text: August Wilhelm von Hofmann, Auguste Laurent and Charles Frédéric Gerhardt challenged Frankland and Kolbe by suggesting that the ethyl radical was in fact a dimer called dimethyl. Frankland and Kolbe countered that ethyl hydride was also a possibility  and in 1864 Carl Schorlemmer proved that dimethyl and ethyl hydride were in fact one and the same compound.
  Radical theory was eventually replaced by a number of theories each advocating specific entities. One adaptation of radical theory was called theory of types (theory of residues), advocated by Charles-Adolphe Wurtz, August Wilhelm von Hofmann and Charles Frédéric Gerhardt. Another was water type as promoted by Alexander William Williamson. Jean-Baptiste Dumas and Auguste Laurent (an early supporter of radical theory) challenged radical theory in 1840 with a Law of Substitution (or Theory of Substitution). This law acknowledged that any hydrogen atom even as part of a radical could be substituted by a halogen.
  Eventually Frankland in 1852  and August Kekulé in 1857  introduced valence theory with the tetravalency of carbon as its central theme, making trivalent carbon obsolete for the time being.