412 Mr. S. A. Sworn on the Constitution 



known facts (JVC, S. 1887, p. 214). Discredit has thus been 

 thrown upon Thomsen's symbol for benzene. If, however, 

 we suppose that the para-linkages are comparatively weak 

 and that these are broken, the facts quoted by Miller cease 

 to be inconsistent with the formulae. Two carbon atoms may 

 be bound together by three kinds of single linkage, which 

 may be called ortho-, para-, and paraffinoid. By this I mean 

 merely a difference in distance between the two atoms. Ortho- 

 carbon atoms are closer together than two consecutive carbon 

 atoms in a paraffinoid chain, and these than para-carbon 

 atoms. The strength of chemical affinity varies inversely as 

 a function of the distance between them. These points I 

 hope to consider more fully in a future paper, and by defining 

 them more precisely to afford a basis for the treatment of the 

 physical properties of benzene. Meanwhile they afford some 

 explanation of the stability of the aromatic nucleus, and of 

 the formation of the additive compounds of benzene, pyri- 

 dine, &c. 



We may first take the quinones. The diketonic formula 

 for benzoquinone seems to be placed beyond all doubt by 

 Pechmann's synthesis of dimethyl-quinone from diacetyl (an 

 undoubted carbonyl compound) {Ber. xxi. p. 1417). Only 

 one benzoquinone is known, and this is a para-compound. 

 From Kekule's theory w T e should expect such a body to be 

 ortho-. Its formation from and reduction to hydroquinone 

 are- best explained by the dissolution and re-establishment of 

 a para-linkage {vide supra, anthraquinone, p. 408). But what 

 becomes of the other two para-linkages in hydroquinone? 

 Is the formula of quinone 



or 



I am inclined to believe that when a para-linkage is broken 

 the nucleus opens out into a hexagonal ring, and the remain- 

 ing para-linkages are severed with the formation of true 

 olefinoid linkages. The work of Baeyer on the additive com- 

 pounds of terephthalic acid lends some support to this view 

 (Annal. ccxlv. pp. 103-185). He has described a series of 

 four compounds — terephthalic acid and its di-, tetra-,and hexa- 

 hydro compounds. It is not impossible to explain his results 

 by the successive setting up of para-linkages in the passage 

 from hexahydroterephthalic acid to terephthalic acid. But a 

 curious fact, and one which seems to indicate the existence of 



