[Gk.] The Egyptians, Greeks, and Romans were acquainted with many of the sub-stances known to us at the present day, and also with the method of their preparation ; but among those nations nothing was known of chemistry as a science. During the Middle Ages the alchemists experimented with numerous substances, more especially with such as were of a metallic nature, with the object of turning them into gold. In this way they discovered some important substances. Dr. Black's discovery of "fixed air," or carbonic acid, in 1756, led the way to the discovery of other gases by Cavendish, Rutherford, Priestley, Scheele. The discovery of oxygen by Priestley in 1774 enabled Lavoisier to explain the true nature of combustion. Next came the discovery of the laws of chemical combination by Dalton, and the publication of his atomic theory. Sir Humphrey Davy, by decomposing potash and soda in 1807, laid the foundation of electro-chemistry. The most important advances in chemistry were made during the nineteenth century, and in organic chemistry progress has been very rapid.