Fire is the result of a chemical reaction that occurs when fuel and oxygen react and release heat energy. When fuel is heated, it releases gas, unless it is already in gas form. In that case, the molecules in the gas separate and react with oxygen. This chemical reaction is known as combustion. Although today we know how substances ignite and burn, the facts were not well understood by 17th and 18th century scientists. At that time, it was believed that anything that burned contained an invisible substance known as phlogiston.
The term "phlogiston" was first used by German physician Johann Joachim Becher in the 1660s and further developed by chemist Georg Ernst Stahl. Stahl developed the theory that phlogiston was materially uniform in all bodies that contained it and was released into the air during combustion and calcination. The phlogiston theory was replaced by Antoine-Laurent Lavoisier's oxidation theory. The concepts discussed by Lavoisier were revolutionary, which is why many refer to him as the founder of modern chemistry.
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