Fire
A flame appears when fuel reaches its ignition point temperature. Flames from hydrocarbon fuels consist primarily of carbon dioxide, water vapor, oxygen, and nitrogen. If hot enough, the gases may become ionized to produce plasma. The color and intensity of the flame depend on the type of fuel and composition of the surrounding gases. Fire is a chemical process in which a fuel and an oxidizing agent react, yielding carbon dioxide and water. This process, known as a combustion reaction, does not proceed directly and involves intermediates. Although the oxidizing agent is typically oxygen, other compounds are able to fulfill the role. For instance, chlorine trifluoride is able to ignite sand.
Fires start when a flammable or combustible material, in combination with a sufficient quantity of an oxidizer such as oxygen gas, is exposed to a source of heat. This is commonly called the fire tetrahedron. Fire cannot exist without all of these elements in place and in the right proportions. For example, a flammable liquid will start burning only if the fuel and oxygen are in the right proportions. Some fuel-oxygen mixes may require a catalyst, a substance that is not consumed, when added, in any chemical reaction during combustion. Once ignited, a chain reaction must take place whereby fires can sustain their own heat by the further release of heat energy in the process of combustion.
The presence of a force of gravity, or of some similar force caused by acceleration, is necessary to produce convection. Without gravity, a fire rapidly surrounds itself with its own combustion products and non-oxidizing gases from the air. Because of this, the risk of fire in a spacecraft is small when it is coasting in inertial flight. Experiments by NASA reveal that diffusion flames in microgravity allow more soot to be completely oxidized after they are produced than diffusion flames on Earth.
The fossil record of fire first appears with the establishment of a land-based flora in the Middle Ordovician period. These land plants contributed large amounts of oxygen to the atmosphere when they released it as a waste product. When this concentration rose above 13%, it permitted the possibility of wildfire. Wildfire is first recorded in the Late Silurian fossil record, by fossils of charred plants. Apart from a controversial gap in the Late Devonian, charcoal is present ever since. The level of atmospheric oxygen is closely correlated with the amount of charcoal in the fossil record, clearly pointing to oxygen as the key factor in the prevalence of wildfire.
Fire also became more abundant when grasses became the dominant component of many ecosystems, around 50 million years ago. This widespread emergence of wildfire may have initiated a positive feedback process, whereby they produced a warmer, drier climate more conducive to fire. Fire made it possible for humans to live at colder places and dark caves. It also protected humans against dangerous animals. It caused nutritional changes, it enabled us to eat with more variation. Evidence of occasional cooked food is found from 250,000 years ago, suggesting it was used in a controlled fashion.
The ability to control fire was a dramatic change in the habits of early humans. Making fire to generate heat and light made it possible for people to cook food, simultaneously increasing the variety and availability of nutrients and reducing disease by killing pathogenic microorganisms in the food. The heat produced would also help people stay warm in cold weather, enabling them to live in cooler climates. Fire also kept nocturnal predators at bay. Evidence becomes widespread around 50 to 100 thousand years ago, suggesting regular use from this time; resistance to air pollution started to evolve in human populations at a similar point in time.
Throughout much of history, cultures attempted to explain nature and the properties of matter by proposing a set of four classical elements, of which fire formed one of the components. As scientific understanding developed following the Middle Ages, this philosophy was replaced by a set of chemical elements and their interactions. During the 17th century, a study of combustion was made by Jan Baptist van Helmont who discovered that burning charcoal released a gas sylvestris, or wild spirit. This was subsequently incorporated into Phlogiston theory by Johann Joachim Becher in 1667 and more formally by Georg Ernst Stahl in 1697. It was Antoine Lavoisier who demonstrated that combustion did not involve the release of a substance, but rather something was being taken up. In 1777, Lavoisier proposed a new theory of combustion based on the reaction of a material with a component of air, which he termed oxygène.
Every natural ecosystem on land has its own fire regime, and the organisms in those ecosystems are adapted to or dependent upon that fire regime. Fire creates a mosaic of different habitat patches, each at different stage of succession. Different species of plants, animals, and microbes specialize in exploiting a particular stage, and by creating these different types of patches, fire allows a greater number of species to exist within a landscape. The positive effects of fire include stimulating plant growth and maintaining ecological balance.
Its negative effects include hazards to life and property, atmospheric pollution, and water contamination. When fire removes protective vegetation, heavy rainfall can cause soil erosion. The burning of vegetation releases nitrogen into the atmosphere, unlike other plant nutrients such as potassium and phosphorus which remain in the ash and are quickly recycled into the soil. This loss of nitrogen produces a long-term reduction in the fertility of the soil, though it can be recovered by nitrogen-fixing plants such as clover, peas, and beans. Globally today, as much as 5 million square kilometres burns in a given year.
The use of fire in warfare has a long history. Fire was the basis of all early thermal weapons, including incendiary devices, heated projectiles, and the use of smoke. This class of weapons was particularly evident during naval battles and siege warfare. The Byzantine fleet used Greek fire to attack ships and men. The invention of gunpowder in China led to the fire lance, a flame-thrower weapon dating to around 1000 CE which was a precursor to projectile weapons driven by burning gunpowder. The earliest modern flamethrowers were used by infantry in the First World War, first used by German troops against entrenched French troops near Verdun in February 1915.
Incendiary bombs were dropped by Axis and Allies during the Second World War, notably on Coventry, Tokyo, Rotterdam, London, Hamburg and Dresden. In the latter two cases, firestorms were deliberately caused in which a ring of fire surrounding each city was drawn inward by an updraft created by a central cluster of fires. The United States Army Air Force extensively used incendiaries against Japanese targets in the latter months of the war, devastating entire cities constructed primarily of wood and paper houses. The incendiary fluid napalm was used in July 1944, towards the end of the Second World War.
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Common questions
What is the chemical process of fire called?
Fire is a chemical process known as a combustion reaction. This process involves fuel and an oxidizing agent reacting to yield carbon dioxide and water.
When did wildfire first appear in the fossil record?
Wildfire is first recorded in the Late Silurian fossil record by fossils of charred plants. The level of atmospheric oxygen rose above 13% during this period, permitting the possibility of wildfire.
How long ago did humans begin using fire for cooking?
Evidence of occasional cooked food is found from 250,000 years ago, suggesting it was used in a controlled fashion. Evidence becomes widespread around 50 to 100 thousand years ago, indicating regular use from that time.
Who proposed the theory of combustion based on oxygen in 1777?
Antoine Lavoisier demonstrated that combustion involved taking up something rather than releasing a substance. In 1777, Lavoisier proposed a new theory of combustion based on the reaction of a material with a component of air which he termed oxygène.
What year were the earliest modern flamethrowers used in warfare?
The earliest modern flamethrowers were used by infantry in the First World War. German troops first used them against entrenched French troops near Verdun in February 1915.