II. BENZENE DERIVATIVES (AROMATIC 

 COMPOUNDS). 



THE aromatic compounds are derived from benzene, 

 C 6 H 6 , and the hydrocarbons homologous with it, just 

 as the fatty bodies are derived from marsh-gas and its 

 homologues. Benzene is the common nucleus of all 

 these bodies. The carbon-atoms in benzene are com- 

 bined in the form of a ring, in such a manner that 

 they are united alternately with first one and then 

 two affinities with each other, and the fourth affinity 

 of each carbon-atom is saturated with a hydrogen- 

 atom : 



(6)HC:CH(1) 



(5)110 CH(2) 

 (4)HC.CH(3). 



By the displacement of one or more hydrogen-atoms 

 by methyl CH 3 , ethyl C 2 H 5 , etc., the homologues of 

 benzene are formed ; by the displacement of hydrogen- 

 atoms by hydroxyl groups, the real aromatic alcohols 

 (phenols); by the displacement of hydrogen atoms by 

 OHO, the aldehydes ; and, finally, by the displacement 

 of hydrogen-atoms by CO.OH, the aromatic acids. All 

 facts lead us to the conviction, that, just as we have 

 observed in the case of marsh-gas that all the hydrogen- 

 atoms had the same relative value, the six hydrogen- 

 atoms in benzene also have the same value. Hence, of 

 all the compounds, formed by the displacement of one 

 hydrogen-atom in benzene by a monovalent element or 

 monovalent group of atoms, only one modification can 



