MOLECULAR ARCHITECTURE 711 



pseudo-cubic symmetry. The following are the data for paradi- 

 bromobenzenesulphanilide : 



The close similarity between the equivalence parameters of 

 the two forms is again noticeable. Now when benzenesulphani- 

 lide is examined, it is also found to be pseudo-cubic ; in fact it 

 crystallises in the tetragonal system, the constants being : 



a : c. X : y : z. 



Benzenesulphanilide . . . 1:2-4314 7 '380 : 3*035 : 3-035 



The parameters reveal the close connection between the 

 crystal structures of benzenesulphanilide and the stable form of 

 the dibromo-derivative and again we observe how small is the 

 change involved in substituting hydrogen by bromine. 



It may be remarked here that Jerusalem ^ has studied the 

 crystals of a large number of picrates and styphnates and finds 

 that all of them can be regarded as derived crystallographically 

 from one or other form of the benzene assemblage. 



From what has been said and from the examples which have 

 been given, it will be obvious that the remarkable work of Barlow 

 and Pope has made it possible to connect together in a wonderful 

 way the crystallographic data concerning derivatives of benzene. 

 Although but a limited amount of work has yet been done in 

 this direction, the results so far obtained indicate that, at any 

 rate in simple derivatives, the crystalline structure of benzene is 

 retained in that of these derivatives modified by the introduction 

 of substituent groups in place of hydrogen atoms. It does not 

 appear that the relative positions of the carbon atoms of the ring 

 suffer displacement ; or in other words, there is no reshuffling of 

 the whole structure in order to accommodate new groups. The 

 number of cases in which the positions of the substituent groups 

 have been completely worked out is of course small ; but then 

 there are few men who can grapple with such involved geo- 

 metrical problems in the way that Barlow and Pope do. 



It should be possible now to solve the problems of the 

 crystalline structure of other ring compounds such as pyridine, 



' J.C.S. Trans. 1909, 1275, 



