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THREE:Chemistry also received valuable extensions of its field. Dr. John Mayow published new facts respecting nitre, and on the phenomena of respiration and combustion, as revealed by experiments on this and other substances. At the commencement of the eighteenth century Stahl, a German chemist, propounded his theory of phlogiston as the principle of combustion, which was only exploded by the further discoveries of Dr. Black, Cavendish, and Priestley. Soon after, Dr. Hales threw new light on a?riform bodies, or, as they are now termed, gases; and finally, Dr. Black demonstrated the presence of a gas in magnesia, lime, and the alkalies, which had long before been noticed by Van Helmont, but had been forgotten. This was then termed fixed air, but has now acquired the name of carbonic acid gas, or carbon dioxide. At the end of this period chemistry was extensively studied, and was rapidly revealing its secrets.