LECTURE XIII.] HISTORY OF CHEMISTRY. 271 



Kekule showed that all these substances may be regarded 

 as derivatives of benzene, and that their chemical nature is 

 dominated by the constitution of this hydrocarbon. 88 A large 

 number of earlier observations told in favour of this view, but 

 some synthetical investigations which had been carried out a 

 short time previously by Fittig in conjunction with Tollens so 

 and others- 10 were also of importance. These chemists em- 

 ployed a method which originated with Wurtz ; 91 that is, they 

 treated mixtures of the alkyl iodides and the bromine substi- 

 tution products of aromatic hydrocarbons with sodium, whereby 

 they succeeded in preparing homologues of the hydrocarbons 

 in question. They were thus able to show that methyl-benzene, 

 obtained from bromo-benzene and methyl iodide, is identical 

 with toluene, but that ethyl-benzene is different from xylene, 

 which, however, approaches very closely in its character to 

 methyl-toluene or dimethyl-benzene. I do not need to enter 

 more fully here into the further results of these interesting 

 researches, as they were only obtained subsequent to the publi- 

 cation of Kekule's paper, and in this paper they were partially 

 foreseen. On the other hand, some of the results of Beilstein's 

 researches were of fundamental importance with respect to the 

 theoretical investigations now to be discussed. Of this char- 

 acter was the proof, carried out in conjunction with Reichen- 

 bach, 92 that the so-called salylic acid, which was regarded as a 

 benzene-carbonic acid, isomeric with benzoic acid, 93 was simply 

 impure benzoic acid ; and so was the fact that the chloro- 

 benzoic acids prepared up to that period, could be reduced in 

 number to three. 94 



Benzene, as the fundamental substance in the aromatic group 

 attains to quite a special significance in consequence of the views 

 of Kekule, and the latter therefore makes a special study of its 

 constitution. I shall deal with this matter in the next lecture. 



88 Annalen. 137, 129. 89 Ibid. 131, 303. 90 Ibid. 136, 303, etc. 

 91 Ann. Chim. [3] 44, 275. * 2 Annalen. 132, 309. 93 Kolbe and 



Lautemann, ibid. 115, 183; Kekule, ibid. 117, 158; Griess, ibid. 117, 

 34 ; Cannizzaro, ibid.'Supplementband I, 274. 94 Beilstein and Schlun, 

 ibid. 133, 239. 



