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In its earliest phases it was the same as alchemy. About the middle of the 18th century, Dr. Black discovered that gas differs from atmospheric air, and soon after two chemists, Priestly ana Schede, discovered oxygen, which discovery revolutionized chemistry. Sir H. Davy founded electro-chemistry. the smallest quantity which can enter or be expelled from combination. existing in two gaseous volumes, or the smallest quantity obtainable in a separate state. Compounds are cla ified both according to the element they contain and to their special chemical functions, e.g. acids, alkalies, and salts. Sulphides are an important cla of compounds. They are compounds of metals 3 ith sulphur.