The inauguration of synthetic chemistry is understood to date from the year 1828 when Wöhler, then a professor of chemistry at Berlin, produced a supposed cyanate of ammonium by the action of ammonium chloride on silver cyanate. Wöhler was surprised to find the cyanate of ammonium which he had obtained did not correspond with other ammonium salts, but resembled, and as he afterwards proved, was identical with the organic substance, urea, a crystalline compound which constitutes about half of the solid matter dissolved in urine. Wöhler and Liebig next collaborated in a study of organic substances, and one of the early results of their investigations was the discovery of the compound radical, benzoyl, as they termed it, C_{7}H_{5}O, which they found could be combined with chlorine, bromine, iodine, sulphur, ammonium, and other substances, always retaining its own individuality. It was, in fact, a compound radical, and though it has never been isolated, its compounds prove its character. Berzelius was so struck by this discovery that he suggested the name of proine or orthrine, either meaning the dawn, in substitution for benzoyl. [Illustration: FRIEDRICH WÖHLER. (From the Royal Collection of Etchings at Munich.) Born at Eschersheim, near Frankfort, 1800; died at Göttingen, 1882. Wöhler’s notable discovery of the artificial production of urea in 1828 is famous as the starting point of synthetic chemistry. ] Henceforward discoveries and theories based on them, or propounded to explain them, so crowd the field that even in bulky volumes the story is only told in outline. But several of the famous theories or laws or expositions, on which modern chemistry relies, have been so fertile in consequences that they must be very briefly mentioned. SUBSTITUTION. Before 1840 the famous French chemist J. B. A. Dumas developed the theory of substitution, or “metalepsy,” showing that the hydrogen atoms in organic substances can be removed one by one from their molecules, other atoms being substituted for them. A simple illustration of this process is manifest in the action of potassium on water, though this is not an example of organic substitution. The water, H_{2}O takes up one atom of potassium, K, in place of one of its hydrogen atoms, becoming caustic potash, KOH. It is further possible by an indirect method to replace the remaining hydrogen atom by another of potassium, yielding potassium oxide, K_{2}O. Changes of organic bodies are always proceeding on these lines, and Frankland said the recognition of the process had contributed more to the progress of the science than any other generalisation. HOMOLOGUES.