406 Mr. S. A. Sworn on the Constitution 



not mean to imply by this model that the atoms are rigidly 

 fixed, but that a given atom is unable to shift its mean position 

 without altering the mean positions of each of the others. 



The arguments by which I propose to distinguish between 

 these octahedral formulae and Kekule's symbol may be thus 

 classified: — 



I. Evidence of direct linkage between symmetrically dis- 

 posed carbon atoms (para-linkage) will be brought forward. 

 Arguments derived from this evidence will support the 

 symbols of Meyer and Thomsen as opposed to those of 

 Kekule and Armstrong. 



II. Arguments for Thomsen's as opposed to Meyer's 

 formula will be based upon the constitutions of conine, of 

 fluorene, and of the conjugated bodies, and upon the analogies 

 of ortho- and para-compounds. 



III. The symbol of Thomsen will then be further developed. 

 This development will be supported by the crj'stallographic 

 character of benzene, and will afford a rational explanation 

 of the meta- and para- laws of substitution. A similar 

 consideration of Meyer's symbol will fail to give this expla- 

 nation. 



Arguments derived from the study of pyridine derivatives 

 will be applied by analogy to the derivatives of benzene. 

 Recent research entirely warrants such an assumption. 

 Hartley found the selective absorption of the ultra-violet 

 rays, characteristic of benzene and its derivatives, to be very 

 strongly marked in the case of pyridine, picoline, quinoline, 

 &c. (J. C. S. 1881, p. 153; 1882, p. 45). The recent paper 

 by Horstmann, on the physical properties of benzene, fully 

 bears out the analogy between the benzene and the pyridine 

 nucleus (Ber. xxi. p. 2220, footnote). We shall see also that 

 the independent consideration of benzene and pyridine deri- 

 vatives leads to the same conclusion. 



I should propose by the term " aromatic nucleus " to indi- 

 cate an octahedral arrangement of six carbon or nitrogen 

 atoms, characterized by a compactness of molecular structure 

 which is due to the existence of para-linkage. 



It will be sufficient in most cases to use one of the pro- 

 jections of each octahedral symbol, viz. the diagonal symbol 

 of Claus and the star symbol of Ladenburg. 



