Kekulé proposed a new type, marsh gas or methane. Shortly afterwards, his well-known formula for benzene, the starting-point and foundation of the vast class of aromatic bodies, was proposed. He insisted that the time had come when chemists must ask what those ultimate particles, or atoms, of the elements themselves were doing in these compounds of various types. The answer was a grand one, and the result, our magnificent store of information concerning the _constitution_ of organic compounds, or the way in which the atoms are connected with each other. It is not to be inferred that our knowledge on this subject, in any one case, is complete. Far from it! Much that is most interesting and important is apparently as remote from our grasp as ever. But we do know something about the general relations of the atoms in the molecule, and our knowledge, so far as it goes, is definite and precise. Somewhat later, Van’t Hoff and Lebel, at the same time but independently, introduced the study of the space relations of organic compounds by suggesting the simplest possible space formula (the tetrahedron) for marsh gas or methane, of which all other organic compounds may, theoretically at least, be regarded as derivatives. Many inexplicable relations, especially among isomers, now became clear. The theory was at first bitterly assailed, especially by Kolbe. It found an able champion in Wislicenus (1838–), however, and has so thoroughly established itself, that it may be safely said that at the present day it is the controlling idea in the large majority of organic investigations. The carbon atom is characterized by a wonderful facility in uniting not only with other elements, but with itself. It would even appear as though its influence in this regard extended to other elements united with it, as nitrogen, for instance, shows an unexpected ability to unite with nitrogen in organic compounds. Further, the carbon atom is characterized by an unusually constant valency, namely, four. These two characteristics account for homology, that is, for a series of similar compounds differing in composition one from the other by—CH2, and enables us to trace back all organic compounds to one mother substance—marsh gas or methane.