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Publié le : 11/5/2013 -Format: Document en format HTML protégé

Fire - chemistry.
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Fire - chemistry.
I

INTRODUCTION

Fire, reaction involving fuel and oxygen that produces heat and light. Early humans used fire to warm themselves, cook food, and frighten away predators. Sitting
around a fire may have helped unite and strengthen family groups and speed the evolution of early society. Fire enabled our human ancestors to travel out of warm,
equatorial regions and, eventually, spread throughout the world. But fire also posed great risks and challenges to early people, including the threat of burns, the
challenge of controlling fire, the greater challenge of starting a fire, and the threat of wildfires.
As early civilizations developed, people discovered more uses for fire. They used fire to provide light, to make better tools, and as a weapon in times of war. Early
religions often included fire as a part of their rituals, reflecting its importance to society. Early myths focused on fire's power. One such myth related the story of Vesta,
the Roman goddess of the hearth. To honor Vesta, the high priest of the Roman religion periodically chose six priestesses, called Vestal Virgins, to keep a fire going in a
community hearth. Keeping a controlled fire burning played a central part in communal life. Before the invention of modern implements, starting a fire, especially in
adverse weather, usually required much time and labor to generate sufficient friction to ignite kindling. If people let their fire go out, they had to spend considerable
time to start it again before they could eat and get warm.
Today people naturally focus not on starting fires but on using fire productively and on preventing or extinguishing unwanted fires. We use fire to cook food and to heat
our homes. Industries use fire to fuel power plants that produce electricity. At the same time, fire remains a potentially destructive force in people's lives. Natural fires
started by lightning and volcanoes destroy wildlife and landscapes. Careless disposal of cigarettes and matches or carelessness with campfires leads to many wildfires.
Fires in the home and workplace damage property and cause injury and death. Fires usually cost the United States and Canada more each year than floods, tornadoes,
and other natural disasters combined.
Scientists and fire protection engineers work together to help people use fire safely and productively. Smoke detectors and automatic sprinklers in homes and the
workplace have reduced property loss, deaths, and injuries due to fire. Engineers continue to develop more fire-resistant materials for use in furniture, buildings,
automobiles, subway cars, and ships. The development of new engineering approaches and new building codes and standards has led to safer buildings without
dramatically increasing costs of construction.

II

EARLY USE OF FIRE

The earliest use of fire by humans may have occurred as early as 1.4 million years ago. Evidence for this was found in Kenya--a mound of burned clay near animal
bones and crude stone tools, suggesting a possible human campsite. However, this fire could have resulted from natural causes. Homo erectus, a species of human who
lived from about 1.8 million to about 30,000 years ago, was the first to use fire on a regular basis. Evidence of a fire tended continuously by many generations of Homo
erectus, dating to about 460,000 years ago, has been found in China. Scientists have also found evidence of tended hearths dating back as many as 400,000 years in
several parts of France.
Homo erectus was the first human species to leave equatorial Africa in large numbers and spread to other continents. Many scientists believe that the use of fire
enabled Homo erectus to adapt to new environments by providing light, heat, and protection from dangerous animals. Tending fires probably helped foster social
behavior by bringing early humans together into a small area. Fires may have tightened family groups as the families congregated around a fire to protect their young.
Homo erectus may have used fire to cook food.
The use of fire became widespread throughout Africa and Asia about 100,000 years ago. By this time anatomically modern humans, Homo sapiens, had evolved and
existed alongside their near relatives, the Neandertals (Homo neanderthalensis). Clear indications of hearths have been found in Israel in Neandertal settlements that
date from 60,000 years ago. The Neandertals died out about 28,000 years ago.

III

EARLY FIRE-MAKING TECHNIQUES

Sometime after people began to use stone for tools, they found that by rubbing together pieces of flint they could produce sparks that would set fire to wood shavings.
Scientists have found evidence that people used pieces of flint and iron to produce sparks for fires 25,000 to 35,000 years ago.
Early people also learned to make fires by rubbing together pieces of wood until the wood produced a hot powder that could light kindling. Later, people made fires by
using wood devices that had been developed for other purposes. The fire drill was an adaptation of the bow and the drill. It consisted of a block of wood and a stick that
was fixed in the looped string of a small, curved bow. The fire builder moved the bow in a sawing motion, with one end of the stick against the block of wood. This
motion rotated the stick rapidly against the wood block, creating friction between the end of the stick and the block of wood. The friction produced a glowing wood
powder that could be fanned into a flame and used to light a fire.
Early people of southeastern Asia produced fire another way. They used a wood piston to compress air inside a bamboo tube that contained wood shavings. The
compressed air became increasingly hotter, eventually igniting the shavings.
The people of ancient civilizations improved on methods of fire-making. Glassmaking among the Greeks led to the development of lenses, which the Greeks used to
focus sunlight on, and thereby ignite, bundles of dry sticks. As the use of metals in toolmaking increased, people developed the tinderbox. This moisture-proof, metal
carrying case held tinder, usually charred cotton or linen cloth, and pieces of steel and flint. Striking the steel and flint together produced a spark that lighted the tinder.
Later the Japanese devised a tinderbox that operated like a present-day cigarette lighter, in which the rotary motion of a metal wheel against flint set off sparks in
tinder. Finally, in the mid-19th century, a reliable form of the phosphorus match was developed.

IV

FIRE AND THE ADVANCE OF CIVILIZATION

As early people began to live in larger communities and to develop more advanced technologies, fire became a central part of their lives. Fire continues to be essential
to humans today, although its presence may be hidden in gas-fired ovens and furnaces and thus less noticeable than before.

A

Prehistoric Uses of Fire

Thousands of years ago hunter-gatherers (people who lived by hunting and gathering wild food) developed a number of valuable uses for fire. With fire they could
remain active after the sun set, protect themselves from predators, warm themselves, cook, and make better tools.
People began using fire as a source of light by taking advantage of the glow of wood-burning fires to continue their activities after dark and inside their dwellings, which

were usually natural caves. Eventually people learned to dip branches in pitch to form torches. They created crude lamps by filling a hollowed out piece of stone with
moss soaked in oil or tallow (a substance derived from animal fat).
By cooking with fire, prehistoric people made the meat of the animals they killed more palatable and digestible. They learned to preserve meat by smoking it over a fire,
vastly decreasing the danger of periodic starvation. Cooking also enabled them to add some formerly inedible plants to their food supplies.
Fire enabled people to make better weapons and tools. In prehistoric times, hunters formed spears from tree branches by burning the tips of the branches and then
scraping the charred ends into a point. They used fire to straighten and harden tools made of green wood. People eventually learned to control the spread of a fire by
blowing at it through reed pipes. They then used this technique to burn hollows in logs to create cradles, bowls, and canoes.

B

Fire in Early Civilizations

When prehistoric people developed the ability to cultivate crops and raise animals, they began to form permanent communities. These communities amassed food
surpluses, enabling some people to devote their time to becoming skilled artisans. The artisans first used fire to make pottery and bricks. The first potters worked
around 6500

BC

in Mesopotamia, one of the earliest centers of civilization, located in modern-day Iraq and eastern Syria. They placed wet clay vessels in open fires to

harden and waterproof them. By 3000

BC,

Egyptian potters used fire in earthen kilns, or ovens, to bake bricks out of a mixture of mud and straw. Later, potters in

Babylonia and Assyria, in the area now known as Iraq, used fire in stone kilns to create high temperatures that produced extremely durable pottery.
Fire became the center of daily life in the ancient civilizations. Most of the mud, thatch, or wood houses in which ancient people lived contained a hearth, or fireplace, in
the center. Smoke escaped through a hole directly overhead in the roof. Some of the houses, as well as tenements in crowded cities such as Rome and Athens, were
heated by braziers (metal pans that held charcoal fires). The large houses of the rich in the Roman Empire were heated by movable stoves, or even furnaces, from
which hot air flowed to a heat chamber under some of the rooms. Modern household stoves and furnaces stem from these developments.
Ancient peoples developed improved devices for using fire to provide light. By 2000

BC

they began using candles made of yarn or dry rushes dipped in animal fat. The

Egyptians and Greeks introduced more advanced forms of the oil lamp, filling a shell or carved stone with animal or vegetable oil and introducing a floatin...


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