Cavendish explained the results in terms of phlogiston and assumed the water was present in each of the two airs before ignition.įor Lavoisier, combustion meant combining with oxygen however, until he could explain the combustion of inflammable air, some would still doubt his new chemistry. When Cavendish repeated the experiment, he found that the dew was actually water. Priestley noted that when inflammable air and common air were ignited with a spark in a closed vessel, a small amount of "dew" formed on the glass walls. In 1766, Englishman Henry Cavendish isolated a gas that he called "inflammable air" because it burned readily. Lavoisier began his full-scale attack on phlogiston in 1783, claiming that "Stahl's phlogiston is imaginary." 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 called it oxygène, from the two Greek words for acid generator. 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 breathable air. By 1777, Lavoisier was ready to propose a new theory of combustion that excluded phlogiston. 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. He eventually concluded that common air was not a simple substance. In Paris, the intrigued Lavoisier repeated Priestley's experiment with mercury and other metal calces. For this reason, he called the gas that he obtained from decomposing mercury calx “dephlogisticated air.” Priestley believed his "pure air" enhanced respiration and caused candles to burn longer because it was free of phlogiston. He described how he had recently heated mercury calx (a red powder) and collected a gas in which a candle burned vigorously. In August 1774, the eminent English natural philosopher Joseph Priestley met with Lavoisier in Paris. To a suspicious Lavoisier, these results were not explained by phlogiston.Īlthough Lavoisier now realized that combustion actually involved air, the exact composition of air at that time was not clearly understood. With lead calx, he was able to capture a large amount of air that was liberated when the calx was heated. In experiments with phosphorus and sulfur, both of which burned readily, Lavoisier showed that they gained weight by combining with air. By 1772, having abandoned law to pursue a career in science, Lavoisier turned his curiosity to the study of combustion. Lavoisier was introduced to phlogiston by Guillaume Franåois Rouelle, whose lectures he attended while pursuing a law degree. This inconsistency caused some phlogistonists to suggest that phlogiston might even have a negative weight. When metals were strongly heated in air, the resulting calx weighed more than the original metal, not less, as would be expected if the lead had lost the phlogiston component. The difficulty with this scheme was the reverse reaction. Therefore, metals, which were thought to contain phlogiston, were also classified as combustibles. He proposed that the phlogiston of the charcoal had united with the calx. Turning from organic substances to metals, Stahl knew that a metal calx (known today as an oxide) heated with charcoal formed the original metal. It followed that the less residue a substance left after burning, the greater its phlogiston content. ![]() Because a combustible substance such as charcoal lost weight when it burned, Stahl reasoned that this change was due to the loss of its phlogiston component to the air. Stahl believed that every combustible substance contained a universal component of fire, which he named phlogiston, from the Greek word for inflammable. ![]() Developed by the German scientist Georg Ernst Stahl early in the 18th century, phlogiston was a dominant chemical concept of the time because it seemed to explain so much in a simple fashion. Thrown into this mix was the concept of phlogiston. The four elements of Aristotle - earth, air, fire, and water - had been slowly modified by the medieval alchemists, who added their own arcane language and symbolism. Unlike physics, which had come of age through the work of Isaac Newton a century earlier, chemistry was still mired in the legacy of the Greek philosophers. When the 17-year-old Lavoisier left Mazarin College in Paris in 1761, chemistry hardly could be called a true science.
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