|Fire is a rapid, self-sustaining oxidation process of combustible
gases ejected from a fuel. It starts by subjecting the fuel to heat or another
energy source, e.g. a match or lighter, and is sustained by the further
release of heat energy.
Controlling fire was one of humankind's first great achievements. It made possible migration to colder climes which otherwise would have remained out of reach for colonization. It also allowed for cooking food and using flame and heat to process materials. Archeology indicates that ancestors of modern humans such as Homo erectus seem to have been using controlled fire as early as some 790,000 years ago. The Cradle of Humankind site has evidence for controlled fire 1 million years ago.
Fires and burning have often been used in religious sacrifices, as the smoke of the fire disperses into the heavens. Fire is one of the four classical elements, as well as one of the five Chinese elements. In Christianity, fire is a symbol of the Holy Spirit, and is also often used in descriptions of Hell.
The burning of wood is often the first association to the word fire, and trees have since ancient times supplied much of the energy needed by humans. In the past, metal smelting and charcoal production consumed large quantities of wood for their production. Nowadays, large scale energy is usually not produced by fires of burning wood, but has been replaced by hydrocarbon oil and coal, and in some cases nuclear energy or renewable energy sources. Wood burning remains a heat source in many third world countries and where other sources of energy are unavailable.
Uncontrolled, fire can be extremely hazardous both to living things and to structures. Exposure to uncontrolled fire can result in severe burns and the symptoms of smoke inhalation. Fire in a building can be started by cooking accidents, electrical faults, fuel leaks, and accidents involving other fire sources such as candles and cigarettes. Fire can propagate rapidly to other structures, especially where proper building standards are not met. For this reason, most municipalities offer fire fighting services to quickly extinguish fire and deal with other problems. Forest fires, though sometimes necessary for the renewal of forests, can cause severe property damage over a wide area.
The glow of a flame is somewhat complex, due to a mix of black-body radiation emitted from soot, gas, and fuel particles (though the soot particles are too small to behave like perfect blackbodies), and from photon emission by de-excited atoms and molecules in the gases. Much of the radiation is emitted in the visible and infrared bands. The color depends on temperature for the black-body radiation, and chemical