448 Mr. S. A. Sworn on the Constitution 



p. 2720) in favour of ortho- and para-linkage. It must be 

 confessed that these arguments depend upon hypothetical 

 views as to the cause of such analogies, and are therefore not 

 very conclusive. Stuart has similarly brought forward the 

 results of his experiments on the benzolmalonic acids (J. C. S. 

 1886, p. 357) in support of Kekule^s formula — a formula 

 which is at variance with so many facts. 



III. Thomsen's Symbol. 



The preceding discussion affects merely the nature of 

 the atomic linkage, and shows that the diagonal symbol of 

 Claus is alone consistent with all the facts. The argument 

 is not merely based on hypothetical analogies between ben- 

 zene and pyridine derivatives. The independent consideration 

 of these groups of bodies clearly shows that they must be 

 formulated on the same type. 



(1) Thomsen's symbol is a development of that of Claus, 

 but the positions of the hydrogen atoms are not considered. 



It is most natural to suppose that any given hydrogen atom 

 is attached to its carbon atom, in such a manner that the 

 direction of the valency falls within the solid angle formed 

 by the three other valencies which unite that carbon atom to its 

 ortho- and para- neighbours. If, for example, we assume that 

 this valency is equally inclined to the other three, it will make 

 an* angle of 148° 36' with them (or 31° 24' with the diagonal 

 of the octahedron). 



But whatever may be the true angle of deviation it is 

 evident that, whilst the meta- and ortho-carbon atoms are 

 equidistant, the meta-hydrogen atoms (or rather their mean 

 positions) would be closer together than the ortho-. 



If the configuration of the benzene molecule as a whole 

 were octahedral, we should expect the crystals of benzene to 

 belong to the regular system. But they are rhombic. Schrauff 

 has considered this point (Wiedemann's Annal. Neue Folge, 

 xxxi. p. 540), but the positions which he assigns to the 

 hydrogen atoms give a symbol which would indicate the 

 existence of two isomeric mono-substitution derivatives. 



Further, any space formula for benzene, which represents 

 all the atoms in one plane (see Claus, Ber. xx. p. 1425), 

 would lead us to suppose that benzene would crystallize in 

 the hexagonal system. 



(2) The development of Thomsen's symbol which I have 

 proposed in no way affects the questions which were pre- 

 viously discussed (I. and II.). 



It affords a basis for an explanation of the so-called para- 



