About 1850 C. F. Gerhardt, one of Liebig’s pupils who settled in France (and died in 1856 at the age of 40), gave the next great impetus to the development of organic chemistry, or the chemistry of carbon compounds, as it was coming to be termed, by showing how vast numbers of organic compounds could be classified and grouped into homologous series. Starting, for example, with marsh gas, CH_{4}, which is chemically known as methane, he showed how from this type methyl alcohol, CH_{4}O, and formic acid, CH_{2}O_{2}, are formed. Ethane, C_{2}H_{6}, comes next in the series and ethyl alcohol and acetic acid follow just as methyl alcohol and formic acid follow from methane. The addition of CH_{2} to ethane gives propane; propyl alcohol and propionic acid following; another addition of CH_{2} results in butane with butyl alcohol and butyric acid; and the next type is pentane, with amyl alcohol and valeric acid in its train. Thus it was perceived that all the multitude of complex bodies included in the organic kingdom were compounded in an orderly system. VALENCY. The English chemist Edward Frankland next put forward the doctrine of valency. According to this theory atoms possess one, two, three, four, or more links each, and require that number of other atoms of minimum combining capacity to “saturate” them in a molecule. Carbon, for example, is usually considered to be quadrivalent, and as shown in the instance of methane, requires four hydrogen atoms to saturate it. But how is it then that in the case of the next type, ethane, C_{2}H_{6}, the conditions are satisfied? The explanation is that the molecule is arranged in this manner: H H | | H--C--C--H | | H H 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 STRUCTURAL FORMULAS, 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 C_{2}H_{6}O; 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 C_{2}H_{6}; alcohol is thus formulated--C_{2}H_{5}OH. That is its “rational formula.” Alcohol is a comparatively simple substance; we shall deal with some formulas of much greater complexity presently. [Illustration: AUGUST KEKULÉ.