2. What is phlogiston? On what theoretical and experimental grounds did Lavoisier reject it? According to Bowler's Making Modern Science, A Historical Survey, the theory of phlogiston was first stated by Johann Joachim Becher in 1667. In 1703, Georg Ernst Stahl, professor of medicine and chemistry at Halle, proposed a variant of the theory in which he renamed Becher's terra pinguis as phlogiston theory and it is in this form that the theory had its influence. Phlogiston was a substance similar to fire, without color, odor, taste or mass, that any combustible substance was partly composed of and it was released during combustion (Bowler 56). A substance rich in phlogiston was called phlogiston and once burned, it became dephlogisticated and returned to its true form, calx (residual substance in the form of fine powder). In general, substances that burned in the air were considered phlogiston. After attempting various experiments with burning substances in an enclosed space, it became apparent that combustion stopped once in an enclosed space. This was sufficient proof that air was only capable of absorbing a certain amount of phlogiston, and that once it was fully phlogiston it could no longer support combustion, nor support any form of life since its purpose in the breathing process was to eliminate the phlogiston. of the body; thus, a callus would never form. In conclusion, according to the phlogiston theory, the role of phlogiston in combustion is opposite to that of oxygen in combustion. The theory of phlogiston and the use of phlogiston in the vocabulary of many chemists remained dominant until the French chemist Antoine-Laurent Lavoisier refuted it with his caloric theory of combustion (Bowler 56). In his theory, Lavoisier showed that combustion requires a gaseous substance that has weight and that its weight can be measured. In his experiments with phosphorus and sulfur, both of which burn easily in air, Lavoisier showed that they both gained weight when combining with air. Using lead calx, Lavoisier was also able to capture a large quantity of air which, according to phlogiston theory, was released when the calx was heated. These results had not been explained by phlogiston theory. Although Lavoisier understood that combustion involved air, he remained perplexed as to the exact composition of air, which was not then understood. It was not until 1774 that Lavoisier met the English natural philosopher and phlogiston Joseph Priestly, who had experimented with a calx of mercury and collected a gas that supported the burning of a candle and the respiratory process of a rat (Bowler 63-66).