ABSTRACT
Tree 3 has a centre and a bicentroid, whereas tree 4 has a centroid and a bicentre. In other cases, the (bi)centre coincides with the (bi)centroid.
The first discussion of different substances which contain the same atoms bonded in various manners was presented in 1811 by GayLussac [24]. Experimental evidence was presented by Liebig after he introduced his precise analytical methods [25]; having discovered silver fulminate in 1823, Liebig showed that it had the same formula as silver cyanate which had been characterized earlier as a different
compound by Wohler. In 1825 Earaday prophesied that this phenomenon would become the rule rather than the exception [26]. Berzelius who had investigated tartaric and racemic acids (both isolated from wine), proposed in 1830 the words ‘Isomerism” and "isomer” (Greek, isos = equal, meros = part) [27]. Many more examples of isomers were discovered in the fdlowing decades but since the notions of atom and molecule, and of ionic and covalent bonds, were still poorly understood, the theory could not advance. Structure theory originated from ideas put forward by Erankland, Gerhardt, Laurent, Butlerov [28], and, above all, by Kekuld [29]. At the first international Chemistry Congress organized by Kekul^ in 1860 in Karlsruhe, Cannizzaro drew attention to Avogadro’s work which had clarified the concepts of atom and molecule. The graphical notation system still in use in chemistry today resulted from chemical diagrams due to Kekul^ [29], Couper [30] and Crum Brown [31]. Kekuld’s Lehrbuch der organischen Chemie appeared in 1861 and contained the first systematic exposition of the theory of chemical structure [29], a lthou^ present day chemical diagrams came to be used only a few years later. The discovery of the two predicted isomeric alcohols C3H7OH (Eriedel prepared isopropanol) and of the four alcohols C4 H9OH (Butlerov prepared t-butanol) served as brilliant and convindng arguments for the correctness of structure theory (initially, the term isomer embraced all compounds with the same empirical formula, including polymers, but later was substituted for the term metamer).
