As with so many other instances in science, the phlogiston theory fell into disrepute only when someone appeared on the scene who could reject traditional thinking almost entirely and propose a radically new view of the phenomenon. That person was the great French chemist Antoine Laurent Lavoisier (1743-1794). Having knowledge of some recent critical discoveries in chemistry, especially the discovery of oxygen by Karl Wilhelm Scheele (1742-1786) in 1771 and Joseph Priestley (1733-1804) in 1774, Lavoisier framed a new definition of combustion. Combustion, he said, is the process by which some material combines with oxygen. By making the best use of precise quantitative experiments, Lavoisier provided such a sound basis for his new theory that it was widely accepted in a relatively short period of time.
Lavoisier initiated another important line of research related to combustion, one involving the amount of heat generated during oxidation. His earliest experiments involved the study of heat lost by a guinea pig during respiration, which Lavoisier called "a combustion." In this work, he was assisted by a second famous French scientist, Pierre Simon Laplace (1749-1827). As a result of their research, Lavoisier and Laplace laid down one of the fundamental principles of thermochemistry, namely that the amount of heat needed to decompose a compound is the same as the amount of heat liberated during its formation from its elements. This line of research was later developed by the Swiss-Russian chemist Henri Hess (1802-1850) in the 1830s. Hess' development and extension of the work of Lavoisier and Laplace has earned him the title of father of thermochemistry.
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