Fire

• 1. Introduction

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  Fire is the rapid oxidation of a fuel, an exothermic process called combustion that releases heat, light, and a mix of reaction products. The most visible part of it, the flame, only appears once the fuel reaches its ignition point temperature. Heat that high can strip electrons from gases and turn them into plasma. It feels elemental, and for most of human history that is exactly how people understood it. Fire was counted among the four classical elements, a fundamental piece of the world itself. Yet the same thing that warms a hearth can raze a city, and the same thing that clears a field can poison the soil beneath it. How did a single chemical reaction become a tool, a weapon, a god, and a hazard all at once? What has to be true for a flame to exist at all, and what happens to that flame when you take gravity away?

• 2. The Fire Tetrahedron

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  Chlorine trifluoride is able to ignite sand, which sounds impossible until you understand what fire actually requires. A combustion reaction is a fuel meeting an oxidizing agent, yielding carbon dioxide and water, and it does not proceed directly but through intermediates. Oxygen is the usual oxidizer, but it is not the only one. Chlorine and other oxygen-rich compounds can play the same role.

  A fire starts when flammable material, enough oxidizer, and heat above the fuel mix's flash point combine to sustain rapid oxidation in a chain reaction. This set of conditions is called the fire tetrahedron, and fire cannot exist unless every element is present in the right proportions. A flammable liquid will only burn when fuel and oxygen are balanced correctly. Some mixtures even need a catalyst, a substance that is not consumed but lets the reactants combust more readily.

  Gravity matters more than most people realize. When the oxidizer is oxygen from the surrounding air, a force such as gravity is needed to drive convection, which carries away combustion products and brings fresh oxygen to the flame. Without it, a fire wraps itself in its own exhaust and non-oxidizing gases, which exclude oxygen and snuff it out. This is why the risk of fire on a spacecraft coasting in inertial flight is small.

  Removing any single element of the tetrahedron extinguishes a fire. Picture a stove-top natural gas burner. Turning off the gas removes the fuel. Covering the flame smothers it by displacing oxygen with carbon dioxide. An inert gas like carbon dioxide does the same. Water pulls heat away faster than the fire can make it. A retardant chemical such as Halon, now largely banned in some countries, slows the reaction until combustion can no longer sustain the chain.

• 3. Anatomy Of A Flame

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  Soot, those incandescent solid particles produced when hydrocarbons like wood burn, is what gives fire its familiar red-orange glow. That light has a continuous spectrum. By contrast, complete combustion of gas glows a dim blue, because excited molecules emit single-wavelength radiation from electron transitions. A diffusion flame is a mixture of reacting gases and solids emitting visible, infrared, and sometimes ultraviolet light, with a spectrum set by the burning material and its intermediates.

  Oxygen is not required for a flame at all. Hydrogen burning in chlorine produces a flame and yields hydrogen chloride. Fluorine with hydrogen works too, as does hydrazine with dinitrogen tetroxide. Hydrogen and hydrazine flames glow pale blue. Burning boron and its compounds, studied in the mid-20th century as a high-energy fuel for jet and rocket engines, emits an intense green flame that earned it the nickname Green Dragon.

  Under normal gravity, the shape of a flame depends on convection. Soot rises to the top, which is what makes a candle flame yellow. In microgravity or zero gravity, convection stops, and the flame becomes spherical, tending toward blue and burning more efficiently. The likely reason is that the temperature grows uniform enough that soot never forms and combustion completes. NASA experiments showed that microgravity diffusion flames oxidize more of their soot than flames on Earth, findings with potential uses in fuel efficiency.

  Temperatures vary enormously by fuel and oxidizer pairing. An oxy-dicyanoacetylene flame reaches 4990 C, while a candle burning paraffin in air sits around 1000 C. A Bunsen burner running air and natural gas falls between 1300 and 1600 C.

• 4. Learning To Burn

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  Around 400,000 years ago, by some accounts, humans began using fire regularly, though evidence of occasional cooked food appears earlier and suggests controlled use. Widespread evidence emerges between 50 and 100 thousand years ago, and resistance to air pollution started evolving in human populations around the same time. Cooking increased the variety and availability of nutrients while killing pathogenic microorganisms that caused disease.

  The ability to control fire reshaped how early humans lived. Heat let them stay warm and settle in colder climates. Flames kept nocturnal predators away. Over time the use grew more sophisticated, turning fire into a means to make charcoal and to control wildlife from tens of thousands of years ago.

  By the Neolithic Revolution, as grain-based agriculture spread, people worldwide used fire to manage the landscape. These were typically controlled burns, or cool fires, set in spring and autumn to clear undergrowth before it grew dense enough to feed a hot fire. Cool fires created a greater variety of environments, encouraging game and plant diversity, and made dense forests passable. Hot fires did the opposite, destroying plants and animals and endangering communities.

  Clearing land for farming gave fire another job. Slash-and-burn agriculture remains common across much of tropical Africa, Asia, and South America, letting small farmers clear overgrown areas and release nutrients back into the soil. The strategy carries a cost. A growing population, forest fragmentation, and a warming climate make ever-larger escaped fires more likely, harming ecosystems and infrastructure and sending carbon and soot skyward in a loop that may drive more warming. Globally, as much as 5 million square kilometres, an area more than half the size of the United States, burns in a given year.

• 5. From Phlogiston To Oxygen

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  During the 17th century, Jan Baptist van Helmont studied combustion and found that burning charcoal released a gas he called sylvestris, or wild spirit. Johann Joachim Becher folded this into phlogiston theory in 1667, and Georg Ernst Stahl formalized it in 1697. The idea held that burning released a substance, and it shaped alchemical thinking for nearly two centuries.

  Antoine Lavoisier overturned that picture by showing that combustion did not release a substance but took one up. In 1777 he proposed a new theory built on a material reacting with a component of air, which he named oxygene. By 1791 his concepts had spread widely among young scientists, and phlogiston theory had been rejected.

  This scientific turn dissolved an older worldview. For much of history, cultures explained matter through a set of four or five classical elements, with fire as one component. As understanding advanced after the Middle Ages, chemical elements replaced that philosophy. The classical elements found a new equivalence in the states of matter: solid, liquid, gas, and plasma.

• 6. Fire As Weapon

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  The Byzantine fleet used Greek fire to attack ships and men, one of the clearest examples of fire as the basis of all early thermal weapons. These included incendiary devices, heated projectiles, and smoke, and they were especially common in naval battles and siege warfare.

  The invention of gunpowder in China produced the fire lance, a flame-thrower dating to around 1000 CE that preceded projectile weapons driven by burning gunpowder. The earliest modern flamethrowers came in the First World War, first used by German troops against entrenched French troops near Verdun in February 1915. By the Second World War they were mounted on armoured vehicles.

  Hand-thrown incendiary bombs improvised from glass bottles, later known as Molotov cocktails, appeared during the Spanish Civil War in the 1930s. In that same war, Fascist Italian and Nazi German air forces, created specifically to support Franco's Nationalists, dropped incendiary bombs on Guernica.

  Axis and Allied forces dropped incendiary bombs throughout the Second World War, on Coventry, Tokyo, Rotterdam, London, Hamburg, and Dresden. At Hamburg and Dresden, firestorms were deliberately caused, with a ring of fire around each city drawn inward by an updraft from a central cluster of fires. The United States Army Air Force used incendiaries heavily against Japanese cities built largely of wood and paper. The incendiary fluid napalm was used in July 1944, near the end of the war, though its use drew little public attention until the Vietnam War.

• 7. Land Of Patches

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  Every natural ecosystem on land has its own fire regime, and the organisms living there are adapted to or dependent on it. Fire creates a mosaic of habitat patches, each at a different stage of succession. Different species of plants, animals, and microbes specialize in exploiting a particular stage, so by carving out these patches, fire lets more species coexist within a landscape.

  The burning of vegetation releases nitrogen into the atmosphere. Other plant nutrients such as potassium and phosphorus stay in the ash and recycle quickly into the soil, but lost nitrogen produces a long-term reduction in soil fertility. That loss can be recovered by nitrogen-fixing plants like clover, peas, and beans, by the decomposition of animal waste and corpses, and by natural events such as lightning. When fire strips away protective vegetation, heavy rainfall can wash the bare soil into erosion.

  The fossil record of fire begins with land-based flora in the Middle Ordovician period. Those early plants released large amounts of oxygen as waste, and once atmospheric oxygen rose above 13%, wildfire became possible. Charred plant fossils first record wildfire in the Late Silurian, and apart from a controversial gap in the Late Devonian, charcoal has been present ever since. The amount of charcoal tracks closely with oxygen levels, pointing to oxygen as the key factor. Fire grew more abundant once grasses came to dominate ecosystems, providing excellent tinder, and that spread may have begun a feedback loop toward a warmer, drier, more fire-prone climate.

• 8. Sacred Flame, Safe Flame

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  Fire worship has been widely practiced since prehistory, with dedicated structures dating from at least the Chalcolithic period. The religion of Zoroastrianism is closely tied to it. In some societies fire was itself a deity, in others a manifestation of the divine, and the hearth flame was seen as symbolic of a Heavenly Fire. Its origin became a subject of mythology, and in ancient Greek culture the Titan Prometheus stole heavenly fire and gave it to humanity.

  The pyre as a funerary practice dates to at least the Ancient Roman period in the West and to about 4,000 years ago on the Indian subcontinent. Cremation has long been practiced in some cultures, including Hindu tradition, and after early religious resistance it became widespread in the 19th century. Fire still carries cultural weight today, from the bonfires of Guy Fawkes Night in England to the barbecue in the United States and the fireworks that mark a New Year. As protest, fire turns destructive, in book burning or in the act of burning in effigy, as in the annual burning of Judas.

  For all its grip on human imagination, people lack an instinctual fascination with fire. In societies that use fire daily, children lose interest in it around age 7 through regular exposure. The relationship can distort into pyromania, an impulse-control disorder of repeated deliberate fire-setting, or invert into pyrophobia, an uncommon irrational fear.

  Modern fire protection turns these instincts into policy. Functioning smoke detectors reduce the risk of death in a fire by 50%, and sprinkler systems can reduce mortality by 100%. In the United States, the leading cause of residential fires is cooking equipment left unattended. Cigarettes and other smoking materials cause about 28% of home fires that ignite upholstered furniture and roughly 58% of the fatalities in such fires, which is why the adoption of fire-safe cigarettes has been tied to a 45% reduction in cigarette-caused fires.