Pre-Lavoisier: Phlogiston and Combustion Theory

One of the first valuable contributors to Dalton's Atomic Theory was Antoine Lavoisier. Lavoisier's experiments began at the end of the 18th century with the dis-proving of Phlogiston theory, the dominant explanation of combustion and the rusting of metal at the time. Phlogiston theory was first proposed by Johann Joachim Becher in 1667.

Phlogiston Theory[]

The theory held that all flammable substances contained a combustible substance called phlogiston. When burnt, the phlogiston was released as a flame and the substance returned to its elemental state. This state was called a calx. Because some metals gained mass when burnt, chemists thought that phlogiston had a negative mass.

Joseph Priestley[]

Phlogiston theory also regarded air as a pure element. Then, Joseph Priestley unknowingly discovered oxygen in the mid 1770s and started a series of changes that revolutionized chemistry.

PSM V05 D400 Joseph Priestley — Joseph Priestley (1733-1804)

At the time, a popular way of testing air quality was to see how long a mouse could survive in it. Gases were commonly collected in jars inverted over experiments and sealed. Priestley's first indication about the existence of oxygen came when he lit a flame inside a jar until it burned out. He discovered that mice died very quickly in this habitat, however, when a plant was placed inside and the jar was exposed to sunlight, the mouse could survive again. His second famous experiment was conducted in 1774. Priestley focused a beam of sunlight on red mercuric oxide (the chemical formula for mercuric oxide is HgO; you can learn more about it at http://en.wikipedia.org/wiki/HgO [1]). The substance was under an inverted glass jar in a pool of mercury. To his surprise, he found that the resulting gas could keep a mouse alive four times as long in this "new species of air."

Joseph_Priestley's_Experimental_Apparatus — Joseph Priestley's Experimental Apparatus

A scientific historian describes the experimental apparatus Joseph Priestley used to make his breakthrough discoveries about the nature of gases.

Not recognizing his discovery of oxygen, Priestly called the substance "dephlogisticated air," reasoning that it could absorb the maximum amount during burning. He met with the French Chemist Antoine Lavosier to share his findings. It was Lavoisier who later repeated the experiments and discovered the elementary nature of oxygen.

You can learn more about the life and discoveries of Joseph Priestley at: http://www.spaceship-earth.de/Biograph/Priestley.htm#4A4FW

Sources

http://www.newadvent.org/cathen/09052a.htm

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