of the Aromatic Nucleus. 403 



structure unknown in other compounds. This view is sup- 

 ported by the fact that the heat of combustion of benzene is 

 much less than that of dipropargyl. The persistency with 

 which the aromatic nucleus holds together throughout a long 

 series of chemical changes leads us to the same conclusion. 



These considerations led the author to suppose that six 

 carbon atoms have attained to their most stable condition of 

 equilibrium only when they have taken up the most sym- 

 metrical positions in space, viz. those at the points of a 

 regular octahedron. Two such symbols have been proposed. 

 That of Meyer is a modification of Ladenburg's prism 

 formula. That of Thomsen is an adaptation of the diagonal 

 symbol of Ciaus, and a special case of Koerner's glyptic 

 symbol. 



The symbols of Thomsen and Meyer are^complementary to 

 each other. The ortho-linkages of the one and the meta- 

 linkages of the other together form a complete octahedron. 



Kff. 1. 



Tliomsen's symbol (continuous line). 

 Meyer's symbol (dotted line). 

 Fig. 2. Fig. 3 



a 



Thomsen's symbol. 



2G2 



Meyer's symbol, 



