towards which Kekulé, of Heidelberg, was the principal contributor. “Rational formulæ” as distinguished from “empiric formulæ” were already recognised as shown by the homologous series of Gerhardt. Let this be illustrated by the instance of alcohol. The atomic composition of compound bodies was ascertained by many of the earlier chemists. Lavoisier analysed alcohol, and assigned to it almost the same composition as we know it to be. Its empirical formula is C2H6O; but that does not explain how it is built up. By deductive reasoning it is established that alcohol is ethane with one hydrogen atom in each molecule replaced by hydroxyl (OH). Ethane is C2H6; alcohol is thus formulated—C2H5OH. That is its “rational formula.” Alcohol is a comparatively simple substance; we shall deal with some formulas of much greater complexity presently.
August Kekulé.
Born at Darmstadt, 1829; died at Bonn, 1896.
But these explanations were by no means sufficient to meet all the cases which were coming before chemists, and now Kekulé’s brilliant “closed ring” theory was conceived, and on this most of the wonderful building up of the synthetic compounds has been planned. Kekulé was puzzling over the formula C6H6 which had been found to represent benzene, now so famous as the starting point of the aromatic series. He stated that the solution of the problem came to his mind on the top of a London omnibus in 1865, when he was an assistant in the chemical laboratory of St. Bartholomew’s Hospital Medical School. He conceived the idea of a hexagonal structure with an atom of carbon at each angle, each united to one atom of hydrogen, and on one side a double link or bond, and on the other a single one, connecting it with the next carbon atom, the quadrivalency of each atom being thereby satisfied.
The formula is depicted in the margin, and is generally accepted; but it ought to be stated that it has rivals, though all are founded on the necessity of providing for the saturation of the four links of the carbon atoms.
Aniline.
Among the events which gradually led to the production of artificial compounds for which physiological properties and action have been claimed, the discovery of aniline is prominent. The substance, now so well known by that name, was first separated from indigo in 1826 in the course of a dry distillation of that dye by a pharmacist of Erfurt, named Unverdorben. He named his product “crystalline,” from its character. In 1834 the same substance, as it was later known to be, was obtained from coal-tar by Runge, who, observing the violet colour which bleaching powder caused in its aqueous solution, designated the product “kyanol.” Ten years subsequently Hofmann continued the investigations which Runge had pioneered. Meanwhile Fritzsche had obtained anthranilic acid from indigo, and from that he had produced an oily base which he called “aniline.” This term was derived from the specific name of the indigofera anil, which was the Sanskrit designation of the famous blue dye. Hofmann’s researches ultimately proved that Unverdorben’s crystalline, Runge’s kyanol, and Fritzsche’s aniline were all chemically identical. Hofmann would have preferred to retain the first of these names, but the more definite aniline prevailed.
The colour producing power of aniline had been observed (as has been already mentioned) by Runge in 1834, but it was not until 1856 that this property became of practical importance, when W. H. Perkin, at the time a pupil of Hofmann’s, commenced the investigation which resulted in such a complete revolution in the dyeing industry. Perkin’s patent for his “mauve” dye was obtained in 1858. It is an interesting circumstance that he made his discovery as a consequence of experiments he was conducting with the view of manufacturing an artificial quinine. Now we may turn to the