LECTURE XIV.] HISTORY OF CHEMISTRY. 277 



compounds. Finally, it was pointed out by Ladenburg, 15 

 taking into account the experiments of Hiibner and Petermann, 16 

 that terephthalic acid and para-oxybenzoic acid belong to the 

 para-series. A very neat and original idea with respect to the 

 solution of this problem originated with Korner, 17 who showed 

 that, by the introduction of a third atomic group into the 

 di-substitution products containing two similar substituting 

 atoms or groups, three isomeric tri-substitution products are 

 possible when the original substance belongs to the meta-series, 

 or two when it is an ortho-compound, while in the case of a 

 para-derivative, only a single tri-substitution product is possible. 

 By employing this method he determined the constitution of 

 the dibromo-benzenes, and Griess 18 determined that of the 

 phenylene diamines. 



After the constitution had thus been determined in some 

 compounds, it was still necessary to establish the relations 

 between these and other compounds by means of simple reac- 

 tions, so as to have the problem solved in the cases of all the 

 doubly substituted benzenes. Not only has this been quite 

 possible, but the position of the substituting groups has also been 

 ascertained in the higher substitution products. In the whole 

 of these often very extensive investigations, which were only 

 practicable by the co-operation of many hands, the reactions 

 of Griess (see above) rendered very important services. 



It is likewise of considerable importance for the theory of 

 aromatic compounds that, starting from the quadrivalence of 

 carbon, and a series of accurately determined facts, it has proved 

 possible to establish the two fundamental principles as to the 

 constitution of benzene, as follow: i. The equivalence of 

 the hydrogen atoms of benzene ; and 2. The symmetry of two 

 pairs of hydrogen atoms in benzene, with respect to the third 

 pair of hydrogen atoms. 19 



It must further be pointed out here that a prolonged con- 



15 Berichte. 2, 140. ]G Annalen. 149, 129. 17 Gazzetta Chimica 



Italiana. 4, 305 ; Journ. Chem. Soc. 29, 204. ]S Berichte. 7, 1226. 



19 Ladenburg, Theorie der aromatischen Verbindungen, Braunschweig 

 1876; Berichte. 10, 1224; Wroblewsky, Annalen. 192, 196. 



