Combustion and the attack on phlogiston


In experiments with phosphorus and sulfur, both of which burned readily, Lavoisier showed that they gained weight by combining with air. With lead calx, he was able to capture a large amount of air that was liberated when the calx was heated. To a suspicious Lavoisier, these results were not explained by phlogiston.

Although Lavoisier now realized that combustion actually involved air, the exact composition of air at that time was not clearly understood. In August 1774, the eminent English natural philosopher and phlogistonist Joseph Priestley met with Lavoisier in Paris. He described how he had recently heated mercury calx (a red powder) and collected a gas in which a candle burned vigorously. After returning from Paris, Priestley found that a mouse could breathe this air and live much longer than a mouse that breathed "common air." At the time, respiration was thought to involve the exhalation of phlogiston, which eventually saturated the common air. Thus, in a confined space, candles were extinguished and mice died. Priestley believed his "pure air" enhanced respiration and caused candles to burn longer because it was free of phlogiston. For this reason, he called the gas that he obtained from decomposing mercury calx dephlogisticated air.

In Paris, the intrigued Lavoisier repeated Priestley's experiment with mercury and other metal calces. He eventually concluded that common air was not a simple substance. Instead, he argued, there were two components: one that combined with the metal and supported respiration and the other an asphyxiant that did not support either combustion or respiration. By 1777, Lavoisier was ready to propose a new theory of combustion that excluded phlogiston. Combustion, he said, was the reaction of a metal or an organic substance with that part of common air he termed "eminently respirable." Two years later, he announced to the Royal Academy of Sciences in Paris that he found that most acids contained this eminently respirable air. Lavoisier called it oxygène, from the two Greek words for acid generator.

Lavoisier began his full-scale attack on phlogiston in 1783, claiming that "Stahl's phlogiston is imaginary." According to biographer Douglas McKie, the paper Lavoisier read before the Academy of Sciences "exposed . . . the many weaknesses of the accepted chemical philosophy" and "reveals . . . Lavoisier's great powers of reasoning and exposition." Calling phlogiston "a veritable Proteus that changes its form every instant," Lavoisier asserted that it was time "to lead chemistry back to a stricter way of thinking" and "to distinguish what is fact and observation from what is system and hypothesis." As a starting point, he offered his theory of combustion, in which oxygen now played the central role. Lavoisier did not expect his ideas to be adopted at once, because those who believed in phlogiston would "adopt new ideas only with difficulty." Lavoisier put his faith in the younger generation who would be more open to new concepts. He was not disappointed.


 

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The chemical heritage of Antoine-Laurent Lavoisier | Combustion and the attack on phlogiston
A new chemistry emerges | Lavoisier's American legacy
The life of Antoine-Laurent Lavoisier (1743-1794) | Landmark designation

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