278 HISTORY OF CHEMISTRY. [LECTURE XIV. 



troversy arose concerning the formula of benzene (that is, as 

 to the mutual linkings of the carbon atoms which it contains) 

 after attention had been drawn to the fact that Kekule's for- 

 mula does not altogether take account of the requirements 

 involved in the two principles stated above.' 20 From this con- 

 troversy it appeared that only the so-called prism formula can 

 give a clear idea of the bearings of isomerism in the aromatic 

 series, since it furnishes likewise an accurate expression for 

 the thermal relations, according to Thomsen, and for the mole- 

 cular volume of benzene and its derivatives, according to R. 

 Schiff. 21 Nevertheless Kekule's hexagon formula has been 

 generally retained, because it is superior to the other formula 

 in many respects. 



Amongst the notable researches which were instigated by 

 Kekule's investigations, I only enter into detail here respecting 

 a single one which may probably be looked upon as the most 

 important amongst them. I refer to Grabe's examination of 

 the quinones. 



Kekule propounded a peculiar view respecting quinone, 22 

 a substance which had been discovered by Woskresensky.- 3 

 This substance was supposed to consist of an open chain of 

 six carbon atoms, which were joined to one another by single 

 and double linking alternately. In opposition to this view, 

 Grabe 24 advanced another, in accordance with which quinone 

 is a benzene derivative in which two hydrogen atoms are re- 

 placed by two oxygen atoms ; and these latter are further 

 united to each other. He bases this view especially upon 

 the already well-known relations of quinone to hydroquinone, 

 and upon the conversion of chloranil into hexachlorobenzene 

 by means of phosphorus pentachloride. These grounds were 

 so convincing that Grabe's view was generally adopted, even 



20 Ladenburg, Berichte. 2, 140. 21 Ibid. 13, 1808; see also 

 Thomsen, Thermochemische Untersuchungen. 4; also Schiff, Annalen. 

 220, 303. According to Schroder (Wiedem. Ann. 15, 667) this 

 also holds for the molecular refraction ; whereas, according to Briihl 

 (Annalen. 200, 229), the opposite is the case. ~ Annalen. 137, 134. 

 23 Ibid. 27, 268. >24 Ibid. 146, i. 



