LECTURE XIV.] HISTORY OF CHEMISTRY. 275 



and yields an aldehyde and an acid, whilst the ethers of the 

 former are converted into alkyl-oxybenzoic acids C (5 H 4 (OR) 

 CO 2 H. 6 



Kekule's views concerning the oxidation of aromatic hydro- 

 carbons into acids are very important. " It may be said in 

 general that the alcohol residues (methyl, ethyl, etc.) attached 

 as side-chains to the nucleus C are converted, by sufficiently 

 vigorous oxidation, into the group CO. 2 H. The oxidation 

 products always contain, therefore, just as many side-chains as 

 the substances from which they have been produced ..... 

 When the reactions are more moderate it is possible, in the 

 case of those derivatives of benzene which contain two or 

 more alcohol radicals, to restrict the action to the formation 

 of intermediate products ; thus, one alcohol radical only is 

 oxidised in the first place, while the other remains unchanged. 

 Dimethylbenzene (xylene) in this way yields toluylic acid. . . . 

 On more vigorous oxidation the toluylic acid is then converted 

 into terephthalic acid." 



* ~ ** /~ T 



Xylene. Toluylic acid. Terephthalic acid. 



It is worth while pointing out, lastly, that Kekule in the 

 further elaboration of his views, cleared up the constitution of 

 the azo-compounds 7 discovered by Mitscherlich, and, more 

 particularly, that of the diazo-compounds discovered and 

 minutely investigated by Griess ; 8 besides showing the con- 

 nection existing between these groups. 9 



These researches upon the aromatic compounds exercised 

 an immense influence upon chemistry. The investigation of 

 these substances, which, up to that period, had been rather 

 neglected, was by many chemists almost exclusively worked at 

 during the succeeding ten years. The countless examples of 

 isomerism which previously rendered this branch so difficult to 



6 Korner, Zeitschrift fiir Chemie. II, 326. 7 Annalen. 12, 311. 



8 Ibid. 106, 123 ; 109, 286 ; 113, 334 ; 117, i ; Supplementband I, 100 ; 

 121, 257, etc. ; compare also Phil. Trans. 1864, 667, etc. 9 Lehrbuch, 

 2, 703- 



