ANALYSIS OF CRYSTAL-STRUCTURE BY X-RAYS 387 



that, far from being just " paper " things, certain formulae 

 well known in organic chemistry are founded on actual stable 

 physical structures. 



Bragg starts out with the hypothesis that the benzene and 

 naphthalene rings are actual systems, having each a definite 

 size and form, which are built as a whole into the organic 

 substances in which they occur. Indeed, without some such 

 simplifying hypothesis, the complicated molecules of organic 

 chemistry might well present almost insuperable difficulties. 

 However, Bragg proceeds to give good a priori reasons for 

 such a supposition. The diamond is a very rigid cubic structure 



« f 



Fig. I. 



The fine lines of the diagram show the structure of the graphite. By moving the top layer to 

 the position shown by the brolcen lines the diamond structure is obtained. 



{By permission of the Physical Society of London.'^ 



in which all the carbon atoms are tied together in such a way 

 that each is at the centre of gravity of four others (Proc. Roy. 

 Soc.y A, vol. Ixxxix, p. 277). Its rigidity and open structure 

 imply that great force is needed to alter the orientation of any 

 coupling with respect to the other three belonging to the same 

 atom {Proc. Phys. Soc, vol. xxxiii, August 1921). Viewed as 

 a whole, the structure appeared as a series of puckered layers 

 parallel to the perfect cleavage planes which lie perpendicular 

 to the four triad axes which emerge from the corners of the 

 cube. From this structure that of graphite can be obtained 

 very simply by a certain shift applied to one of two such 

 parallel layers (Fig. i). [In this point, Bragg accepts Hull's 



