The foundation stones of empirical discovery, upon which this science is based, had been crudely shaped by the workmen of preceding centuries, but the classification and laying of them into the structure of an exact science is the work of the Nineteenth Century. The glass of the Phœnicians, and the dyes and metallurgical operations of the Egyptians, involved some chemical knowledge; much more did the operations of the alchemists, who vainly sought to convert the baser metals into gold, but these were only the crude building stones, out of which the great complex modern structure has been raised. In the Sixteenth Century the study of chemistry, apart from alchemy, began, and some attention was given to its application to the uses of medicine. Aristotle’s four elements--fire, air, earth and water--were no longer accepted as representing a correct theory, and new ones were proposed only to be found as erroneous, and to be superseded in time by others. Briefly traversing the more important of the earlier steps, there may be mentioned the phlogiston theory of Stahl in the earlier part of the Eighteenth Century; the discovery of the composition of water by Cavendish in 1766; of oxygen by Priestly and Scheele in 1774; the electro-chemical dualistic theory of Lavoisier in the latter part of the Eighteenth Century, followed by a rational nomenclature established by Guyton de Morveau, Berthollet and Fourcroy; the doctrine of chemical equivalents by Wenzel in 1777 and Richter in 1792; Dalton’s atomic theory; Wollaston’s scale of chemical equivalents; Gay Lussac’s law of combining volumes; Berzelius’ system of chemical symbols and theory of compound radicals; contributions of Sir Humphrey Davy and Faraday in electro-chemistry, and Thenard’s grouping of the metals. These interesting phases of development of the old chemistry have been followed by the new theory of substitution, by Dumas and others. This change, beginning about 1860 and running through a period of nearly twenty years, has gradually supplanted the old electro-chemical dualistic theory and established the present system. Among the important and interesting achievements of chemistry in the Nineteenth Century is the _artificial production of organic compounds_. All such compounds had heretofore been either directly or indirectly derived from plants or animals. In 1828 Wohler produced urea from inorganic substances, which was the first example of the synthetic production of organic compounds, and it was for many years the only product so formed. Berthelot, of Paris, by heating carbonic oxide with hydrate of potash produced formiate of potash, from which formic acid is obtained; by agitating olefiant gas with oil of vitriol a compound is produced from which, upon the addition of water and distillation, alcohol is formed; he also re-combined the fatty acids with glycerine to form the original fats.