Fire. The earliest use people made of fire was to keep warm, provide light, and cook food. As civilizations advanced, people learned to use fire in many other ways. People learned to use fire to shape weapons and tools and to change clay into pottery. Today, we also use fire in many more ways. Fire furnishes the energy to drive machines, and keeps industries running. It supplies the power to drive trains, ships, and planes; and it generates electricity. Fire is also used to remove and destroy waste materials. In addition, fire is used in separating most metals from their ores, as well as in forging and shaping metals into useful things.
Controlled fire is useful. But fire can also be destructive. Uncontrolled fire kills thousands of people and destroys billions of dollars worth of property each year. Fires burned down large parts of London in 1666, Chicago in 1871, and Tokyo in 1923. Fires also destroy large areas of trees and brush every year.
What is fire?
Fire is the heat and light that comes from burning substances. In 1777, Antoine Lavoisier, a French chemist, proved that burning is the result of the rapid union of oxygen with other substances. As a substance burns, heat and light are produced. Burning is also called combustion. Often oxygen unites with other substances at such a slow rate that little heat and no light are given off. When this happens, we call the process oxidation, rather than burning or combustion. Oxidation takes place whenever oxygen unites with other substances either rapidly or slowly. For example, when oxygen unites with gasoline, the action takes place rapidly and heat and light are given off. This process may be described by any of the three words, burning, combustion, or oxidation. When oxygen unites with iron and causes it to rust, burning, or combustion, does not take place, but oxidation does.
Kinds of fire.
All substances do not burn in the same manner. Charcoal, for example, gives off heat with a faint glow. But other substances, such as coal, gas, magnesium, oil, and wood, give off heat with a flame. The color of the flame depends chiefly on the kind of material being burned and on the temperature.
Substances may burn in different ways, but they all require oxygen to burn. Sometimes old rags soaked with oil or paint are thrown aside and forgotten. Oxygen from the air may slowly unite with the oil in the rags. At first, there will not be a fire. But as oxidation gradually takes place, enough heat accumulates to set the rags on fire. This type of burning, called spontaneous combustion, causes many fires.
Very rapid burning may cause explosions like those produced by gunpowder and dynamite. Here, oxidation takes place so rapidly that great volumes of gases are produced. These require many hundreds of times the space that was formerly occupied by the gunpowder or dynamite before it was oxidized. These gases expand so rapidly and violently that they produce an explosion.
How fire is produced.
Three conditions must exist before a fire can be made. There must be a fuel or a substance that will burn. The fuel must be heated to its ignition temperature. This is the lowest temperature at which combustion can begin and continue. Finally, there must be plenty of oxygen, which usually comes from the surrounding air.
Fuels are of three classes, solids, liquids, and gases. Coal and wood are examples of solids. Oil and gasoline are liquid fuels. Natural gas and hydrogen are gaseous fuels. 
The burning of a solid fuel often depends on the form of the fuel. For example, you may not be able to light a large log with a match, but a small twig from the same tree may catch fire easily with the same match. This is because heat flows to the inside of the log, and the log cannot maintain a high enough temperature to keep burning. But when several logs are burned together, heat also flows from each log to the others and keeps the fire going. This explains why it is easy to start a fire with splinters or shavings.
The ignition temperatures of fuels differ. For a solid or liquid fuel to ignite, some of the fuel must first be heated to the temperature at which it vaporizes (turns to a gas). Solids generally have higher ignition temperatures than liquids because they vaporize at higher temperatures. For example, the ignition temperatures of most woods and plastics range from about 500 to 900 °F (260 to 480 °C). A liquid fuel such as gasoline can ignite at a temperature as low as –36 °F (–38 °C).
How fires behave.
A candle burning in a room without drafts produces a steady flame. The flame’s heat vaporizes just enough candle wax to keep the flame burning at the same height.
Uncontrolled fires, on the other hand, fuel themselves by vaporizing the solid or liquid materials they find in their path. A house fire or forest fire may begin with easily ignitable materials. As the fire grows, it radiates more heat. The heat contributes to further growth, and the process accelerates as long as fuel and oxygen remain available. In a house fire, a phenomenon known as flashover occurs when all the surfaces in a room reach their ignition temperature. At this point, a relatively small fire suddenly ignites the remaining materials, filling the room with flames. Because fires in structures can grow quickly and suddenly, professional firefighters should immediately be called to control them. In a forest fire, leaves, twigs, and other materials along the ground usually make up the fuel. But wind and certain types of terrain may cause a forest fire to spread rapidly through the crowns of trees. In some forests, if a fire has not occurred for a long time, fuel materials may build up. People sometimes intentionally set prescribed fires to reduce fuels and prevent dangerous crown fires. Wildfires occur naturally in most places on Earth. Some species and ecological processes depend on them. Natural wildfires are a problem only if they threaten buildings or public safety.
We can control the fire in a furnace by regulating the supply of fuel and oxygen it receives. But only winds and the flow of air created by the fire regulate the rate of burning of an uncontrolled fire.
Fireproof materials.
The term fireproof suggests that a material has been treated with a substance that will prevent it from burning. But no material is truly fireproof. Even such noncombustible materials as concrete and stone can become damaged by an intense fire.
Materials can, however, be treated with a fire retardant to reduce their ability to burn. Most fire retardants act to raise the ignition temperature of a material or to reduce the heat produced by combustion. Such treatments can slow combustion but they do not eliminate it. See Fireproofing.
Methods of starting fires.
There are several methods of starting a fire, but in each of them the three necessary conditions for a fire must be present. Before matches were invented, the flint and steel method was used. This method required a piece of steel, a flint (hard rock), and a tinder. The tinder was generally made from cotton or linen cloth, or from dried, powdered bark from certain trees. It was heated in an oven until it was nearly ready to burn. It was then placed in a tinderbox to keep it perfectly dry. When the fire was to be started, tinder was placed on the ground and the flint struck against the steel. Some of the sparks made by the flint and steel would fly into the tinder and light it.
Another early method of starting fires was by friction. This method consisted of whirling a stick in a notch in a board until the wood powder that was produced began to glow. Enough oxygen to turn the glow into a blaze was supplied by blowing carefully on the glowing powder.
The first match was invented in 1827 by the English pharmacist John Walker. The tip of this match was coated with a mixture of antimony sulfide and potassium chlorate that was held on the wooden matchstick by gum arabic and starch. When this tip was rubbed on a rough surface, friction produced enough heat to ignite the chemicals. The burning chemicals then produced enough heat to ignite the matchstick. Safer and more efficient matches were developed later. See Match (History).
What fire produces
An entire piece of wood or coal will not burn, even if there is sufficient oxygen present. Most of us have taken the ashes from a charcoal grill or fireplace. The ash, generally a mixture of minerals, is present in the fuel, but will not unite with the oxygen. Some fuels have a lower ash content than others. This is important to remember when buying charcoal or wood because you want the fuel with the lowest ash content, provided that it is good in other respects.
Often the bottom of a pan or a skillet becomes black when it is placed over a fire. This discoloration occurs because of soot. Soot is primarily unburned carbon. The skillet becomes coated because it cools the flame, preventing the temperature from getting high enough to burn the fuel completely. If a furnace produces great quantities of soot, some of the carbon of the fuel is not being burned, and is wasted. This problem can be remedied by seeing that sufficient air is supplied to burn all the carbon in the fuel.
Gases.
Substances that burn in air are nearly always composed of two elements, carbon and hydrogen, or their compounds. For example, coal, coke, and charcoal are mostly carbon. Natural gas, gasoline, and fuel oils consist of many compounds of hydrogen and carbon. When these fuels burn, the oxygen of the air unites with the carbon and hydrogen to form carbon dioxide gas and water vapor. These usually mix with the air and disappear. The uniting of the oxygen with the hydrogen and the carbon is what produces the heat and flame of the fire.
Often, a deadly gas called carbon monoxide forms when there is not enough oxygen to burn the fuel completely. For example, when gasoline burns in an automobile engine, some of this gas forms and comes out the exhaust pipe. If you are in a closed garage when this happens, you are in danger of breathing this gas. Death may result. A person should never run the engine of an automobile in a closed garage.
Most people who are killed in fires in buildings die from inhaling carbon monoxide. Both smoldering fires and too little oxygen following flashover can promote the production of this gas.
Smoke is a mixture of soot and other particles with the gases produced by combustion. Smoke from fires can contain carbon monoxide and other poisonous gases. The soot and particles hamper vision and thus can make it difficult to escape from fires. In general, smoke results from incomplete combustion, which wastes energy and pollutes the environment.
Light.
Most of the energy caused by a fire goes into heat, but some of it goes into light. The light results either because the carbon particles in the flame become so hot that they give off light energy, or because the gas that is burning is a type that gives off light.
Ever since fire was discovered, people have been trying to convert more energy from heat into light energy. People first used flaming pieces of wood as torches. They later discovered that if the wood was dipped into pitch before lighting it, the light lasted longer and was much brighter. 
Years afterward, people poured oil in a dish, placed a wick in it, and lighted the wick. This approach gave a better light. Later, the tallow candle, which people could conveniently carry around, was invented. The kerosene lamp, with its chimney to help control the air currents, was a big improvement over the candle. After electricity was made usable, the American inventor Thomas A. Edison sent an electric current through a carbon filament (wire) until the filament became so hot that it gave off light.
Fire in legend and religion
We can only guess that prehistoric people may have gained a knowledge of fire from observing things in nature, such as lightning, the fire of volcanoes, and the heat of the sun. They also must have noticed that sparks fly when stones are struck upon one another, or when the hoofs or claws of an animal strike some hard substance.
In Persian literature, there is a story of the discovery of fire in a fight with a dragon. A stone that the hero used as a weapon missed the monster and struck a rock. Light shone forth and human beings saw fire for the first time. The mythology of nearly all early peoples contains some account of accidental or supernatural events that first revealed fire to human beings. Early peoples regarded fire as a true gift of the gods.
Fire was considered sacred because it was so essential to the welfare of people. Fire worship and sun worship have existed since very early times. Because fire was so hard to produce, the custom soon became common of keeping a public fire, which was never allowed to die out. These fires were kept in every village among the Egyptians, Persians, Greeks, and Romans. They were often in the civic center of the community.
The Temple of Vesta in Rome was an outstanding example of the importance of fire to the Romans. Vesta was originally the goddess of the hearth, and her shrine was in every home. But when religion became an affair of state, a temple was erected in which the sacred fire was kept burning at all times.
