404 Mr. S. A. Sworn on the Constitution 



The two symbols above given are the only octahedral sym- 

 bols which will account for the relationships of the benzene 

 substitution derivatives. Moreover, these relationships can 

 be explained only oil the assumption that the properties of the 

 derivatives in question are dependent both upon the positions 

 in space of the carbon atoms and upon the nature of the 

 atomic interactions. 



(1-2) Ortho- = a, b &c. } 



(1-3) Meta- = a, c &c. > in each diagram. 



(1-4) Para- = a,d &c. J 



The symbol of Kekule should give rise to four isomeric sub- 

 stitution derivatives when the introduced radicals are similar, 

 and to five when they are dissimilar (Wroblewsky Ber. xv. 

 p. 1023). The researches of Wroblewsky on the tolui dines 

 (Annal. cxcii. p. 196) have proved that only one ortho- and 

 one meta-toluidine exist. (See also Lobry de Bruyn, J. C. S. 

 1885, abstracts, p. 972.) Kekule has given an explanation of 

 the non-existence of two isomeric ortho-derivatives, which 

 is, however, very unsatisfactory. We shall not discuss this 

 point, because there are so many others which are in conflict 

 with his theory. 



It may be pointed out that the angles a b c and a b e are 

 respectively 60° and 45°, whilst the angle enclosed by any 

 pair of valencies directed from the centre of a regular tetra- 

 hedron to its apices is 109° 28', and it may therefore be 

 argued that the octahedral formula? are in direct opposition 

 to the Yan't Hoff theory. But Van't Hoff himself states 

 that the tetrahedron is not necessarily regular (see Dix annees 

 dans Vhistoire oVune theorie, p. 27). The author's view of the 

 " tetrahedral theory " involves no arbitrary assumptions as 

 to the nature of chemical affinity or the shape of the atoms. 

 It is briefly as follows : — By means of the forces of chemical 

 affinity the carbon atom is able to unite with other groups. 

 These forces must act in four directions in space, which we 

 may call valency-directions. The directions are dependent 

 upon the nature of the associated groups. Only when they 

 are precisely similar will the valency-directions be perfectly 

 symmetrical. 



In the octahedral formula for benzene we have one hydro- 

 gen atom on the one side of a plane drawn through a carbon 

 atom a perpendicular to a d (fig. 1). On the other side 



