388 SCIENCE PROGRESS 



determination of the structure of graphite {Phys. Rev., vol. x, 

 p. 692, 191 7) in preference to that of Debye {Physik. Zeitsch., 

 vol. xiiij p. 297, 191 7), which maintains that the layers of atoms 

 are not puckered but flat. In any case, in the light of more 

 recent considerations, it is probable that neither analysis is 

 complete, though almost undoubtedly Hull's is nearer the 

 truth.] Whereas in diamond all the atoms were equidistant, 

 (i'54x ID"* cm. between centres), we have now in graphite 

 layers of even more rigidly connected atoms (1*50 x lO"^ cm. 

 between centres), each layer being loosely connected to the 

 next across the still more perfect cleavage by bonds of length 

 3-25 X io~^ cm. Now these stable puckered layers consist 

 of distorted hexagons of carbon atoms, alternately directed 

 upwards and downwards. Fig. i shows them clearly. They 

 are perpendicular to the hexagonal axis of graphite crystals, 

 and it is parallel to them that the crystals cleave so readily 

 into shiny black flakes. The distorted but stable hexagons 

 of carbon atoms which constitute these flakes are conceived 

 by Bragg to be the physical basis of the well-known benzene 

 ring. Benzene is derived from one such hexagon, naph- 

 thalene from two adjacent ones, anthracene from three, and 

 so on. From Hull's measurements we have now a means of 

 calculating with sufficient approximation the actual shape 

 and dimensions of these various rings. 

 Naphthalene, C^oHg, 



was chosen first for experimental investigation for the reason 

 that it forms well-defined crystals solid at ordinary tempera- 

 tures. But in the case of benzene, though it would seem 

 more natural and desirable to tackle its structure first, we are 

 at once confronted with the difficulties of producing and 

 working at low temperatures with crystals about which very 

 few reliable crystallographic facts are known. At present, 

 not even an accurate estimate of so fundamental a quantity 

 as its specific gravity is available. With regard to naphthalene, 

 X-ray observation, assisted by the data given in vol. v of 

 Groth's Chemische Krystallographie, shows at once that in its 

 ultimate monoclinic space-lattice (/3=i22° 49') 



a =- 8-34 X lo"^ cm. b = 6-05 x io~^ cm. 



c= 8-69 X 10"^ cm., 



and that there are two molecules to each cell. A unit mono- 



