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 each carbon atom having three hydrogen atoms attached to it, the fourth bond uniting it with the other carbon atom. This and other difficulties led to the theory of

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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.

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