remained constant, no matter the condition or form of the uranium. The rays, she theorized, came from the element's atomic structure. This revolutionary idea created the field of atomic physics and the Curies coined the word "radioactivity" to describe the phenomena.


title: History of chemistry

text: Pierre and Marie further explored radioactivity by working to separate the substances in uranium ores and then using the electrometer to make radiation measurements to ‘trace’ the minute amount of unknown radioactive element among the fractions that resulted. Working with the mineral pitchblende, the pair discovered a new radioactive element in 1898. They named the element polonium, after Marie's native country of Poland. On December 21, 1898, the Curies detected the presence of another radioactive material in the pitchblende. They presented this finding to the French Academy of Sciences on December 26, proposing that the new element be called radium. The Curies then went to work isolating polonium and radium from naturally occurring compounds to prove that they were new elements. In 1902, the Curies announced that they had produced a decigram of pure radium, demonstrating its existence as a unique chemical element. While it took three years for them to isolate radium, they were never able to isolate polonium. Along with the discovery of two new elements and finding techniques for isolating radioactive isotopes, Curie oversaw the world's first studies into the treatment of neoplasms, using radioactive isotopes. With Henri Becquerel and her husband, Pierre Curie, she was awarded the 1903 Nobel Prize for Physics. She was the sole winner of the 1911 Nobel Prize for Chemistry. She was the first woman to win a Nobel Prize, and she is the only woman to win the award in two different fields.


title: History of chemistry

text: While working with Marie to extract pure substances from ores, an undertaking that really required industrial resources but that they achieved in relatively primitive conditions, Pierre himself concentrated on the physical study (including luminous and chemical effects) of the new radiations. Through the action of magnetic fields on the rays given out by the radium, he proved the existence of particles that were electrically positive, negative, and neutral; these Ernest Rutherford was afterward to call alpha, beta, and gamma rays. Pierre then studied these radiations by calorimetry and also observed the physiological effects of radium, thus opening the way to radium therapy. Among Pierre Curie's discoveries were that ferromagnetic substances exhibited a critical temperature transition, above which the substances lost their ferromagnetic behavior – this is known as the "Curie point." He was elected to the Academy of Sciences (1905), having in 1903 jointly with Marie received the Royal Society's prestigious Davy Medal and jointly with her and Becquerel the Nobel Prize for Physics. He was run over by a carriage in the rue Dauphine in Paris in 1906 and died instantly. His complete works were published in 1908.


title: History of chemistry

text: New Zealand-born chemist and physicist Ernest Rutherford is considered to be "the father of nuclear physics." Rutherford is best known for devising the names alpha, beta, and gamma to classify various forms of radioactive "rays" which were poorly understood at his time (alpha and beta rays are particle beams, while gamma rays are a form of high-energy electromagnetic radiation). Rutherford deflected alpha rays with both electric and magnetic fields in 1903. Working with Frederick Soddy, Rutherford explained that radioactivity is due to the transmutation of elements, now known to involve nuclear reactions.


title: History of chemistry

text: He also observed that the intensity of radioactivity of a radioactive element decreases over a unique and regular amount of time until a point of stability, and he named the halving time the "half-life." In 1901 and 1902 he worked with Frederick Soddy to prove that atoms of one radioactive element would spontaneously turn into another, by expelling a piece of the atom at high velocity. In 1906 at the University of Manchester, Rutherford oversaw an experiment conducted by his students Hans Geiger (known for the Geiger counter) and Ernest Marsden. In the Geiger–Marsden experiment, a beam of alpha particles, generated by the radioactive decay of radon, was directed normally onto a sheet of very thin gold foil in an evacuated chamber. Under the prevailing plum pudding model, the alpha particles should all have passed through the foil and hit the detector screen, or have been deflected by, at most, a few degrees.


title: History of chemistry

text: However, the actual results surprised Rutherford. Although many of the alpha particles did pass through as expected, many others were deflected at small angles while others were reflected back to the alpha source. They observed that a very small percentage of particles were deflected through angles much larger than 90 degrees. The gold foil experiment showed large deflections for a small fraction of incident particles. Rutherford realized that, because some of the alpha particles were deflected or reflected, the atom had a concentrated centre of positive charge and of relatively large mass – Rutherford later termed this positive center the "atomic nucleus". The alpha particles had either hit the positive centre directly or passed by it close enough to be affected by its positive charge. Since many other particles passed through the gold foil, the positive centre would have to be a relatively small size compared to the rest of the atom – meaning that the atom is mostly open space. From his results, Rutherford developed a model of the atom that was similar to the solar system, known as the Rutherford model. Like planets, electrons orbited a central, sun-like nucleus. For his work with radiation and the atomic nucleus, Rutherford received the 1908 Nobel Prize in Chemistry.


title: History of chemistry

text: In 1903, Mikhail Tsvet invented chromatography, an important analytic technique. In 1904, Hantaro Nagaoka proposed an early nuclear model of the atom, where electrons orbit a dense massive nucleus. In 1905, Fritz Haber and Carl Bosch developed the Haber process for making ammonia, a milestone in industrial chemistry with deep consequences in agriculture. The Haber process, or Haber-Bosch process, combined nitrogen and hydrogen to form ammonia in industrial quantities for the production of fertilizer and munitions. The food production for half the world's current population depends on this method for producing fertilizer. Haber, along with Max Born, proposed the Born–Haber cycle as a method for evaluating the lattice energy of an ionic solid. Haber has also been described as the "father of chemical warfare" for his work developing and deploying chlorine and other poisonous gases during World War I.


title: History of chemistry

text: In 1905, Albert Einstein explained Brownian motion in a way that definitively proved atomic theory. Leo Baekeland invented bakelite, one of the first commercially successful plastics. In 1909, American physicist Robert Andrews Millikan – who had studied in Europe under Walther Nernst and Max Planck – measured the charge of individual electrons with unprecedented accuracy through the oil drop experiment, in which he measured the electric charges on tiny falling water (and later oil) droplets. His study established that any particular droplet's electrical charge is a multiple of a definite, fundamental value — the electron's charge — and thus a confirmation that all electrons have the same charge and mass. Beginning in 1912, he spent several years investigating and finally proving Albert Einstein's proposed linear relationship between energy and frequency, and providing the first direct photoelectric support for Planck's constant. In 1923 Millikan was awarded the Nobel Prize for Physics.


title: History of chemistry

text: In 1909, S. P. L. Sørensen invented the pH concept and developed methods for measuring acidity. In 1911, Antonius Van den Broek proposed the idea that the elements on the periodic table are more properly organized by positive nuclear charge rather than atomic weight. In 1911, the first Solvay Conference was held in Brussels, bringing together most of the most prominent scientists of the day. In 1912, William Henry Bragg and William Lawrence Bragg proposed Bragg's law and established the field of X-ray crystallography, an important tool for elucidating the crystal structure of substances. In 1912, Peter Debye used the concept of a molecular dipole to describe asymmetric charge distribution in some molecules.


title: History of chemistry

text: In 1913, Niels Bohr, a Danish physicist, introduced the concepts of quantum mechanics to atomic structure by proposing what is now known as the Bohr model of the atom, where electrons exist only in strictly defined circular orbits around the nucleus similar to rungs on a ladder. The Bohr Model is a planetary model in which the negatively charged electrons orbit a small, positively charged nucleus similar to the planets orbiting the Sun (except that the orbits are not planar) – the gravitational force of the solar system is mathematically akin to the attractive Coulomb (electrical) force between the positively charged nucleus and the negatively charged electrons.


title: History of chemistry

text: In the Bohr model, however, electrons orbit the nucleus in orbits that have a set size and energy – the energy levels are said to be "quantized", which means that only certain orbits with certain radii are allowed; orbits in between simply don't exist. The energy of the orbit is related to its size – that is, the lowest energy is found in the smallest orbit. Bohr also postulated that electromagnetic radiation is absorbed or emitted when an electron moves from one orbit to another. Because only certain electron orbits are permitted, the emission of light accompanying a jump of an electron from an excited energy state to ground state produces a unique emission spectrum for each element. Bohr later received the Nobel Prize in physics for this work.


title: History of chemistry

text: Niels Bohr also worked on the principle of complementarity, which states that an electron can be interpreted in two mutually exclusive and valid ways. Electrons can be interpreted as wave or particle models. His hypothesis was that an incoming particle would strike the nucleus and create an excited compound nucleus. This formed the basis of his liquid drop model and later provided a theory base for nuclear fission after its discovery by chemists Otto Hahn and Fritz Strassman, and explanation and naming by physicists Lise Meitner and Otto Frisch.


title: History of chemistry

text: In 1913, Henry Moseley, working from Van den Broek's earlier idea, introduced the concept of atomic number to fix some inadequacies of Mendeleev's periodic table, which had been based on atomic weight. The peak of Frederick Soddy's career in radiochemistry was in 1913 with his formulation of the concept of isotopes, which stated that certain elements exist in two or more forms which have different atomic weights but which are indistinguishable chemically. He is remembered for proving the existence of isotopes of certain radioactive elements, and is also credited, along with others, with the discovery of the element protactinium in 1917. In 1913, J. J. Thomson expanded on the work of Wien by showing that charged subatomic particles can be separated by their mass-to-charge ratio, a technique known as mass spectrometry.


title: History of chemistry

text: American physical chemist Gilbert N. Lewis laid the foundation of valence bond theory; he was instrumental in developing a bonding theory based on the number of electrons in the outermost "valence" shell of the atom. In 1902, while Lewis was trying to explain valence to his students, he depicted atoms as constructed of a concentric series of cubes with electrons at each corner. This "cubic atom" explained the eight groups in the periodic table and represented his idea that chemical bonds are formed by electron transference to give each atom a complete set of eight outer electrons (an "octet").


title: History of chemistry

text: Lewis's theory of chemical bonding continued to evolve and, in 1916, he published his seminal article "The Atom of the Molecule", which suggested that a chemical bond is a pair of electrons shared by two atoms. Lewis's model equated the classical chemical bond with the sharing of a pair of electrons between the two bonded atoms. Lewis introduced the "electron dot diagrams" in this paper to symbolize the electronic structures of atoms and molecules. Now known as Lewis structures, they are discussed in virtually every introductory chemistry book.


title: History of chemistry

text: Shortly after the publication of his 1916 paper, Lewis became involved with military research. He did not return to the subject of chemical bonding until 1923, when he masterfully summarized his model in a short monograph entitled Valence and the Structure of Atoms and Molecules. His renewal of interest in this subject was largely stimulated by the activities of the American chemist and General Electric researcher Irving Langmuir, who between 1919 and 1921 popularized and elaborated Lewis's model. Langmuir subsequently introduced the term "covalent bond". In 1921, Otto Stern and Walther Gerlach established the concept of quantum mechanical spin in subatomic particles.


title: History of chemistry

text: For cases where no sharing was involved, Lewis in 1923 developed the electron pair theory of acids and base: Lewis redefined an acid as any atom or molecule with an incomplete octet that was thus capable of accepting electrons from another atom; bases were, of course, electron donors. His theory is known as the concept of Lewis acids and bases. In 1923, G. N. Lewis and Merle Randall published "Thermodynamics and the Free Energy of Chemical Substances", first modern treatise on chemical thermodynamics.


title: History of chemistry

text: The 1920s saw a rapid adoption and application of Lewis's model of the electron-pair bond in the fields of organic and coordination chemistry. In organic chemistry, this was primarily due to the efforts of the British chemists Arthur Lapworth, Robert Robinson, Thomas Lowry, and Christopher Ingold; while in coordination chemistry, Lewis's bonding model was promoted through the efforts of the American chemist Maurice Huggins and the British chemist Nevil Sidgwick.


title: History of chemistry

text: In 1924, French quantum physicist Louis de Broglie published his thesis, in which he introduced a revolutionary theory of electron waves based on wave–particle duality. In his time, the wave and particle interpretations of light and matter were seen as being at odds with one another, but de Broglie suggested that these seemingly different characteristics were instead the same behavior observed from different perspectives — that particles can behave like waves, and waves (radiation) can behave like particles. Broglie's proposal offered an explanation of the restricted motion of electrons within the atom. The first publications of Broglie's idea of "matter waves" had drawn little attention from other physicists, but a copy of his doctoral thesis chanced to reach Einstein, whose response was enthusiastic. Einstein stressed the importance of Broglie's work both explicitly and by building further on it.


title: History of chemistry

text: In 1925, Austrian-born physicist Wolfgang Pauli developed the Pauli exclusion principle, which states that no two electrons around a single nucleus in an atom can occupy the same quantum state simultaneously, as described by four quantum numbers. Pauli made major contributions to quantum mechanics and quantum field theory – he was awarded the 1945 Nobel Prize for Physics for his discovery of the Pauli exclusion principle – as well as solid-state physics, and he successfully hypothesized the existence of the neutrino. In addition to his original work, he wrote masterful syntheses of several areas of physical theory that are considered classics of scientific literature.


title: History of chemistry

text: In 1926 at the age of 39, Austrian theoretical physicist Erwin Schrödinger produced the papers that gave the foundations of quantum wave mechanics. In those papers he described his partial differential equation that is the basic equation of quantum mechanics and bears the same relation to the mechanics of the atom as Newton's equations of motion bear to planetary astronomy. Adopting a proposal made by Louis de Broglie in 1924 that particles of matter have a dual nature and in some situations act like waves, Schrödinger introduced a theory describing the behaviour of such a system by a wave equation that is now known as the Schrödinger equation. The solutions to Schrödinger's equation, unlike the solutions to Newton's equations, are wave functions that can only be related to the probable occurrence of physical events. The readily visualized sequence of events of the planetary orbits of Newton is, in quantum mechanics, replaced by the more abstract notion of probability. (This aspect of the quantum theory made Schrödinger and several other physicists profoundly unhappy, and he devoted much of his later life to formulating philosophical objections to the generally accepted interpretation of the theory that he had done so much to create.)


title: History of chemistry

text: German theoretical physicist Werner Heisenberg was one of the key creators of quantum mechanics. In 1925, Heisenberg discovered a way to formulate quantum mechanics in terms of matrices. For that discovery, he was awarded the Nobel Prize for Physics for 1932. In 1927 he published his uncertainty principle, upon which he built his philosophy and for which he is best known. Heisenberg was able to demonstrate that if you were studying an electron in an atom you could say where it was (the electron's location) or where it was going (the electron's velocity), but it was impossible to express both at the same time. He also made important contributions to the theories of the hydrodynamics of turbulent flows, the atomic nucleus, ferromagnetism, cosmic rays, and subatomic particles, and he was instrumental in planning the first West German nuclear reactor at Karlsruhe, together with a research reactor in Munich, in 1957. Considerable controversy surrounds his work on atomic research during World War II.


title: History of chemistry

text: Some view the birth of quantum chemistry in the discovery of the Schrödinger equation and its application to the hydrogen atom in 1926. However, the 1927 article of Walter Heitler and Fritz London is often recognised as the first milestone in the history of quantum chemistry. This is the first application of quantum mechanics to the diatomic hydrogen molecule, and thus to the phenomenon of the chemical bond. In the following years much progress was accomplished by Edward Teller, Robert S. Mulliken, Max Born, J. Robert Oppenheimer, Linus Pauling, Erich Hückel, Douglas Hartree and Vladimir Aleksandrovich Fock, to cite a few.


title: History of chemistry

text: Still, skepticism remained as to the general power of quantum mechanics applied to complex chemical systems. The situation around 1930 is described by Paul Dirac:


title: History of chemistry

text: Hence the quantum mechanical methods developed in the 1930s and 1940s are often referred to as theoretical molecular or atomic physics to underline the fact that they were more the application of quantum mechanics to chemistry and spectroscopy than answers to chemically relevant questions. In 1951, a milestone article in quantum chemistry is the seminal paper of Clemens C. J. Roothaan on Roothaan equations. It opened the avenue to the solution of the self-consistent field equations for small molecules like hydrogen or nitrogen. Those computations were performed with the help of tables of integrals which were computed on the most advanced computers of the time.


title: History of chemistry

text: In the 1940s many physicists turned from molecular or atomic physics to nuclear physics (like J. Robert Oppenheimer or Edward Teller). Glenn T. Seaborg was an American nuclear chemist best known for his work on isolating and identifying transuranium elements (those heavier than uranium). He shared the 1951 Nobel Prize for Chemistry with Edwin Mattison McMillan for their independent discoveries of transuranium elements. Seaborgium was named in his honour, making him the only person, along with Albert Einstein and Yuri Oganessian, for whom a chemical element was named during his lifetime.


title: History of chemistry

text: By the mid 20th century, in principle, the integration of physics and chemistry was extensive, with chemical properties explained as the result of the electronic structure of the atom; Linus Pauling's book on "The Nature of the Chemical Bond" used the principles of quantum mechanics to deduce bond angles in ever-more complicated molecules. However, though some principles deduced from quantum mechanics were able to predict qualitatively some chemical features for biologically relevant molecules, they were, till the end of the 20th century, more a collection of rules, observations, and recipes than rigorous ab initio quantitative methods.


title: History of chemistry

text: This heuristic approach triumphed in 1953 when James Watson and Francis Crick deduced the double helical structure of DNA by constructing models constrained by and informed by the knowledge of the chemistry of the constituent parts and the X-ray diffraction patterns obtained by Rosalind Franklin. This discovery lead to an explosion of research into the biochemistry of life.


title: History of chemistry

text: In the same year, the Miller–Urey experiment demonstrated that basic constituents of protein, simple amino acids, could themselves be built up from simpler molecules in a simulation of primordial processes on Earth. This first attempt by chemists to study hypothetical processes in the laboratory under controlled conditions helped kickstart bountiful research, within the natural sciences, into the origins of life.


title: History of chemistry

text: In 1983 Kary Mullis devised a method for the in-vitro amplification of DNA, known as the polymerase chain reaction (PCR), which revolutionized the chemical processes used in the laboratory to manipulate it. PCR could be used to synthesize specific pieces of DNA and made possible the sequencing of DNA of organisms, which culminated in the huge human genome project.


title: History of chemistry

text: An important piece in the double helix puzzle was solved by one of Pauling's students Matthew Meselson and Frank Stahl, the result of their collaboration (Meselson–Stahl experiment) has been called as "the most beautiful experiment in biology".


title: History of chemistry

text: They used a centrifugation technique that sorted molecules according to differences in weight. Because nitrogen atoms are a component of DNA, they were labelled and therefore tracked in replication in bacteria.


title: History of chemistry

text: In 1970, John Pople developed the Gaussian program greatly easing computational chemistry calculations. In 1971, Yves Chauvin offered an explanation of the reaction mechanism of olefin metathesis reactions. In 1975, Karl Barry Sharpless and his group discovered stereoselective oxidation reactions including Sharpless epoxidation, Sharpless asymmetric dihydroxylation, and Sharpless oxyamination.


title: History of chemistry

text: In 1985, Harold Kroto, Robert Curl and Richard Smalley discovered fullerenes, a class of large carbon molecules superficially resembling the geodesic dome designed by architect R. Buckminster Fuller. In 1991, Sumio Iijima used electron microscopy to discover a type of cylindrical fullerene known as a carbon nanotube, though earlier work had been done in the field as early as 1951. This material is an important component in the field of nanotechnology. In 1994, K. C. Nicolaou with his group and Robert A. Holton and his group, achieved the first total synthesis of Taxol. In 1995, Eric Cornell and Carl Wieman produced the first Bose–Einstein condensate, a substance that displays quantum mechanical properties on the macroscopic scale.


title: History of chemistry

text: Before the 20th century, chemistry was defined as the science of the nature of matter and its transformations. It was therefore distinct from physics which was not concerned with such dramatic transformation of matter. Moreover, in contrast to physics, chemistry remained predominantly a descriptive and empirical science until the end of the 19th century. Though they developed a consistent quantitative foundation based on notions of atomic and molecular weights, combining proportions, and thermodynamic quantities, chemists had less use of advanced mathematics. Some even expressed reluctance about the use of mathematics within chemistry. For example, the philosopher Auguste Comte wrote in 1830:


title: History of chemistry

text: However, in the second part of the 19th century, the situation began to change as August Kekulé wrote in 1867:


title: History of chemistry

text: As understanding of the nature of matter has evolved, so too has the self-understanding of the science of chemistry by its practitioners. This continuing historical process of evaluation includes the categories, terms, aims and scope of chemistry. Additionally, the development of the social institutions and networks which support chemical enquiry are highly significant factors that enable the production, dissemination and application of chemical knowledge. (See Philosophy of chemistry)


title: History of chemistry

text: The later part of the nineteenth century saw a huge increase in the exploitation of petroleum extracted from the earth for the production of a host of chemicals and largely replaced the use of whale oil, coal tar and naval stores used previously. Large-scale production and refinement of petroleum provided feedstocks for liquid fuels such as gasoline and diesel, solvents, lubricants, asphalt, waxes, and for the production of many of the common materials of the modern world, such as synthetic fibers, plastics, paints, detergents, pharmaceuticals, adhesives and ammonia as fertilizer and for other uses. Many of these required new catalysts and the utilization of chemical engineering for their cost-effective production.


title: History of chemistry

text: In the mid-twentieth century, control of the electronic structure of semiconductor materials was made precise by the creation of large ingots of extremely pure single crystals of silicon and germanium. Accurate control of their chemical composition by doping with other elements made the production of the solid state transistor in 1951 and made possible the production of tiny integrated circuits for use in electronic devices, especially computers.


title: Icelandic króna

text: The króna or krona (sometimes called Icelandic crown; sign: kr; code: ISK) is the currency of Iceland. Iceland is the second-smallest country by population, after the Seychelles, to have its own currency and monetary policy.


title: Icelandic króna

text: Like the Nordic currencies (such as the Danish krone, Swedish krona and Norwegian krone) that participated in the historical Scandinavian Monetary Union, the name "króna" (meaning "crown") comes from the Latin word "corona" ("crown").


title: Icelandic króna

text: The Danish krone was introduced to Iceland in 1874, replacing the earlier Danish currency, the rigsdaler. In 1885, Iceland began issuing its own banknotes.


title: Icelandic króna

text: The Icelandic krona separated from the Danish krone after the dissolution of the Scandinavian Monetary Union at the start of World War I and Icelandic sovereignty from Denmark in 1918. The first coins were issued in 1922.


title: Icelandic króna

text: Iceland was forced to devalue the Icelandic krona in 1922, by 23% against the Danish krone, which saw the beginning of an independent monetary policy in Iceland, and was to be the first of many subsequent devaluations of the krona.


title: Icelandic króna

text: In 1925 the krona was pegged to the British pound for the next 14 years until the spring of 1939. Later in 1939 the currency was pegged to the US Dollar, which was maintained until 1949.


title: Icelandic króna

text: Iceland's first coins were 10 and 25 aurar pieces introduced in 1922. These were followed in 1925 by 1 krona and 2 krona pieces and in 1926 by 1, 2 and 5 aurar pieces. In 1946, the coins' designs were altered to remove the royal monogram (CXR), following Icelandic independence from Denmark in 1944.


title: Icelandic króna

text: Starting in 1967, new coins were introduced due to a considerable fall in the value of the krona. 10 krona coins were introduced in that year, followed by 50 aurar and 5 krona pieces in 1969 and 50 krona pieces in 1970.


title: Icelandic króna

text: The first notes issued in 1885 by the "Landssjóður Íslands" were in denominations of 5, 10 and 50 krona. In 1904, the Bank of Iceland ("Íslands Banki") took over note production and introduced 100 krona notes. In 1921, the "Ríkissjóður Íslands" began issuing paper money, with notes for 1, 5, 10 and 50 krona.


title: Icelandic króna

text: In 1928, another bank, the "Landsbanki Íslands", took over issuance of denominations of 5 krona and above, with the "Ríkissjóður Íslands" continuing to issue 1 krona notes until 1947. The Landsbanki Íslands introduced 500 krona notes in 1935, followed by 25 and 1000 krona notes in 1957.


title: Icelandic króna

text: In 1961, the "Seðlabanki Íslands" became the central bank of Iceland and started issuing paper money, in denominations of 10, 25, 100, and 500 krona. They were manufactured in England by De la Rue.


title: Icelandic króna

text: In 1981, the Icelandic krona was revalued, due to high inflation, with 100 old krona (ISJ) being worth 1 new krona (ISK) and a new 500 krona banknote was first put into circulation in 1981. The 1000 krona was put into circulation in 1984 and the 5000 krona in 1986. The 2000 krona banknote was put into circulation in 1995 but never became very popular. The 10,000 krona banknote was put into circulation in 2013.


title: Icelandic króna

text: Coins of less than one krona have not circulated for many years. In September 2002, Davíð Oddsson, the Icelandic Prime Minister at the time, signed two regulations decreeing that all monetary amounts on invoices and financial claims should be stated and paid in whole krona only and that coins with a value of less than one krona should be withdrawn from circulation.


title: Icelandic króna

text: In 1981, coins were introduced in denominations of 5, 10 and 50 aurar "(cents)", 1 krona and 5 krona. These were followed by 10 krona pieces in 1984, 50 krona in 1987 and 100 krona in 1995. Since 2003, Icelandic banks no longer accept any coins denominated in aurar "(cents)".


title: Icelandic króna

text: Icelandic banknotes are printed with the dates from which the legal basis of the currency derives. In 1981, notes were issued in denominations of 10, 50, 100 and 500 krona based on the law of 29 March 1961. 1000 krona notes were introduced in 1984, followed by 5000 krona notes in 1986 with the same law.


title: Icelandic króna

text: 100, 500, and 1000 krona notes were reissued in 1994 under the law of 5 May 1986. In the following year, a new denomination of 2000 krona was issued for the first time. The 2000 krona note is subtly different from the other notes. For example, the underprint pattern extends all the way upward and downward, while the other denominations had white margins on every side. The number "2000" is printed in multi color for 3 of the 4 occurrences. And the numeral "2000" on the lower left corner of reverse is vertical. The "shadow" of the numeral is printed with SÍ in microprint.


title: Icelandic króna

text: The 22 May 2001 series, saw substantial changes. The underprint and microprint features of the 2000 krona note were extended to other denominations. The 1000 and 5000 krona notes also received metallic foils next to the portrait.


title: Icelandic króna

text: In 2013, a new 10000 krona banknote was introduced. The face of natural scientist and poet Jónas Hallgrímsson appears on the bill, as well as the Eurasian golden plover. Notes of 100 krona or less are no longer in circulation, as they have been withdrawn by the central bank.


title: Icelandic króna

text: The Icelandic currency is a low-volume world currency, strongly managed by its central bank. Its value in terms of other currencies has historically been swift to change, for example against the US and Canadian dollars, and the other Nordic currencies (Swedish krona, Norwegian krone, Danish krone), and the euro. For example, during the first half of 2006, the Icelandic krona ranged between 50 and 80 per US dollar. Prior to the currency's collapse in October 2008, the krona was considered overvalued.


title: Icelandic króna

text: In most shops electronic payment is accepted. Other currencies are very rarely accepted in Iceland. A notable exception is Keflavík International Airport (which has many transfer passengers), where the US dollar, euro and some other currencies are accepted by all merchants. Certain stores in central Reykjavík accept some foreign currencies.


title: Icelandic króna

text: Iceland's per capita computer usage is among the highest in the world, far higher than the UK or US. The saturation of technology in Iceland has had ramifications in the monetary system: a very high proportion of payments in Iceland are made electronically, e.g. by debit or credit cards or online bank transfers. The largest denomination banknote, the 10000 krona note (around €83 in July 2017), has a relatively low value; therefore, most of Iceland's high value trades are done in electronic transfers and in other currencies.


title: Icelandic króna

text: In October 2008, the financial crisis of 2007–2008 brought about a collapse of the Icelandic banking sector. The value of the Icelandic krona dropped, and on 7 October 2008 the Icelandic Central Bank attempted to peg it at 131 against the euro. This peg was abandoned the next day. The krona later dropped again and to 340 against the euro before trade in the currency was suspended (by comparison, the rate at the start of 2008 was about 90 krona to the euro). After a period of tentative, very low-volume international trading in the krona, activity had been expected to pick up again throughout November 2008, albeit still with low liquidity, as Iceland secured an International Monetary Fund loan. However, as of January 2009 the krona was still not being traded regularly, with the European Central Bank (ECB) reference rate being set only intermittently, the last time on 3 December 2008 at 290 krona per euro.


title: Icelandic króna

text: The Icelandic krona similarly fell in value against the US dollar, from around 50 to 80 per dollar to about 110–115 per dollar; by mid-November 2008 it had continued to lower to 135 to the dollar. As of 2 April 2009, the value hovered around 119 per dollar, roughly maintaining that value over the next two years with 23 March 2011, prices around 114 per dollar. With this, the previously high costs for foreign traders and tourists dropped, which Iceland's trade and tourism industry undertook to exploit. In July 2008, a Big Mac cost the equivalent of nearly US$6, versus $3.57 in the US.


title: Icelandic króna

text: Theoretically, the adoption of the euro could have several advantages. Adopting what is perceived by some as a historically stronger currency might help Iceland to "avoid the turbulence surrounding speculations in international financial markets". In addition, Icelandic economists listed several arguments in favour of the Euro before the crisis. "In terms of growth potentials and welfare, the euro could be expected to bring lower long-term interest rates [...]. This would of course increase capital investment and labour productivity. The euro might lower consumer prices by facilitating a comparison with other euro countries."


title: Icelandic króna

text: Because of the volatility between the euro and the krona, former Foreign Minister Valgerður Sverrisdóttir considered the idea that Iceland might dollarize itself into the Eurozone without joining the European Union.


title: Icelandic króna

text: Opinion regarding the euro is mixed among Icelanders. An opinion poll about Iceland joining the European Union released on 11 September 2007 showed that 53% of respondents were in favour of adopting the euro, 37% opposed and 10% undecided. Another poll produced for the Icelandic newspaper "Fréttablaðið" and released on 30 September 2007 showed 56% opposed to euro adoption and 44% in favour. In January 2008, a poll by the Icelandic Chamber of Commerce put support for Iceland to abandon the krona for another currency at 63%. A number of companies in Iceland, such as Össur, have started to pay their employees in euros or US dollars, mainly due to the high inflation and high volatility.


title: Icelandic króna

text: The financial crisis prompted further calls for Iceland to join the Eurozone. In January 2009, one senior Icelandic official stated that due to the crisis "the krona is dead. We need a new currency. The only serious option is the euro." In March 2009, a report by Iceland's Minister for Foreign Affairs, Össur Skarphédinsson, considered three options: retaining the krona, adopting the euro without joining the EU and adopting the euro through EU membership. The report recommended the third option.


title: Icelandic króna

text: An economic study of the impact of the adoption of the euro by Iceland found that the Icelandic krona acts both as a barrier and buffer to international trade, and that by joining the EU and adopting the euro, Icelandic international trade might be 60% higher.


title: Icelandic króna

text: In July 2009, the Alþingi (parliament) narrowly voted to apply for EU membership, but that application has been frozen since 2013 (see accession of Iceland to the European Union).


title: Icelandic króna

text: In March 2015, Icelandic authorities announced by letter to the Presidency of the Council of the European Union that Iceland should not be seen as candidate state, and that there were no specific plans to continue any membership process. Leaders of the ruling government parties have also stated that the krona will remain as Iceland's currency for at least the foreseeable future.


title: Icelandic króna

text: Some small countries such as El Salvador, Ecuador and Montenegro have unilaterally adopted the use of a more stable foreign currency as a means of controlling inflation. The cost of this is generally very high, as the adopting country loses all control over monetary policy, and all the benefits of seignorage. A currency board is a second tier solution, where the exchange rate of the currency is fixed to that of another country, or a "basket" of currencies.


title: Icelandic króna

text: There was a discussion in 2012 about the feasibility of adopting a foreign currency. The idea of adopting the Canadian dollar was popular and the Canadian ambassador to Iceland stated that Iceland could adopt the currency if it so wished. Canada was favoured due to its northern geography and similar resource based economy, in addition to its relative economic stability.


title: Icelandic króna

text: Arnór Sighvatsson, Deputy Governor of the Central Bank of Iceland, at a meeting of the Icelandic Federation of Labour on 10 January 2012, said:


title: Tarawa-class amphibious assault ship

text: The "Tarawa" class is a ship class of Landing Helicopter Assault (LHA) type amphibious assault ships operated by the United States Navy (USN). Five ships were built by Ingalls Shipbuilding between 1971 and 1980; another four ships were planned, but later canceled; instead they were joined by the s.


title: Tarawa-class amphibious assault ship

text: , all vessels had been decommissioned. The "Tarawa" class were replaced by the s from 2014 onward while the "Wasp" class remains in service.


title: Tarawa-class amphibious assault ship

text: Propulsion is provided by two Combustion Engineering boilers, connected to two Westinghouse turbines. These supply to the ship's two propeller shafts. A "Tarawa"-class vessel can reach a maximum speed of , and has a maximum range of at . In addition to the main propulsion system, the ships are fitted with a bow thruster.


title: Tarawa-class amphibious assault ship

text: As of 1998, the ships' armament consisted of a Mark 49 RAM surface-to-air missile system, two Vulcan Phalanx close-in weapons systems, six Mark 242 25 mm automatic cannons, and eight 12.7 mm machine guns. Previously, the amphibious warships were fitted with 2 Mark 25 Sea Sparrow missile systems (which were replaced by the Phalanx units), and three 5-inch (127 mm) Mk 45 lightweight guns in bow and starboard aft sponsons (the guns were removed across the class during 1997 and 1998). Countermeasures and decoys include four Mark 36 SRBOC launchers, a SLQ-25 Nixie towed torpedo decoy, a Sea Gnat unit, SLQ-49 chaff decoys.


title: Tarawa-class amphibious assault ship

text: The number of helicopters carried by each vessel was up to 19 Sikorsky CH-53 Sea Stallions, 26 Boeing Vertol CH-46 Sea Knight, or a mix of the two. The flight deck is fitted with two aircraft lifts, and up to nine Sea Stallions or 12 Sea Knights can be operated simultaneously. With a small amount of modification, the ships could carry and operate up to six McDonnell Douglas AV-8B Harrier II jump-jets.


title: Tarawa-class amphibious assault ship

text: The "Tarawa"-class ships are designed to embark a reinforced battalion of the United States Marine Corps and their equipment. Onboard accommodation is provided for up to 1,703 marines, while is provided for the battalion's vehicles, and is allocated for stores and other equipment. As well as deploying by helicopters, personnel and equipment can be embarked or offloaded via a well deck in each ship's stern. Up to four LCU 1610 landing craft can be transported in and operated from the well deck, along with other designs and combinations of landing craft (two LCU and two LCM-8, or 17 LCM-6, or 45 LVT).


title: Tarawa-class amphibious assault ship

text: The "Tarawa" design was later repeated for the s, with some changes. The main changes to the latter eight-ship class include the lower placement of the ship's bridge aboard the "Wasp"s, the relocation of the command and control facilities to inside the hull, modifications to allow the operation of Harrier jump-jets and Landing Craft Air Cushion hovercraft, and removal of the 5-inch guns and their sponsons to increase the overall size of the flight deck.


title: Tarawa-class amphibious assault ship

text: All five warships were built by Ingalls Shipbuilding, at this company's shipyard in Pascagoula, Mississippi. "Tarawa" was approved for construction during fiscal year 1969, with two more ships of this class ordered by Congress in the 1970 and 1971 fiscal years.


title: Tarawa-class amphibious assault ship

text: Design problems emerged early in the LHA program and contrary to the intent of the Total Package Procurement concept, the Navy became heavily involved in the design process. Nine ships were originally contracted for the "Tarawa" class, but that number was reduced to five in January 1971. The other four ships were never built for the Navy.


title: Tarawa-class amphibious assault ship

text: Work on the first warship of this class, , began on 15 November 1971, and she was commissioned into the Navy on 29 May 1976. The last of the five ships, , was completed on 3 May 1980.


title: Tarawa-class amphibious assault ship

text: The "Tarawa"s began leaving service in 2005. By April 2011, four of the five amphibious assault ships had been decommissioned, leaving only "Peleliu" in active service. "Peleliu" was decommissioned on 31 March 2015 in San Diego.


title: Tarawa-class amphibious assault ship

text: The "Tarawa" class is to be replaced by the . The first "America"-class vessel was delivered and commissioned in 2014.


title: Charles Hallahan

text: Charles John Hallahan (July 29, 1943 – November 25, 1997) was an American film, television, and stage actor known for his performances in "Going in Style", "The Thing", "Cast a Deadly Spell", and "Dante's Peak". He was also best known as Capt. Charlie Devane on "Hunter" from 1986 to 1991 and Chet Wilke in "Lou Grant" (1979–1982).


title: Charles Hallahan

text: Hallahan was born in Philadelphia and graduated from Rutgers University-Camden, and earned an MFA from Temple University. During his acting career he was often cast as a police officer, and may have been best known as LAPD Captain Charlie Devane on "Hunter". He was memorable for his portrayal of the nameless "Coach" in "Vision Quest", opposite Matthew Modine. He also served in the US Navy in the early 1960s, including time as a Navy hospital corpsman stationed in Puerto Rico.


title: Charles Hallahan

text: In 1982, he portrayed geologist Vance Norris in the remake of "The Thing". His most notable role came when he portrayed volcanologist Dr. Paul Dreyfus in the 1997 disaster-thriller film "Dante's Peak". Hallahan also co-starred as a regular cast member in two popular television series, "Grace Under Fire" and the long-running "The Paper Chase". His brother, Fr. Kenneth Hallahan, was a Roman Catholic priest in New Jersey.


title: Charles Hallahan

text: In 1997, Hallahan was cast as Liam Bilby for the "" episode "". On November 25, 1997, he died of an apparent heart attack while driving his car in Los Angeles. Nick Tate assumed the role of Liam Bilby in Hallahan's memory.


title: Maximum power point tracking

text: Maximum power point tracking (MPPT) or sometimes just power point tracking (PPT), is a technique used with variable power sources to maximize energy extraction as conditions vary. The technique is most commonly used with photovoltaic (PV) solar systems, but can also be used with wind turbines, optical power transmission and thermophotovoltaics.


title: Maximum power point tracking

text: PV solar systems have varying relationships to inverter systems, external grids, battery banks, and other electrical loads. The central problem addressed by MPPT is that the efficiency of power transfer from the solar cell depends on the amount of available sunlight, shading, solar panel temperature and the load's electrical characteristics. As these conditions vary, the load characteristic (impedance) that gives the highest power transfer changes. The system is optimized when the load characteristic changes to keep power transfer at highest efficiency. This optimal load characteristic is called the "maximum power point" (MPP). MPPT is the process of adjusting the load characteristic as the conditions change. Circuits can be designed to present optimal loads to the photovoltaic cells and then convert the voltage, current, or frequency to suit other devices or systems.


title: Maximum power point tracking

text: Solar cells' non-linear relationship between temperature and total resistance can be analyzed based on the Current-voltage (I-V) curve and the power-voltage (P-V) curves. MPPT samples cell output and applies the proper resistance (load) to obtain maximum power. MPPT devices are typically integrated into an electric power converter system that provides voltage or current conversion, filtering, and regulation for driving various loads, including power grids, batteries, or motors. Solar inverters convert DC power to AC power and may incorporate MPPT.


title: Maximum power point tracking

text: In general, the P-V curve of a partially shaded solar array can have multiple peaks, and some algorithms can get stuck in a local maximum rather than the global maximum of the curve.


title: Maximum power point tracking

text: Photovoltaic cells have a complex relationship between their operating environment and the power they produce. The fill factor (FF) characterizes the cell's non-linear electrical behavior. Fill factor is defined as the ratio of the maximum power from the cell to the product of open circuit voltage V and short-circuit current I. Tabulated data is often used to estimate the maximum power that a cell can provide with an optimal load under given conditions:


title: Maximum power point tracking

text: For most purposes, FF, V, and I are enough information to give a useful approximate view of the cell's electrical behavior under typical conditions.


title: Maximum power point tracking

text: For any given set of conditions, cells have a single operating point where the values of the current ("I") and voltage ("V") of the cell allow maximum power output. These values correspond to a particular load resistance, which is equal to "V" / I as specified by Ohm's law. The power P is given by:


title: Maximum power point tracking

text: A photovoltaic cell, for the majority of its useful curve, acts as a constant current source. However, at a photovoltaic cell's MPP region, its curve has an approximately inverse exponential relationship between current and voltage. From basic circuit theory, the power delivered to a device is optimized (MPP) where the derivative (graphically, the slope) "dI/dV" of the I-V curve is equal and opposite the "I/V" ratio (where d"P/dV"=0) and corresponds to the "knee" of the curve.


title: Maximum power point tracking

text: A load with resistance "R=V/I" equal to the reciprocal of this value draws the maximum power from the device. This is sometimes called the 'characteristic resistance' of the cell. This is a dynamic quantity that changes depending on the level of illumination, as well as other factors such as temperature and cell condition. Lower or higher resistance reduces power output. Maximum power point trackers utilize control circuits or logic to identify this point.


title: Maximum power point tracking

text: If a full power-voltage (P-V) curve is available, then the maximum power point can be obtained using a bisection method.


title: Maximum power point tracking

text: When directly connecting a load to cell, the operating point of the panel is rarely at peak power. The impedance seen by the panel determines its operating point. Setting the impedance correctly achieves peak power. Since panels are DC devices, DC-DC converters transform the impedance of one circuit (source) to the other circuit (load). Changing the duty ratio of the DC-DC converter changes the impedance (duty ratio) seen by the cell. The I-V curve of the panel can be considerably affected by atmospheric conditions such as irradiance and temperature.


title: Maximum power point tracking

text: MPPT algorithms frequently sample panel voltages and currents, then adjust the duty ratio accordingly. Microcontrollers implement the algorithms. Modern implementations often utilize more sophisticated computers for analytics and load forecasting.


title: Maximum power point tracking

text: Controllers can follow several strategies to optimize power output. MPPTs may switch among multiple algorithms as conditions dictate.


title: Maximum power point tracking

text: In this method the controller adjusts the voltage from the array by a small amount and measures power; if the power increases, further adjustments in that direction are tried until power no longer increases. This is called "perturb and observe" (P&O) and is most common, although this method can cause power output to oscillate. It is also referred to as a "hill climbing" method, because it depends on the rise of the curve of power against voltage below the maximum power point, and the fall above that point. Perturb and observe is the most commonly used method due to its ease of implementation. Perturb and observe method may result in top-level efficiency, provided that a proper predictive and adaptive hill climbing strategy is adopted.


title: Maximum power point tracking

text: In this method, the controller measures incremental current and voltage changes to predict the effect of a voltage change. This method requires more computation in the controller, but can track changing conditions more rapidly than P&O. Power output does not oscillate. It utilizes the incremental conductance (formula_3) of the photovoltaic array to compute the sign of the change in power with respect to voltage (formula_4). The incremental conductance method computes MPP by comparison of the incremental conductance (formula_5) to the array conductance (formula_6). When these two are the same (formula_7), the output voltage is the MPP voltage. The controller maintains this voltage until the irradiation changes and the process is repeated.


title: Maximum power point tracking

text: The incremental conductance method is based on the observation that at MPP, formula_8, and that formula_9. The current from the array can be expressed as a function of the voltage:


title: Maximum power point tracking

text: Therefore, formula_11. Setting this equal to zero yields: formula_12. Therefore, MPP is achieved when the incremental conductance is equal to the negative of the instantaneous conductance. The power-voltage curve characteristic shows that: when the voltage is smaller than MPP, formula_13, so formula_14; when the voltage is bigger than MPP, formula_15 or formula_16. Thus, a tracker can know where it is on the power-voltage curve by calculating the relation of the change of current/voltage and the current voltage themselves.


title: Maximum power point tracking

text: The "current sweep" method uses a sweep waveform for the array current such that the I-V characteristic of the PV array is obtained and updated at fixed time intervals. MPP voltage can then be computed from the characteristic curve at the same intervals.


title: Maximum power point tracking

text: "Constant voltage" methods include one in which the output voltage is regulated to a constant value under all conditions and one in which the output voltage is regulated based on a constant ratio to the measured open circuit voltage (formula_17). The latter technique may also be labeled the "open voltage" method. If the output voltage is held constant, there is no attempt to track MPP, so it is not strictly a MPPT technique, though it does function in cases when MPP tracking tends to fail, and thus it is sometimes used supplementally. In the open voltage method, power delivery is momentarily interrupted and the open-circuit voltage with zero current is measured. The controller then resumes operation with the voltage controlled at a fixed ratio, such as 0.76, of the open-circuit voltage formula_17. This is usually a value that has been predetermined to be the MPP, either empirically or based on modelling, for expected operating conditions. The array's operating point is thus kept near MPP by regulating the array voltage and matching it to the fixed reference voltage formula_19. The value of formula_20 may be chosen to give optimal performance relative to other factors as well as the MPP, but the central idea is that formula_20 is determined as a ratio to formula_17. One of the inherent approximations in the method is that the ratio of MPP voltage to formula_17 is only approximately constant, so it leaves room for further possible optimization.


title: Maximum power point tracking

text: This method estimates the MPP voltage (formula_24) by measuring the temperature of the solar module and comparing it against a reference. Since changes in irradiation levels have a negligible effect on the MPP voltage, its influences may be ignored - the voltage is assumed to vary linearly with temperature.


title: Maximum power point tracking

text: Both P&O and incremental conductance are examples of "hill climbing" methods that can find the local maximum of the power curve for the array's operating condition, and so provide a true MPP.


title: Maximum power point tracking

text: P&O produces power output oscillations around the maximum power point even under steady state irradiance.


title: Maximum power point tracking

text: Incremental conductance it can determine the maximum power point without oscillating. It can perform MPPT under rapidly varying irradiation conditions with higher accuracy than P&O. However, this method can produce oscillations and can perform erratically under rapidly changing atmospheric conditions. The sampling frequency is decreased due to the higher complexity of the algorithm compared to P&O.


title: Maximum power point tracking

text: In the constant voltage ratio (or "open voltage") method, energy may be lost during the time the current is set to zero. The approximation of 76% as the formula_31 ratio is not necessarily accurate. Although simple and low-cost to implement, the interruptions reduce array efficiency and do not ensure finding the actual MPP. However, efficiencies of some systems may reach above 95%.


title: Maximum power point tracking

text: Traditional solar inverters perform MPPT for the entire array. In such systems the same current, dictated by the inverter, flows through all modules in the string (series). Because different modules have different I-V curves and different MPPs (due to manufacturing tolerance, partial shading, etc.) this architecture means some modules will be performing below their MPP, costing efficiency.


title: Maximum power point tracking

text: Instead, MPPTs can be deployed for individual modules, allowing each to operate at peak efficiency despite uneven shading, soiling or electrical mismatch.


title: Maximum power point tracking

text: Data suggest having one inverter with one MPPT for a project that has identical number of east and west-facing modules presents no disadvantages when compared to having two inverters or one inverter with more than one MPPT.


title: Maximum power point tracking

text: At night, an off-grid PV system may use batteries to supply loads. Although the fully charged battery pack voltage may be close to the PV panel's MPP voltage, this is unlikely to be true at sunrise when the battery is partially discharged. Charging may begin at a voltage considerably below the PV panel MPP voltage, and an MPPT can resolve this mismatch.


title: Maximum power point tracking

text: When the batteries are fully charged and PV production exceeds local loads, an MPPT can no longer operate the panel at its MPP as the excess power has no load to absorb it. The MPPT must then shift the PV panel operating point away from the peak power point until production matches demand. (An alternative approach commonly used in spacecraft is to divert surplus PV power into a resistive load, allowing the panel to operate continuously at its peak power point in order to keep the panel as cool as possible.)


title: Maximum power point tracking

text: In a grid-connected system, all delivered power from solar modules is sent to the grid. Therefore, the MPPT in a grid connected system always attempts to operate at MPP.


title: Lake Maggiore

text: Lake Maggiore (, ; ; ; ; literally 'Greater Lake') or Verbano (; ) is a large lake located on the south side of the Alps. It is the second largest lake in Italy and the largest in southern Switzerland. The lake and its shoreline are divided between the Italian regions of Piedmont and Lombardy and the Swiss canton of Ticino. Located halfway between Lake Orta and Lake Lugano, Lake Maggiore extends for about between Locarno and Arona.


title: Lake Maggiore

text: The climate is mild in both summer and winter, producing Mediterranean vegetation, with many gardens growing rare and exotic plants. Well-known gardens include those of the Borromean and Brissago Islands, that of the Villa Taranto in Verbania, and the Alpinia Botanical Garden above Stresa.


title: Lake Maggiore

text: Lake Maggiore is drained by the Ticino, a main tributary of the Po. Its basin also collects the waters of several large lakes, notably Lake Lugano (through the Tresa), Lake Orta (through the Toce) and Lake Varese (through the Bardello).