MOLECULAR ARCHITECTURE 703 



numbered in the same way as in Kekule's formula. Considera- 

 tion of this figure will show that three di- derivatives are possible, 

 namely, the 1:2, 1:3 and i 14 isomerides. If the first sub- 

 stituent be in position i, then the second may enter into 

 position 3 or 5 in the same layer as the first, forming a metadi- 

 derivative ; or it may enter, in the alternate layers, into 

 positions 2 or 6, forming ortho- or into 4 forming para- di- 

 derivatives. We thus see here a distinct geometrical difference 

 between the derivation on the one hand of meta- di- derivatives 

 and on the other hand of ortho- or para- di- derivatives. Sup- 

 posing that the substituent X displaces the hydrogen atoms 

 «i, bi, Cs, de, Ciffi, etc., then one kind of partitioning leads solely 

 to the production of a i : 4 di- derivative and the other kind to the 

 I : 2 di- derivative only. This is, of course, completely in accord 

 with the known chemical laws of substitution in the benzene 

 nucleus and gives an entirely novel geometrical explanation of 

 the known simultaneous production of i : 2 and 1:4 di- deriva- 

 tives without any appreciable quantity of i : 3 isomeride. It 

 may therefore be said with truth that the Celis units arrived 

 at by geometrical partitioning of the assemblage described 

 above conform to the known chemical properties of benzene. 

 And as the structure as a whole agrees in symmetry and 

 dimensions with crystalline benzene, it furnishes us with a very 

 vivid picture of the molecular condition of this hydrocarbon in 

 the solid state. 



Before proceeding to discuss the crystalline form of benzene 

 derivatives with respect to that of the parent substance, it is 

 necessary to mention two alternative methods of packing 

 together the benzene units to form a homogeneous close-packed 

 assemblage. The arrangement depicted in fig. 3 possesses 

 orthorhombic symmetry ; one alternative arrangement possesses 

 rhombohedral symmetry (J.C.S. Trans. 1906, p. 1699), the 

 other is pseudo-cubic {J.C.S. Trans. 1910, p. 2380). Only 

 one crystalline modification of benzene is known but of the 

 three assemblages referred to, two, viz. the first and the third, 

 present nearly the same dimensions, so that indeed it is not at 

 present certain which of them is exemplified by the known 

 crystal (cf. J.C.S. Trans. 1906, fig. 3, p. 1694, and ib. 1910, fig. 57, 

 p. 2386). The reason of this resemblance is that in two respects 

 they are practically alike in that in the direction of the y axis 

 strings of large (carbon) spheres in contact are present and that 



