Emergence
In philosophy, systems theory and art, emergence occurs when an entity is observed to have properties its parts do not have on their own. These properties or behaviors emerge only. For example, smooth forward motion emerges when a bicycle and its rider interoperate, but neither part can produce the behavior on their own. Emergence plays a central role of complex systems. For instance, the phenomenon of life as studied in biology is an emergent property of chemistry, psychological phenomena emerge from the neurobiological phenomena of living things. In philosophy, theories that emphasize emergent properties have been called emergentism. All accounts of emergentism include a form of epistemic or ontological irreducibility to the lower levels. Philosophers understand emergence as a claim about the etiology of a system's properties. An emergent property of a system, in this context, is one, not a property of any component of that system, but is still a feature of the system as a whole. Nicolai Hartmann, one of the first modern philosophers to write on emergence, termed this a categorial novum.
This idea of emergence has been around since at least the time of Aristotle. The many scientists and philosophers who have written on the concept include John Stuart Mill and Julian Huxley; the philosopher G. H. Lewes coined the term "emergent", writing in 1875: Every resultant is either a sum or a difference of the co-operant forces. Further, every resultant is traceable in its components, because these are homogeneous and commensurable, it is otherwise with emergents, instead of adding measurable motion to measurable motion, or things of one kind to other individuals of their kind, there is a co-operation of things of unlike kinds. The emergent is unlike its components insofar as these are incommensurable, it cannot be reduced to their sum or their difference. In 1999 economist Jeffrey Goldstein provided a current definition of emergence in the journal Emergence. Goldstein defined emergence as: "the arising of novel and coherent structures and properties during the process of self-organization in complex systems".
In 2002 systems scientist Peter Corning described the qualities of Goldstein's definition in more detail: The common characteristics are: radical novelty. Corning suggests a narrower definition, requiring that the components be unlike in kind, that they involve division of labor between these components, he says that living systems, while emergent, cannot be reduced to underlying laws of emergence: Rules, or laws, have no causal efficacy. They serve to describe regularities and consistent relationships in nature; these patterns may be illuminating and important, but the underlying causal agencies must be separately specified. But that aside, the game of chess illustrates... why any laws or rules of emergence and evolution are insufficient. In a chess game, you cannot use the rules to predict'history' – i.e. the course of any given game. Indeed, you cannot reliably predict the next move in a chess game. Why? Because the'system' involves more than the rules of the game, it includes the players and their unfolding, moment-by-moment decisions among a large number of available options at each choice point.
The game of chess is inescapably historical though it is constrained and shaped by a set of rules, not to mention the laws of physics. Moreover, this is a key point, the game of chess is shaped by teleonomic, feedback-driven influences, it is not a self-ordered process. Usage of the notion "emergence" may be subdivided into two perspectives, that of "weak emergence" and "strong emergence". In terms of physical systems, weak emergence is a type of emergence in which the emergent property is amenable to computer simulation. Crucial in these simulations is. If not, a new entity is formed with new, emergent properties: this is called strong emergence, which cannot be simulated by a computer; some common points between the two notions are that emergence concerns new properties produced as the system grows, to say ones which are not shared with its components or prior states. It is assumed that the properties are supervenient rather than metaphysically primitive. Weak emergence describes new properties arising in systems as a result of the interactions at an elemental level.
However, it is stipulated that the properties can be determined only by observing or simulating the system, not by any process of a reductionist analysis. As a consequence the emerging properties are scale dependent: they are only observable if the system is large enough to exhibit the phenomenon. Chaotic, unpredictable behaviour can be seen as an emergent phenomenon, while at a microscopic scale the behaviour of the constituent parts can be deterministic. B
Thermionic emission
Thermionic emission is the thermally induced flow of charge carriers from a surface or over a potential-energy barrier. This occurs because the thermal energy given to the carrier overcomes the work function of the material; the charge carriers can be electrons or ions, in older literature are sometimes referred to as thermions. After emission, a charge, equal in magnitude and opposite in sign to the total charge emitted is left behind in the emitting region, but if the emitter is connected to a battery, the charge left behind is neutralized by charge supplied by the battery as the emitted charge carriers move away from the emitter, the emitter will be in the same state as it was before emission. The classical example of thermionic emission is that of electrons from a hot cathode into a vacuum in a vacuum tube; the hot cathode can be a metal filament, a coated metal filament, or a separate structure of metal or carbides or borides of transition metals. Vacuum emission from metals tends to become significant only for temperatures over 1,000 K.
The term "thermionic emission" is now used to refer to any thermally-excited charge emission process when the charge is emitted from one solid-state region into another. This process is crucially important in the operation of a variety of electronic devices and can be used for electricity generation or cooling; the magnitude of the charge flow increases with increasing temperature. Because the electron was not identified as a separate physical particle until the work of J. J. Thomson in 1897, the word "electron" was not used when discussing experiments that took place before this date; the phenomenon was reported in 1853 by Edmond Becquerel. It was rediscovered in 1873 by Frederick Guthrie in Britain. While doing work on charged objects, Guthrie discovered that a red-hot iron sphere with a negative charge would lose its charge, he found that this did not happen if the sphere had a positive charge. Other early contributors included Johann Wilhelm Hittorf, Eugen Goldstein, Julius Elster and Hans Friedrich Geitel.
The effect was rediscovered again by Thomas Edison on February 13, 1880, while he was trying to discover the reason for breakage of lamp filaments and uneven blackening of the bulbs in his incandescent lamps. Edison built several experimental lamp bulbs with an extra wire, metal plate, or foil inside the bulb, separate from the filament and thus could serve as an electrode, he connected a galvanometer, a device used to measure current, to the output of the extra metal electrode. If the foil was put at a negative potential relative to the filament, there was no measurable current between the filament and the foil; when the foil was raised to a positive potential relative to the filament, there could be a significant current between the filament through the vacuum to the foil if the filament was heated sufficiently. We now know that the filament was emitting electrons, which were attracted to a positively charged foil, but not a negatively charged one; this one-way current was called the Edison effect.
He found that the current emitted by the hot filament increased with increasing voltage, filed a patent application for a voltage-regulating device using the effect on November 15, 1883. He found; this was exhibited at the International Electrical Exposition in Philadelphia in September 1884. William Preece, a British scientist, took back with him several of the Edison effect bulbs, he presented a paper on them in 1885, where he referred to thermionic emission as the "Edison Effect." The British physicist John Ambrose Fleming, working for the British "Wireless Telegraphy" Company, discovered that the Edison Effect could be used to detect radio waves. Fleming went on to develop the two-element vacuum tube known as the diode, which he patented on November 16, 1904; the thermionic diode can be configured as a device that converts a heat difference to electric power directly without moving parts. Following J. J. Thomson's identification of the electron in 1897, the British physicist Owen Willans Richardson began work on the topic that he called "thermionic emission".
He received a Nobel Prize in Physics in 1928 "for his work on the thermionic phenomenon and for the discovery of the law named after him". From band theory, there are one or two electrons per atom in a solid that are free to move from atom to atom; this is sometimes collectively referred to as a "sea of electrons". Their velocities follow a statistical distribution, rather than being uniform, an electron will have enough velocity to exit the metal without being pulled back in; the minimum amount of energy needed for an electron to leave a surface is called the work function. The work function is characteristic of the material and for most metals is on the order of several electronvolts. Thermionic currents can be increased by decreasing the work function; this often-desired goal can be achieved by applying various oxide coatings to the wire. In 1901 Richardson published the results of his experiments: the current from a heated wire seemed to depend exponentially on the temperature of the wire with a mathematical form similar to the Arrhenius equation.
He propose
Xenon arc lamp
A xenon arc lamp is a specialized type of gas discharge lamp, an electric light that produces light by passing electricity through ionized xenon gas at high pressure. It produces a bright white light that mimics natural sunlight, with applications in movie projectors in theaters, in searchlights, for specialized uses in industry and research to simulate sunlight for product testing. Xenon headlamps in automobiles are metal-halide lamps, where a xenon arc is only used during start-up to correct the color temperature. Xenon arc lamps can be divided into three categories: continuous-output xenon short-arc lamps, continuous-output xenon long-arc lamps, xenon flash lamps; each consists of a fused quartz or other heat resistant glass arc tube, with a tungsten metal electrode at each end. The glass tube is first evacuated and re-filled with xenon gas. For xenon flashtubes, a third "trigger" electrode surrounds the exterior of the arc tube; the lifetime of a xenon arc lamp varies according to its design and power consumption, with a major manufacturer quoting average lifetimes ranging from 500 hours to 1,500.
Interest in the xenon discharge was first aroused by P. Schulz in 1944, following his discovery of its near-continuous spectrum and high colour rendering white light. Owing to wartime limitations on the availability of this noble gas, significant progress was not made until John Aldington of the British Siemens lamp company published his research in 1949; this triggered intensive efforts at the German Osram company to further develop the technology as a replacement for carbon arcs in cinema projection. The first successful public projection using xenon light was performed on 30 October 1950, when excerpts from a colour film were shown during the 216th session of the German Cinematographic Society in Berlin; the technology was commercially introduced by German Osram in 1952. First produced in the 2 kW size, these lamps saw wide use in movie projection, where they replaced the older, more labor-intensive carbon arc lamps; the white continuous light generated by the xenon arc is spectrally similar to daylight, but the lamp has a rather low efficacy in terms of lumens of visible light output per watt of input power.
Today all movie projectors in theaters employ these lamps, with power ratings ranging from 900 watts up to 12 kW. Omnimax projection systems use single xenon lamps with ratings as high as 15 kW; as of 2016, laser illumination for digital theater projectors is starting to establish a market presence and has been predicted to supersede the xenon arc lamp for this application. All modern xenon short-arc lamps use a fused quartz envelope with thoriated tungsten electrodes. Fused quartz is the only economically feasible material available that can withstand the high pressure and high temperature present in an operating lamp, while still being optically clear; the thorium dopant in the electrodes enhances their electron emission characteristics. Because tungsten and quartz have different coefficients of thermal expansion, the tungsten electrodes are welded to strips of pure molybdenum metal or Invar alloy, which are melted into the quartz to form the envelope seal; because of the high power levels involved, large lamps are water-cooled.
In those used in IMAX projectors, the electrode bodies are made from solid Invar and tipped with thoriated tungsten. An O-ring seals the tube. In low power applications the electrodes are too cold for efficient electron emission and are not cooled. In high power applications an additional water cooling circuit for each electrode is necessary. To reduce cost, the water circuits are not separated and the water needs to be deionized to make it electrically non-conductive, which lets the quartz or some laser media dissolve into the water. To achieve maximum efficiency, the xenon gas inside short-arc lamps is maintained at an high pressure — up to 30 atmospheres — which poses safety concerns. If a lamp is dropped or ruptures while in service, pieces of the lamp envelope can be thrown at high speed. To mitigate this, large xenon short-arc lamps are shipped in protective shields, which will contain the envelope fragments should breakage occur; the shield is removed once the lamp is installed in the lamp housing.
When the lamp reaches the end of its useful life, the protective shield is put back on the lamp, the spent lamp is removed from the equipment and discarded. As lamps age, the risk of failure increases, so bulbs being replaced are at the greatest risk of explosion. Lamp manufacturers recommend the use of eye protection; some lamps those used in IMAX projectors, require the use of full-body protective clothing. Xenon short-arc lamps come in two distinct varieties: pure xenon. In a pure xenon lamp, the majority of the light is generated within a tiny, pinpoint-sized cloud of plasma situated where the electron stream leaves the face of the cathode; the light generation volume is cone-shaped, the luminous intensity falls off exponentially moving from cathode to anode. Electrons passing through the plasma cloud strike the anode; as a result, the anode in a xenon short-arc lamp either has to be much larger than the cathode or be water-cooled, to dissipate the heat. The output of a pure xenon short-arc lamp offers continuous spectral power distribution with a color temperature of about 6200K and color renderi
Humphry Davy
Sir Humphry Davy, 1st Baronet was a Cornish chemist and inventor, best remembered today for isolating, using electricity, a series of elements for the first time: potassium and sodium in 1807 and calcium, barium and boron the following year, as well as discovering the elemental nature of chlorine and iodine. He studied the forces involved in these separations, inventing the new field of electrochemistry. In 1799 Davy experimented with nitrous oxide and was astonished at how it made him laugh, so he nicknamed it "laughing gas", wrote about its potential anaesthetic properties in relieving pain during surgery. Berzelius called Davy's 1806 Bakerian Lecture On Some Chemical Agencies of Electricity "one of the best memoirs which has enriched the theory of chemistry." Davy was a baronet, President of the Royal Society, Member of the Royal Irish Academy, Fellow of the Geological Society. He invented the Davy lamp and a early form of arc lamp, he joked. Davy was born in Penzance, Cornwall in England on 17 December, 1778.
Davy's brother, writes that the society of their hometown was characterised by "an unbounded credulity respecting the supernatural and monstrous... Amongst the middle and higher classes, there was little taste for literature, still less for science... Hunting, wrestling, cockfighting ending in drunkenness, were what they most delighted in". At the age of six, Davy was sent to the grammar school at Penzance. Three years his family moved to Varfell, near Ludgvan, subsequently, in term-time Davy boarded with John Tonkin, his godfather and his guardian. On leaving Penzance grammar school in 1793, Tonkin paid for Davy to attend Truro Grammar School in 1793 to finish his education under the Rev Dr Cardew, who, in a letter to Davies Gilbert, said dryly: "I could not discern the faculties by which he was afterwards so much distinguished." Yet, Davy entertained his school friends with writing poetry and telling stories from One Thousand and One Nights. Reflecting on his school days, in a letter to his mother, Davy wrote: "Learning is a true pleasure.
Davy said: "I consider it fortunate I was left much to myself as a child, put upon no particular plan of study... What I am I made myself." Davy's brother praises his "native vigour": "there belonged, however, to his mind, it cannot be doubted, the genuine quality of genius, or of that power of intellect which exalts its possessor above the crowd."After Davy's father died in 1794, Tonkin apprenticed him to John Bingham Borlase, a surgeon with a practice in Penzance. Davy's indenture is dated 10 February 1795. In the apothecary's dispensary, Davy became a chemist, conducted his earliest chemical experiments in a garret in Tonkin's house. Davy's friends said: "This boy Humphry is incorrigible, he will blow us all into the air." His elder sister complained of the ravages made on her dresses by corrosive substances. Davy was taught French by a refugee priest, in 1797 read Lavoisier's Traité élémentaire de chimie: much of his future work can be seen as reacting against Lavoisier's work and the dominance of French chemists.
As a poet, over one hundred and sixty manuscript poems were written by Davy, the majority of which are found in his personal notebooks. Most of his written poems were not published, he chose instead to share a few of them with his friends. Eight of his known poems were published, his poems reflected his views on both his career and his pereception of certain aspects of human life. He wrote on human endeavours and aspects of life like death, geology, natural theology and chemistry. John Ayrton Paris remarked that poetry written by the young Davy "bear the stamp of lofty genius". Davy's first preserved poem entitled The Sons of Genius is dated 1795 and marked by the usual immaturity of youth. Other poems written in the following years On the Mount's Bay and St Michael's Mount, are descriptive verses, showing sensibility but no true poetic imagination. Three of Davy's paintings from around 1796 have been donated to the Penlee House museum at Penzance. One is of the view from above Gulval showing the church, Mount's Bay and the Mount, while the other two depict Loch Lomond in Scotland.
While writing verses at the age of 17 in honour of his first love, he was eagerly discussing the question of the materiality of heat with his Quaker friend and mentor Robert Dunkin. Dunkin remarked:'I tell thee what, thou art the most quibbling hand at a dispute I met with in my life.' One winter day he took Davy to the Larigan River, To show him that rubbing two plates of ice together developed sufficient energy by motion, to melt them, that after the motion was suspended, the pieces were united by regelation. It was a crude form of analogous experiment exhibited by Davy in the lecture-room of the Royal Institution that elicited considerable attention; as professor at the Royal Institution, Davy repeated many of the ingenious experiments he learned from his friend and mentor, Robert Dunkin. Though he started writing his poems, albeit haphazardly, as a reflection of his views on his career and on life most of his final poems concentrated on immortality and death; this was after he started experiencing failing a decline both in health and career.
Davies Giddy met Davy in Penzance carelessly swinging on the half-gate of Dr Borlase's house, interested by his talk invited him to his house at Tredrea and offered him the use of his library. This led to an introduction to Dr Edwards. Edwards was a lecturer in
Plasma (physics)
Plasma is one of the four fundamental states of matter, was first described by chemist Irving Langmuir in the 1920s. Plasma can be artificially generated by heating or subjecting a neutral gas to a strong electromagnetic field to the point where an ionized gaseous substance becomes electrically conductive, long-range electromagnetic fields dominate the behaviour of the matter. Plasma and ionized gases have properties and display behaviours unlike those of the other states, the transition between them is a matter of nomenclature and subject to interpretation. Based on the surrounding environmental temperature and density ionized or ionized forms of plasma may be produced. Neon signs and lightning are examples of ionized plasma; the Earth's ionosphere is a plasma and the magnetosphere contains plasma in the Earth's surrounding space environment. The interior of the Sun is an example of ionized plasma, along with the solar corona and stars. Positive charges in ions are achieved by stripping away electrons orbiting the atomic nuclei, where the total number of electrons removed is related to either increasing temperature or the local density of other ionized matter.
This can be accompanied by the dissociation of molecular bonds, though this process is distinctly different from chemical processes of ion interactions in liquids or the behaviour of shared ions in metals. The response of plasma to electromagnetic fields is used in many modern technological devices, such as plasma televisions or plasma etching. Plasma may be the most abundant form of ordinary matter in the universe, although this hypothesis is tentative based on the existence and unknown properties of dark matter. Plasma is associated with stars, extending to the rarefied intracluster medium and the intergalactic regions; the word plasma comes from Ancient Greek πλάσμα, meaning'moldable substance' or'jelly', describes the behaviour of the ionized atomic nuclei and the electrons within the surrounding region of the plasma. Each of these nuclei are suspended in a movable sea of electrons. Plasma was first identified in a Crookes tube, so described by Sir William Crookes in 1879; the nature of this "cathode ray" matter was subsequently identified by British physicist Sir J.
J. Thomson in 1897; the term "plasma" was coined by Irving Langmuir in 1928. Lewi Tonks and Harold Mott-Smith, both of whom worked with Irving Langmuir in the 1920s, recall that Langmuir first used the word "plasma" in analogy with blood. Mott-Smith recalls, in particular, that the transport of electrons from thermionic filaments reminded Langmuir of "the way blood plasma carries red and white corpuscles and germs."Langmuir described the plasma he observed as follows: "Except near the electrodes, where there are sheaths containing few electrons, the ionized gas contains ions and electrons in about equal numbers so that the resultant space charge is small. We shall use the name plasma to describe this region containing balanced charges of ions and electrons." Plasma is a state of matter in which an ionized gaseous substance becomes electrically conductive to the point that long-range electric and magnetic fields dominate the behaviour of the matter. The plasma state can be contrasted with the other states: solid and gas.
Plasma is an electrically neutral medium of unbound negative particles. Although these particles are unbound, they are not "free" in the sense of not experiencing forces. Moving charged particles generate an electric current within a magnetic field, any movement of a charged plasma particle affects and is affected by the fields created by the other charges. In turn this governs collective behaviour with many degrees of variation. Three factors define a plasma: The plasma approximation: The plasma approximation applies when the plasma parameter, Λ, representing the number of charge carriers within a sphere surrounding a given charged particle, is sufficiently high as to shield the electrostatic influence of the particle outside of the sphere. Bulk interactions: The Debye screening length is short compared to the physical size of the plasma; this criterion means that interactions in the bulk of the plasma are more important than those at its edges, where boundary effects may take place. When this criterion is satisfied, the plasma is quasineutral.
Plasma frequency: The electron plasma frequency is large compared to the electron-neutral collision frequency. When this condition is valid, electrostatic interactions dominate over the processes of ordinary gas kinetics. Plasma temperature is measured in kelvin or electronvolts and is, informally, a measure of the thermal kinetic energy per particle. High temperatures are needed to sustain ionisation, a defining feature of a plasma; the degree of plasma ionisation is determined by the electron temperature relative to the ionization energy, in a relationship called the Saha equation. At low temperatures and electrons tend to recombine into bound states—atoms—and the plasma will become a gas. In most cases the electrons are close enough to thermal equilibrium that their temperature is well-defined; because of the large difference in ma
Light
Light is electromagnetic radiation within a certain portion of the electromagnetic spectrum. The word refers to visible light, the visible spectrum, visible to the human eye and is responsible for the sense of sight. Visible light is defined as having wavelengths in the range of 400–700 nanometres, or 4.00 × 10−7 to 7.00 × 10−7 m, between the infrared and the ultraviolet. This wavelength means a frequency range of 430–750 terahertz; the main source of light on Earth is the Sun. Sunlight provides the energy that green plants use to create sugars in the form of starches, which release energy into the living things that digest them; this process of photosynthesis provides all the energy used by living things. Another important source of light for humans has been fire, from ancient campfires to modern kerosene lamps. With the development of electric lights and power systems, electric lighting has replaced firelight; some species of animals generate their own light, a process called bioluminescence.
For example, fireflies use light to locate mates, vampire squids use it to hide themselves from prey. The primary properties of visible light are intensity, propagation direction, frequency or wavelength spectrum, polarization, while its speed in a vacuum, 299,792,458 metres per second, is one of the fundamental constants of nature. Visible light, as with all types of electromagnetic radiation, is experimentally found to always move at this speed in a vacuum. In physics, the term light sometimes refers to electromagnetic radiation of any wavelength, whether visible or not. In this sense, gamma rays, X-rays and radio waves are light. Like all types of EM radiation, visible light propagates as waves. However, the energy imparted by the waves is absorbed at single locations the way particles are absorbed; the absorbed energy of the EM waves is called a photon, represents the quanta of light. When a wave of light is transformed and absorbed as a photon, the energy of the wave collapses to a single location, this location is where the photon "arrives."
This is. This dual wave-like and particle-like nature of light is known as the wave–particle duality; the study of light, known as optics, is an important research area in modern physics. EM radiation, or EMR, is classified by wavelength into radio waves, infrared, the visible spectrum that we perceive as light, ultraviolet, X-rays, gamma rays; the behavior of EMR depends on its wavelength. Higher frequencies have shorter wavelengths, lower frequencies have longer wavelengths; when EMR interacts with single atoms and molecules, its behavior depends on the amount of energy per quantum it carries. EMR in the visible light region consists of quanta that are at the lower end of the energies that are capable of causing electronic excitation within molecules, which leads to changes in the bonding or chemistry of the molecule. At the lower end of the visible light spectrum, EMR becomes invisible to humans because its photons no longer have enough individual energy to cause a lasting molecular change in the visual molecule retinal in the human retina, which change triggers the sensation of vision.
There exist animals that are sensitive to various types of infrared, but not by means of quantum-absorption. Infrared sensing in snakes depends on a kind of natural thermal imaging, in which tiny packets of cellular water are raised in temperature by the infrared radiation. EMR in this range causes molecular vibration and heating effects, how these animals detect it. Above the range of visible light, ultraviolet light becomes invisible to humans because it is absorbed by the cornea below 360 nm and the internal lens below 400 nm. Furthermore, the rods and cones located in the retina of the human eye cannot detect the short ultraviolet wavelengths and are in fact damaged by ultraviolet. Many animals with eyes that do not require lenses are able to detect ultraviolet, by quantum photon-absorption mechanisms, in much the same chemical way that humans detect visible light. Various sources define visible light as narrowly as 420–680 nm to as broadly as 380–800 nm. Under ideal laboratory conditions, people can see infrared up to at least 1050 nm.
Plant growth is affected by the color spectrum of light, a process known as photomorphogenesis. The speed of light in a vacuum is defined to be 299,792,458 m/s; the fixed value of the speed of light in SI units results from the fact that the metre is now defined in terms of the speed of light. All forms of electromagnetic radiation move at this same speed in vacuum. Different physicists have attempted to measure the speed of light throughout history. Galileo attempted to measure the speed of light in the seventeenth century. An early experiment to measure the speed of light was conducted by Ole Rømer, a Danish physicist, in 1676. Using a telescope, Rømer observed one of its moons, Io. Noting discrepancies in the apparent period of Io's orbit, he calculated that light takes about 22 minutes to traverse the diameter of Earth's orbit. However, its size was not known at that time. If Rømer had known the diameter of the Earth's orbit, he would have calculated a speed of 227,000,000 m/s. Another, more accurate, measurement of the speed of light was performed in Europe by Hippolyte Fizeau in 1849.
Fizeau dire
Gas
Gas is one of the four fundamental states of matter. A pure gas may be made up of individual atoms, elemental molecules made from one type of atom, or compound molecules made from a variety of atoms. A gas mixture would contain a variety of pure gases much like the air. What distinguishes a gas from liquids and solids is the vast separation of the individual gas particles; this separation makes a colorless gas invisible to the human observer. The interaction of gas particles in the presence of electric and gravitational fields are considered negligible, as indicated by the constant velocity vectors in the image; the gaseous state of matter is found between the liquid and plasma states, the latter of which provides the upper temperature boundary for gases. Bounding the lower end of the temperature scale lie degenerative quantum gases which are gaining increasing attention. High-density atomic gases super cooled to low temperatures are classified by their statistical behavior as either a Bose gas or a Fermi gas.
For a comprehensive listing of these exotic states of matter see list of states of matter. The only chemical elements that are stable diatomic homonuclear molecules at STP are hydrogen, nitrogen and two halogens: fluorine and chlorine; when grouped together with the monatomic noble gases – helium, argon, krypton and radon – these gases are called "elemental gases". The word gas was first used by the early 17th-century Flemish chemist Jan Baptist van Helmont, he identified the first known gas other than air. Van Helmont's word appears to have been a phonetic transcription of the Ancient Greek word χάος Chaos – the g in Dutch being pronounced like ch in "loch" – in which case Van Helmont was following the established alchemical usage first attested in the works of Paracelsus. According to Paracelsus's terminology, chaos meant something like "ultra-rarefied water". An alternative story is that Van Helmont's word is corrupted from gahst, signifying a ghost or spirit; this was because certain gases suggested a supernatural origin, such as from their ability to cause death, extinguish flames, to occur in "mines, bottom of wells and other lonely places".
In contrast, French-American historian Jacques Barzun speculated that Van Helmont had borrowed the word from the German Gäscht, meaning the froth resulting from fermentation. Because most gases are difficult to observe directly, they are described through the use of four physical properties or macroscopic characteristics: pressure, number of particles and temperature; these four characteristics were observed by scientists such as Robert Boyle, Jacques Charles, John Dalton, Joseph Gay-Lussac and Amedeo Avogadro for a variety of gases in various settings. Their detailed studies led to a mathematical relationship among these properties expressed by the ideal gas law. Gas particles are separated from one another, have weaker intermolecular bonds than liquids or solids; these intermolecular forces result from electrostatic interactions between gas particles. Like-charged areas of different gas particles repel, while oppositely charged regions of different gas particles attract one another. Gaseous compounds with polar covalent bonds contain permanent charge imbalances and so experience strong intermolecular forces, although the molecule while the compound's net charge remains neutral.
Transient, randomly induced charges exist across non-polar covalent bonds of molecules and electrostatic interactions caused by them are referred to as Van der Waals forces. The interaction of these intermolecular forces varies within a substance which determines many of the physical properties unique to each gas. A comparison of boiling points for compounds formed by ionic and covalent bonds leads us to this conclusion; the drifting smoke particles in the image provides some insight into low-pressure gas behavior. Compared to the other states of matter, gases have low viscosity. Pressure and temperature influence the particles within a certain volume; this variation in particle separation and speed is referred to as compressibility. This particle separation and size influences optical properties of gases as can be found in the following list of refractive indices. Gas particles spread apart or diffuse in order to homogeneously distribute themselves throughout any container; when observing a gas, it is typical to specify a frame of length scale.
A larger length scale corresponds to a global point of view of the gas. This region must be sufficient in size to contain a large sampling of gas particles; the resulting statistical analysis of this sample size produces the "average" behavior of all the gas particles within the region. In contrast, a smaller length scale corresponds to a particle point of view. Macroscopically, the gas characteristics measured are either in terms of the gas particles themselves or their surroundings. For example, Robert Boyle studied pneumatic chemistry for a small portion of his career. One of his experiments related the macroscopic properties of volume of a gas, his experiment used a J-tube manometer which looks like a test tube in the shape of the letter J. Boyle trapped an inert gas in the closed end of the test tube with a column of mercury, thereby ma
