Lavoisier is regarded as the founder of modern chemistry. Born in Paris, he studied both law and science, but after graduating concentrated his attention on science. He invested his money in a private tax-collecting company, the Ferme Generale, and thereby became rich enough to build a large and well-equipped laboratory. He then proceeded to study combustion. During the 18th century combustible matter was thought to contain a substance called phlogiston, which was released when combustion took place. This theory, which was developed by George Stahl, had one obvious flaw; substances frequently increase in weight as a result of combustion. Lavoisier performed a number of experiments in which he burned phosphorous, lead, and other elements; he noted that the weight of the container and its contents did not increase though that of the solid did. In 1772 he recorded his observations that phosphorous and sulfur burn with a gain in weight caused by their combination with air. In 1774 he discovered that when a calx was heated with charcoal the gas produced was the fixed air found by Joseph Black.
After returning from Paris, Priestley took up once again his investigation of the air from mercury calx. His results now showed that this air was not just an especially pure form of common air but was "five or six times better than common air, for the purpose of respiration, inflammation, and … every other use of common air." He called the air dephlogisticated air, as he thought it was common air deprived of its phlogiston. Since it was therefore in a state to absorb a much greater quantity of phlogiston given off by burning bodies and respiring animals, the greatly enhanced combustion of substances and the greater ease of breathing in this air were explained.
Lavoisier's researches included some of the first truly quantitative chemical experiments. He carefully weighed the reactants and products of a chemical reaction in