Capillary action is the ability of a liquid to flow in narrow spaces without the assistance of, or in opposition to, external forces like gravity. The effect can be seen in the drawing up of liquids between the hairs of a paint-brush, in a thin tube, in porous materials such as paper and plaster, in some non-porous materials such as sand and liquefied carbon fiber, or in a biological cell, it occurs because of intermolecular forces between surrounding solid surfaces. If the diameter of the tube is sufficiently small the combination of surface tension and adhesive forces between the liquid and container wall act to propel the liquid; the first recorded observation of capillary action was by Leonardo da Vinci. A former student of Galileo, Niccolò Aggiunti, was said to have investigated capillary action. In 1660, capillary action was still a novelty to the Irish chemist Robert Boyle, when he reported that "some inquisitive French Men" had observed that when a capillary tube was dipped into water, the water would ascend to "some height in the Pipe".
Boyle reported an experiment in which he dipped a capillary tube into red wine and subjected the tube to a partial vacuum. He found that the vacuum had no observable influence on the height of the liquid in the capillary, so the behavior of liquids in capillary tubes was due to some phenomenon different from that which governed mercury barometers. Others soon followed Boyle's lead; some thought that liquids rose in capillaries because air could not enter capillaries as as liquids, so the air pressure was lower inside capillaries. Others thought that the particles of liquid were attracted to each other and to the walls of the capillary. Although experimental studies continued during the 18th century, a successful quantitative treatment of capillary action was not attained until 1805 by two investigators: Thomas Young of the United Kingdom and Pierre-Simon Laplace of France, they derived the Young–Laplace equation of capillary action. By 1830, the German mathematician Carl Friedrich Gauss had determined the boundary conditions governing capillary action.
In 1871, the British physicist William Thomson determined the effect of the meniscus on a liquid's vapor pressure—a relation known as the Kelvin equation. German physicist Franz Ernst Neumann subsequently determined the interaction between two immiscible liquids. Albert Einstein's first paper, submitted to Annalen der Physik in 1900, was on capillarity. Capillary penetration in porous media shares its dynamic mechanism with flow in hollow tubes, as both processes are resisted by viscous forces. A common apparatus used to demonstrate the phenomenon is the capillary tube; when the lower end of a glass tube is placed in a liquid, such as water, a concave meniscus forms. Adhesion occurs between the fluid and the solid inner wall pulling the liquid column along until there is a sufficient mass of liquid for gravitational forces to overcome these intermolecular forces; the contact length between the top of the liquid column and the tube is proportional to the radius of the tube, while the weight of the liquid column is proportional to the square of the tube's radius.
So, a narrow tube will draw a liquid column along further than a wider tube will, given that the inner water molecules cohere sufficiently to the outer ones. Capillary action is seen in many plants. Water is brought high up in trees by branching. Capillary action for uptake of water has been described in some small animals, such as Ligia exotica and Moloch horridus. In the built environment, evaporation limited capillary penetration is responsible for the phenomenon of rising damp in concrete and masonry, while in industry and diagnostic medicine this phenomenon is being harnessed in the field of paper-based microfluidics. In physiology, capillary action is essential for the drainage of continuously produced tear fluid from the eye. Two canaliculi of tiny diameter are present in the inner corner of the eyelid called the lacrimal ducts. Wicking is the absorption of a liquid by a material in the manner of a candle wick. Paper towels absorb liquid through capillary action, allowing a fluid to be transferred from a surface to the towel.
The small pores of a sponge act as small capillaries, causing it to absorb a large amount of fluid. Some textile fabrics are said to use capillary action to "wick" sweat away from the skin; these are referred to as wicking fabrics, after the capillary properties of candle and lamp wicks. Capillary action is observed in thin layer chromatography, in which a solvent moves vertically up a plate via capillary action. In this case the pores are gaps between small particles. Capillary action draws ink to the tips of fountain pen nibs from a reservoir or cartridge inside the pen. With some pairs of materials, such as mercury and glass, the intermolecular forces within the liquid exceed those between the solid and the liquid, so a convex meniscus forms and capillary action works in reverse. In hydrology, capillary action describes the attraction of water molecules to soil particles. Capillary action is responsible for moving groundwater from wet areas of the soil to dry a
The European Union is a political and economic union of 28 member states that are located in Europe. It has an area of an estimated population of about 513 million; the EU has developed an internal single market through a standardised system of laws that apply in all member states in those matters, only those matters, where members have agreed to act as one. EU policies aim to ensure the free movement of people, goods and capital within the internal market, enact legislation in justice and home affairs and maintain common policies on trade, agriculture and regional development. For travel within the Schengen Area, passport controls have been abolished. A monetary union was established in 1999 and came into full force in 2002 and is composed of 19 EU member states which use the euro currency; the EU and European citizenship were established when the Maastricht Treaty came into force in 1993. The EU traces its origins to the European Coal and Steel Community and the European Economic Community, established by the 1951 Treaty of Paris and 1957 Treaty of Rome.
The original members of what came to be known as the European Communities were the Inner Six: Belgium, Italy, the Netherlands, West Germany. The Communities and its successors have grown in size by the accession of new member states and in power by the addition of policy areas to its remit; the latest major amendment to the constitutional basis of the EU, the Treaty of Lisbon, came into force in 2009. While no member state has left the EU or its antecedent organisations, the United Kingdom signified the intention to leave after a membership referendum in June 2016 and is negotiating its withdrawal. Covering 7.3% of the world population, the EU in 2017 generated a nominal gross domestic product of 19.670 trillion US dollars, constituting 24.6% of global nominal GDP. Additionally, all 28 EU countries have a high Human Development Index, according to the United Nations Development Programme. In 2012, the EU was awarded the Nobel Peace Prize. Through the Common Foreign and Security Policy, the EU has developed a role in external relations and defence.
The union maintains permanent diplomatic missions throughout the world and represents itself at the United Nations, the World Trade Organization, the G7 and the G20. Because of its global influence, the European Union has been described as an emerging superpower. During the centuries following the fall of Rome in 476, several European States viewed themselves as translatio imperii of the defunct Roman Empire: the Frankish Empire and the Holy Roman Empire were thereby attempts to resurrect Rome in the West; this political philosophy of a supra-national rule over the continent, similar to the example of the ancient Roman Empire, resulted in the early Middle Ages in the concept of a renovatio imperii, either in the forms of the Reichsidee or the religiously inspired Imperium Christianum. Medieval Christendom and the political power of the Papacy are cited as conducive to European integration and unity. In the oriental parts of the continent, the Russian Tsardom, the Empire, declared Moscow to be Third Rome and inheritor of the Eastern tradition after the fall of Constantinople in 1453.
The gap between Greek East and Latin West had been widened by the political scission of the Roman Empire in the 4th century and the Great Schism of 1054. Pan-European political thought emerged during the 19th century, inspired by the liberal ideas of the French and American Revolutions after the demise of Napoléon's Empire. In the decades following the outcomes of the Congress of Vienna, ideals of European unity flourished across the continent in the writings of Wojciech Jastrzębowski, Giuseppe Mazzini or Theodore de Korwin Szymanowski; the term United States of Europe was used at that time by Victor Hugo during a speech at the International Peace Congress held in Paris in 1849: A day will come when all nations on our continent will form a European brotherhood... A day will come when we shall see... the United States of America and the United States of Europe face to face, reaching out for each other across the seas. During the interwar period, the consciousness that national markets in Europe were interdependent though confrontational, along with the observation of a larger and growing US market on the other side of the ocean, nourished the urge for the economic integration of the continent.
In 1920, advocating the creation of a European economic union, British economist John Maynard Keynes wrote that "a Free Trade Union should be established... to impose no protectionist tariffs whatever against the produce of other members of the Union." During the same decade, Richard von Coudenhove-Kalergi, one of the first to imagine of a modern political union of Europe, founded the Pan-Europa Movement. His ideas influenced his contemporaries, among which Prime Minister of France Aristide Briand. In 1929, the latter gave a speech in favour of a European Union before the assembly of the League of Nations, precursor of the United Nations. In a radio address in March 1943, with war still raging, Britain's leader Sir Winston Churchill spoke warmly of "restoring the true greatness of Europe" once victory had been achieved, mused on the post-war creation of a "Council of Europe" which would bring the European nations together to build peace. After World War II, European integration was seen as an antidote to the extreme nationalism which had devastated the continent.
In a speech delivered on 19
Vacuum is space devoid of matter. The word stems from the Latin adjective vacuus for "vacant" or "void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure. Physicists discuss ideal test results that would occur in a perfect vacuum, which they sometimes call "vacuum" or free space, use the term partial vacuum to refer to an actual imperfect vacuum as one might have in a laboratory or in space. In engineering and applied physics on the other hand, vacuum refers to any space in which the pressure is lower than atmospheric pressure; the Latin term in vacuo is used to describe an object, surrounded by a vacuum. The quality of a partial vacuum refers to how it approaches a perfect vacuum. Other things equal, lower gas pressure means higher-quality vacuum. For example, a typical vacuum cleaner produces enough suction to reduce air pressure by around 20%. Much higher-quality vacuums are possible. Ultra-high vacuum chambers, common in chemistry and engineering, operate below one trillionth of atmospheric pressure, can reach around 100 particles/cm3.
Outer space is an higher-quality vacuum, with the equivalent of just a few hydrogen atoms per cubic meter on average in intergalactic space. According to modern understanding if all matter could be removed from a volume, it would still not be "empty" due to vacuum fluctuations, dark energy, transiting gamma rays, cosmic rays and other phenomena in quantum physics. In the study of electromagnetism in the 19th century, vacuum was thought to be filled with a medium called aether. In modern particle physics, the vacuum state is considered the ground state of a field. Vacuum has been a frequent topic of philosophical debate since ancient Greek times, but was not studied empirically until the 17th century. Evangelista Torricelli produced the first laboratory vacuum in 1643, other experimental techniques were developed as a result of his theories of atmospheric pressure. A torricellian vacuum is created by filling a tall glass container closed at one end with mercury, inverting it in a bowl to contain the mercury.
Vacuum became a valuable industrial tool in the 20th century with the introduction of incandescent light bulbs and vacuum tubes, a wide array of vacuum technology has since become available. The recent development of human spaceflight has raised interest in the impact of vacuum on human health, on life forms in general; the word vacuum comes from Latin, meaning'an empty space, void', noun use of neuter of vacuus, meaning "empty", related to vacare, meaning "be empty". Vacuum is one of the few words in the English language that contains two consecutive letters'u'. There has been much dispute over whether such a thing as a vacuum can exist. Ancient Greek philosophers debated the existence of a vacuum, or void, in the context of atomism, which posited void and atom as the fundamental explanatory elements of physics. Following Plato the abstract concept of a featureless void faced considerable skepticism: it could not be apprehended by the senses, it could not, provide additional explanatory power beyond the physical volume with which it was commensurate and, by definition, it was quite nothing at all, which cannot rightly be said to exist.
Aristotle believed that no void could occur because the denser surrounding material continuum would fill any incipient rarity that might give rise to a void. In his Physics, book IV, Aristotle offered numerous arguments against the void: for example, that motion through a medium which offered no impediment could continue ad infinitum, there being no reason that something would come to rest anywhere in particular. Although Lucretius argued for the existence of vacuum in the first century BC and Hero of Alexandria tried unsuccessfully to create an artificial vacuum in the first century AD, it was European scholars such as Roger Bacon, Blasius of Parma and Walter Burley in the 13th and 14th century who focused considerable attention on these issues. Following Stoic physics in this instance, scholars from the 14th century onward departed from the Aristotelian perspective in favor of a supernatural void beyond the confines of the cosmos itself, a conclusion acknowledged by the 17th century, which helped to segregate natural and theological concerns.
Two thousand years after Plato, René Descartes proposed a geometrically based alternative theory of atomism, without the problematic nothing–everything dichotomy of void and atom. Although Descartes agreed with the contemporary position, that a vacuum does not occur in nature, the success of his namesake coordinate system and more implicitly, the spatial–corporeal component of his metaphysics would come to define the philosophically modern notion of empty space as a quantified extension of volume. By the ancient definition however, directional information and magnitude were conceptually distinct. In the medieval Middle Eastern world, the physicist and Islamic scholar, Al-Farabi, conducted a small experiment concerning the existence of vacuum, in which he investigated handheld plungers in water, he concluded that air's volume can expand to fill available space, he suggested that the concept of perfect vacuum was incoherent. However, according to Nader El-Bizri, the physicist Ibn al-Haytham and the Mu'tazili theologians disagreed with Aristotle and Al-Farabi, they supported the existence of a void.
Using geometry, Ibn al-Haytham mathematically demonstrated that place is the imagined three-dimensional void between the inner surfaces of a containing body. According to Ahmad Dallal, Abū Rayhān al-Bīrūnī states that "there is no observable
In physics, a vapor or vapour is a substance in the gas phase at a temperature lower than its critical temperature, which means that the vapor can be condensed to a liquid by increasing the pressure on it without reducing the temperature. A vapor is different from an aerosol. An aerosol is a suspension of tiny particles of both within a gas. For example, water has a critical temperature of 647 K, the highest temperature at which liquid water can exist. In the atmosphere at ordinary temperatures, gaseous water will condense into a liquid if its partial pressure is increased sufficiently. A vapor may co-exist with a liquid; when this is true, the two phases will be in equilibrium, the gas-partial pressure will be equal to the equilibrium vapor pressure of the liquid. Vapor refers to a gas phase at a temperature where the same substance can exist in the liquid or solid state, below the critical temperature of the substance. If the vapor is in contact with a liquid or solid phase, the two phases will be in a state of equilibrium.
The term gas refers to a compressible fluid phase. Fixed gases are gases for which no liquid or solid can form at the temperature of the gas, such as air at typical ambient temperatures. A liquid or solid does not have to boil to release a vapor. Vapor is responsible for the familiar processes of cloud condensation, it is employed to carry out the physical processes of distillation and headspace extraction from a liquid sample prior to gas chromatography. The constituent molecules of a vapor possess vibrational and translational motion; these motions are considered in the kinetic theory of gases. The vapor pressure is a solid at a specific temperature; the equilibrium vapor pressure of a liquid or solid is not affected by the amount of contact with the liquid or solid interface. The normal boiling point of a liquid is the temperature at which the vapor pressure is equal to normal atmospheric pressure. For two-phase systems, the vapor pressure of the individual phases are equal. In the absence of stronger inter-species attractions between like-like or like-unlike molecules, the vapor pressure follows Raoult's law, which states that the partial pressure of each component is the product of the vapor pressure of the pure component and its mole fraction in the mixture.
The total vapor pressure is the sum of the component partial pressures. Perfumes contain chemicals that vaporize at different temperatures and at different rate in scent accords, known as notes. Atmospheric water vapor is found near the earth's surface, may condense into small liquid droplets and form meteorological phenomena, such as fog and haar. Mercury-vapor lamps and sodium vapor lamps produce light from atoms in excited states. Flammable liquids do not burn, it is the vapor cloud above the liquid that will burn if the vapor's concentration is between the lower flammable limit and upper flammable limit, of the flammable liquid. E-Cigarettes allow users to inhale "e-liquid" aerosol/vapor, rather than cigarette smoke. Since it is in the gas phase, the amount of vapor present is quantified by the partial pressure of the gas. Vapors obey the barometric formula in a gravitational field, just as conventional atmospheric gases do. Dilution Evaporation – Type of vaporization of a liquid that occurs from its surface.
Horticulture has been defined as the culture of plants for food and beauty. A more precise definition can be given as "The cultivation and sale of fruits, vegetables, ornamental plants, flowers as well as many additional services", it includes plant conservation, landscape restoration, soil management and garden design and maintenance, arboriculture. In contrast to agriculture, horticulture does not include large-scale crop production or animal husbandry. Horticulturists apply their knowledge and technologies used to grow intensively produced plants for human food and non-food uses and for personal or social needs, their work involves plant propagation and cultivation with the aim of improving plant growth, quality, nutritional value, resistance to insects and environmental stresses. They work as gardeners, therapists and technical advisors in the food and non-food sectors of horticulture. Horticulture refers to the growing of plants in a field or garden; the word horticulture is modeled after agriculture, comes from the Latin hortus "garden" and cultūra "cultivation", from cultus, the perfect passive participle of the verb colō "I cultivate".
Hortus is cognate with the native English word yard and the borrowed word garden. The major areas of Horticulture include: Arboriculture is the study of, the selection, plant and removal of, individual trees, shrubs and other perennial woody plants. Turf management includes all aspects of the production and maintenance of turf grass for sports, leisure use or amenity use. Floriculture includes the marketing of floral crops. Study of flower cultivation. Landscape horticulture includes the production and maintenance of landscape plants. Olericulture includes the marketing of vegetables. Pomology includes the marketing of pome fruits. Viticulture includes the marketing of grapes. Oenology includes all aspects of winemaking. Postharvest physiology involves maintaining the quality of and preventing the spoilage of plants and animals. Horticulture has a long history; the study and science of horticulture dates all the way back to the times of Cyrus the Great of ancient Persia, has been going on since, with present-day horticulturists such as Freeman S. Howlett and Luther Burbank.
The practice of horticulture can be retraced for many thousands of years. The cultivation of taro and yam in Papua New Guinea dates back to at least 6950–6440 cal BP; the origins of horticulture lie in the transition of human communities from nomadic hunter-gatherers to sedentary or semi-sedentary horticultural communities, cultivating a variety of crops on a small scale around their dwellings or in specialized plots visited during migrations from one area to the next. In the Pre-Columbian Amazon Rainforest, natives are believed to have used biochar to enhance soil productivity by smoldering plant waste. European settlers called it Terra Preta de Indio. In forest areas such horticulture is carried out in swiddens. A characteristic of horticultural communities is that useful trees are to be found planted around communities or specially retained from the natural ecosystem. Horticulture differs from agriculture in two ways. First, it encompasses a smaller scale of cultivation, using small plots of mixed crops rather than large fields of single crops.
Secondly, horticultural cultivations include a wide variety of crops including fruit trees with ground crops. Agricultural cultivations however as a rule focus on one primary crop. In pre-contact North America the semi-sedentary horticultural communities of the Eastern Woodlands contrasted markedly with the mobile hunter-gatherer communities of the Plains people. In Central America, Maya horticulture involved augmentation of the forest with useful trees such as papaya, cacao and sapodilla. In the cornfields, multiple crops were grown such as beans, squash and chilli peppers, in some cultures tended or by women. Since 1804 The Royal Horticultural Society, a UK charity, leads on the encouragement and improvement of the science and practice of horticulture in all its branches and shares this knowledge through its community and learning programmes, world class gardens and shows; the oldest Horticultural society in the world, founded in 1768, is the Ancient Society of York Florists. They still have four shows a year in York, UK.
The professional body representing horticulturists in Great Britain and Ireland is the Institute of Horticulture. The IOH has an international branch for members outside of these islands; the International Society for Horticultural Science promotes and encourages research and education in all branches of horticultural science. The American Society of Horticultural Science promotes and encourages research and education in all branches of horticultural science in the Americas; the Australian Society of Horticultural Science was established in 1990 as a professional society for the promotion and enhancement of Australian horticultural science and industry. The National Junior Horticultural Association was established in 1934 and was the first organisation in the world dedicated to youth and horticulture. NJHA programs are designed to help young people obtain a basic understanding of, develop skills in, the ever-expanding art and science of horticulture; the New Zealand Horticulture Institute. The Global Horticulture Initiative (GlobalHo
A magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials, such as iron, attracts or repels other magnets. A permanent magnet is an object made from a material, magnetized and creates its own persistent magnetic field. An everyday example is a refrigerator magnet used to hold notes on a refrigerator door. Materials that can be magnetized, which are the ones that are attracted to a magnet, are called ferromagnetic; these include the elements iron and cobalt, some alloys of rare-earth metals, some occurring minerals such as lodestone. Although ferromagnetic materials are the only ones attracted to a magnet enough to be considered magnetic, all other substances respond weakly to a magnetic field, by one of several other types of magnetism. Ferromagnetic materials can be divided into magnetically "soft" materials like annealed iron, which can be magnetized but do not tend to stay magnetized, magnetically "hard" materials, which do.
Permanent magnets are made from "hard" ferromagnetic materials such as alnico and ferrite that are subjected to special processing in a strong magnetic field during manufacture to align their internal microcrystalline structure, making them hard to demagnetize. To demagnetize a saturated magnet, a certain magnetic field must be applied, this threshold depends on coercivity of the respective material. "Hard" materials have high coercivity, whereas "soft" materials have low coercivity. The overall strength of a magnet is measured by its magnetic moment or, the total magnetic flux it produces; the local strength of magnetism in a material is measured by its magnetization. An electromagnet is made from a coil of wire that acts as a magnet when an electric current passes through it but stops being a magnet when the current stops; the coil is wrapped around a core of "soft" ferromagnetic material such as mild steel, which enhances the magnetic field produced by the coil. Ancient people learned about magnetism from lodestones which are magnetized pieces of iron ore.
The word magnet was adopted in Middle English from Latin magnetum "lodestone" from Greek μαγνῆτις meaning " from Magnesia", a part of ancient Greece where lodestones were found. Lodestones, suspended so they could turn, were the first magnetic compasses; the earliest known surviving descriptions of magnets and their properties are from Greece and China around 2500 years ago. The properties of lodestones and their affinity for iron were written of by Pliny the Elder in his encyclopedia Naturalis Historia. By the 12th to 13th centuries AD, magnetic compasses were used in navigation in China, the Arabian Peninsula and elsewhere; the magnetic flux density is a vector field. The magnetic B field vector at a given point in space is specified by two properties: Its direction, along the orientation of a compass needle, its magnitude, proportional to how the compass needle orients along that direction. In SI units, the strength of the magnetic B field is given in teslas. A magnet's magnetic moment is a vector.
For a bar magnet, the direction of the magnetic moment points from the magnet's south pole to its north pole, the magnitude relates to how strong and how far apart these poles are. In SI units, the magnetic moment is specified in terms of A·m2. A magnet both responds to magnetic fields; the strength of the magnetic field it produces is at any given point proportional to the magnitude of its magnetic moment. In addition, when the magnet is put into an external magnetic field, produced by a different source, it is subject to a torque tending to orient the magnetic moment parallel to the field; the amount of this torque is proportional both to the external field. A magnet may be subject to a force driving it in one direction or another, according to the positions and orientations of the magnet and source. If the field is uniform in space, the magnet is subject to no net force, although it is subject to a torque. A wire in the shape of a circle with area A and carrying current I has a magnetic moment of magnitude equal to IA.
The magnetization of a magnetized material is the local value of its magnetic moment per unit volume denoted M, with units A/m. It is a vector field, rather than just a vector, because different areas in a magnet can be magnetized with different directions and strengths. A good bar magnet may have a magnetic moment of magnitude 0.1 A•m2 and a volume of 1 cm3, or 1×10−6 m3, therefore an average magnetization magnitude is 100,000 A/m. Iron can have a magnetization of around a million amperes per meter; such a large value explains. Two different models exist for magnets: atomic currents. Although for many purposes it is convenient to think of a magnet as having distinct north and south magnetic poles, the concept of poles should not be taken literally: it is a way of referring to the two different ends of a magnet; the magnet does not have distinct south particles on opposing sides. If a bar magnet is broken into two pieces, in an attempt to separate the north and south poles, the result will be two b
The mercury-in-glass or mercury thermometer was invented by physicist Daniel Gabriel Fahrenheit in Amsterdam. It consists of a bulb containing mercury attached to a glass tube of narrow diameter; the volume of mercury changes with temperature. The space above the mercury may be filled with nitrogen gas or it may be at less than atmospheric pressure, a partial vacuum. In order to calibrate the thermometer, the bulb is made to reach thermal equilibrium with a temperature standard such as an ice/water mixture, with another standard such as water/vapour, the tube is divided into regular intervals between the fixed points. In principle, thermometers made of different material might be expected to give different intermediate readings due to different expansion properties; the application of mercury and Fahrenheit scale for liquid-in-glass thermometers ushered in a new era of accuracy and precision in thermometry, is still to this day regarded as one of the most accurate thermometers available. The thermometer was used by the originators of the Celsius scales.
Anders Celsius, a Swedish scientist, devised the Celsius scale, described in his publication The origin of the Celsius temperature scale in 1742. Celsius used two fixed points in his scale: the temperature of melting ice and the temperature of boiling water; this wasn't a new idea, since Isaac Newton was working on something similar. The distinction of Celsius was to use not that of freezing; the experiments for reaching a good calibration of his thermometer lasted for 2 winters. By performing the same experiment over and over again, he discovered that ice always melted at the same calibration mark on the thermometer, he found a similar fixed point in the calibration of boiling water to water vapour. At the moment that he removed the thermometer from the vapour, the mercury level climbed slightly; this was related to the rapid cooling of the glass. When Celsius decided to use his own temperature scale, he defined his scale "upside-down", i.e. he chose to set the boiling point of pure water at 0 °C and the freezing point at -100 °C.
One year Frenchman Jean-Pierre Christin proposed to invert the scale with the freezing point at 0 °C and the boiling point at 100 °C. He named it Centigrade. Celsius proposed a method of calibrating a thermometer: Place the cylinder of the thermometer in melting ice made of pure water and mark the point where the fluid in the thermometer stabilises; this point is the freeze/thaw point of water. In the same manner mark the point where the fluid stabilises when the thermometer is placed in boiling water vapour. Divide the length between the two marks into 100 equal parts; these points are adequate for approximate calibration but both vary with atmospheric pressure. Nowadays, the triple point of water is used instead of the freezing point. Before the discovery of the true thermodynamic temperature, the thermometer defined the temperature. In practice, several materials gave similar temperatures to each other and, when discovered, to the thermodynamic temperature. One special kind of mercury-in-glass thermometer, called a maximum thermometer, works by having a constriction in the neck close to the bulb.
As the temperature rises, the mercury is pushed up through the constriction by the force of expansion. When the temperature falls, the column of mercury breaks at the constriction and cannot return to the bulb, thus remaining stationary in the tube; the observer can read the maximum temperature over the set period of time. To reset the thermometer it must be swung sharply; this design is used in the traditional type of medical thermometer. A maximum minimum thermometer known as Six's thermometer, is a thermometer which registers the maximum and minimum temperatures reached over a period of time 24 hours; the original design contains mercury, but as a way to indicate the position of a column of alcohol whose expansion indicates the temperature. Mercury thermometers cover a wide temperature range from −37 to 356 °C; this introduction of an inert gas increases the pressure on the liquid mercury and therefore its boiling point is increased, this in combination with replacing the Pyrex glass with fused quartz allows the upper temperature range to be extended to 800 °C.
Mercury cannot be used below the temperature at which it becomes solid, −38.83 °C. If the thermometer contains nitrogen, the gas may flow down into the column when the mercury solidifies and be trapped there when the temperature rises, making the thermometer unusable until returned to the factory for reconditioning. To avoid this, some weather services require that all mercury-in-glass thermometers be brought indoors when the temp