The next great step was the introduction of the theory of radicles, first suggested tentatively by Berzelius (1810), but put forward in a definite way as one of the results of the classical investigation on benzoyl by Liebig and Wöhler (1832). That is to say, a group of elements, or radicle, can pass through a series of compounds, from one to the other, as though the group were one single element. For years this idea was the guiding principle in chemical investigations, and was most useful in aiding the classification of chemical compounds and bringing order out of the chaos of accumulating observations.
But the search for radicles was in a sense a vain one. We now know that no radicle exists as such by itself. Meanwhile, Dumas and his pupil Laurent had introduced and developed the theory of types, whereby all chemical compounds could be classified under four types, which marked a distinct step in advance. Laurent, together with his colleague Gerhardt (1816–1856), recognized the shortcomings of both the radicle and type theories in their earlier forms, and showed their inter-relation, when modified so as to do away with certain inconsistencies.
Dumas had before this demonstrated the theory of substitution (1834),—that is, that in certain compounds one or more of the elements can be driven out and replaced by others without changing the essential characteristics of the compound. For instance, chloracetic acid, in which part of the hydrogen of acetic acid has been replaced by chlorine, contains all the essential characteristics of acetic acid; in fact, some of them—its acidic properties, for example—being markedly accentuated. This theory was fiercely assailed at first, notably by Liebig. Like all theories of science, it was in the beginning pushed to the extreme, and put forward to explain things to which it was not applicable. It gradually came to demonstrate its own right to existence, largely as a result of the work of Laurent and Gerhardt, and made its influence felt in the exposition of their ideas, to which reference has just been made.
The development of these theories, about the middle of the century, was greatly hastened by the work of many brilliant investigators, notably Wurtz (1817–1884), Hofmann (1818–1892), Williamson (1824–), Kolbe (1818–1884), and Frankland (1825–) among others.
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.
These ideas have also been more or less successfully applied to the study of the composition of inorganic compounds. The assistance organic chemistry has given to the general subject is incalculable. Finally, it may be said, that while in the nature of the case our ideas of structure in organic compounds cannot be regarded as proved, or as not subject to possible future modifications, we have, at least, a consistent theory and good working hypothesis. A homely illustration of our present ideas may be drawn from the modern high city building. The skeleton of this building is made of iron, about which are grouped the brick, stone, wood, and other materials to form a complete building. So the organic body is built on a chain or frame-work or skeleton of carbon atoms, about which are grouped the atoms of hydrogen, oxygen, and nitrogen, or radicle compounds thereof.
It is not possible here to even name some of the more eminent workers who for a quarter of a century have contributed to our knowledge of organic chemistry. This branch of chemistry has been the vogue, and has been pushed almost to the limit of possibility since 1875. Many almost unexplored fields still remain, but chemists recognize the fact that in theory and practice organic chemistry has reached a high degree of perfection, and they are returning to continue the researches in other fields which have for so long been almost neglected.