276 HISTORY OF CHEMISTRY. [LECTURE XIV. 



investigate (since it was only possible for a few persons to 

 obtain a real grasp of the facts) increased the attractiveness 

 of the investigations, now that a simple explanation of these 

 phenomena was forthcoming. And, what is of the greatest 

 importance, Kekule's views were confirmed by them in the 

 most complete manner, and did not require alteration in any 

 essential particulars isolated statements at variance with them 

 always proving capable of very early refutation as incorrect. 

 Moreover, these hypotheses were widened to a considerable 

 extent and perfected, as a consequence of the immense number 

 of facts afterwards discovered. 



The problem of determining the positions of the substitut- 

 ing atoms and groups deserves to be mentioned here first. 

 Merely referred to by Kekule, 10 it was fully solved afterwards. 



By determining the position in the aromatic series, we 

 understand the ascertaining of the relations to one another and 

 to the carbon nucleus of the atoms or groups which replace 

 the hydrogen in benzene. Obviously the question can, at the 

 earliest, only possess any significance in the case of the di-sub- 

 stitution products. The three isomers here possible, according 

 to Kekule, have had the distinguishing prefixes ortho-, meta- 

 and para- attached to their names, and the question at once 

 arises how these are to be conceived as regards their constitu- 

 tion. The first step in this connection was taken by Baeyer, 11 

 after it had been proved by Fittig 12 that mesitylene is a 

 trimethylbenzene. From the mode of its formation, Baeyer 

 draws the conclusion that the three methyl groups are symmetri- 

 cally arranged with respect to the benzene nucleus ; that is to 

 say, that mesitylene and isophthalic acid are meta-compounds. 

 This hypothesis was afterwards proved 13 by an accurate investi- 

 gation of the substitution products of mesitylene. Grabe was 

 then able to show by detailed discussions and experiments 

 as to the nature of naphthalene u that this substance, and 

 consequently, phthalic acid also, must be regarded as ortho- 



10 Annalen. 137, 174. u Ibid. 140, 306. 12 Zeitschrift flir Chemic. 

 9, 518. ly Ladenburg, Annalen. 179, 163. 14 Ibid. 149, 22. 



