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Although oxygen combined with many other substances, it never behaved as though it were itself a combination of other substances. This principle is now called the law of conservation of mass.Īs Lavoisier continued his experiments with oxygen, he noticed something else.
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Lavoisier hypothesized that this should be true of all chemical changes, and further experiments showed that he was right. That is, there was no change in mass upon formation or decomposition of the calx. Lavoisier’s careful experiments also revealed that the combined masses of mercury and oxygen were exactly equal to the mass of calx of mercury. (A calx is the ash left when a substance burns in air.) At a higher temperature this calx decomposes into mercury and oxygen. In an important series of experiments he showed that when mercury is heated in oxygen at a moderate temperature, a red substance, calx of mercury, is obtained. Eventually he realized that this component was the dephlogisticated air which had been discovered by Joseph Priestly (1733 to 1804) a few years earlier. He became convinced that when a substance is burned in air, it combines with some component of the air. Much of Lavoisier’s work as a chemist was devoted to the study of combustion. It was Lavoisier’s position as a tax collector, not his chemical research, which led to his death by guillotine in 1794, at the height of the French Revolution. The son of a wealthy French lawyer, he was well educated and became a successful businessman, gentleman farmer, economist, and social reformer, as well as the leading chemist of his day. Antoine Lavoisier was born in 1743, the same year as Thomas Jefferson. Neither, for that matter, did the man whose experiments and ideas led directly to the theory itself. (From Lavoisier, “Traite de chemie,” 1789.