Very soon, however, it was found that organic compounds existed having the same percentage composition, but quite dissimilar properties, physical and chemical, as, for instance, sugar and starch. Other striking examples are Faraday’s discovery (1825) of a compound identical in composition with ethylene, but wholly different in properties; and Wöhler’s classical synthesis (1828) of urea by the transformation of ammonium cyanate. Similar facts in the domain of inorganic chemistry, though now well known, were at that time wanting, and thus this most fruitful idea, designated as isomerism, was introduced into the science. 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 For instance, chloracetic acid, in which part of the hydrogen of acetic acid has been replaced by chlorine, contains all the essential characteristics of acetic acid; in fact, some of them—its acidic properties, for example—being markedly accentuated.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.