586 



Sir William Bragg 



[May 19, 



Suppose now that we compare the structures of diamond and 

 graphite. As my son showed long ago, the structure of graphite 

 must be derivable from that of the diamond by separating to nearly 

 double their previous distance the sheets of atoms parallel to one of 

 the cleavage planes of the latter crystal. The question has been 

 very carefully considered more recently by Hull in America and by 

 Debye and Scherrer on the Continent in the hope of rinding more 

 exactly the details of the movement ; they do not quite agree. 

 Fig. 2 represents the change as described by Hull. The bonds 

 between the atoms in each sheet are unaffected apparently, but those 

 between sheet and sheet are replaced by something much weaker. 

 The diamond is typical of hardness ; the graphite is used as a lubri- 

 cant. If the hexagonal rings of which the sheets are formed have 

 survived this violent change, why not suppose that they may survive 

 the further change when the sheets break up into ring structures ? 



Fig. 2. — The fine lines of the diagram show the structure of 

 graphite. By moving the top layer to the position shown 

 by the broken lines the diamond structure is obtained. 



In other words, suppose that the benzene ring is really a fact, not 

 merely a b diagram ; the distance between atom and atom in the ring 

 is 1 * 54 A.U., as in the diamond, and perhaps we may add that the 

 atoms are not all in one plane, but are arranged as may be seen in 

 Fig. 3. We then proceed to test this hypothesis by finding whether 

 we can fit together molecules of the assumed size and shape into the 

 cells which hold them. From X-ray studies we know the exact 

 form and dimensions of the cells, and can learn also much concerning 

 the relative distributions of the molecules within them. It appears 

 at once that in the few simple cases which have been examined an 

 excellent fit is possible, and, more than that, we find encouraging 

 signs that the structural idea has been chosen rightly. For instance, 

 the comparison of the cells of naphthalene and anthracene, one a two- 

 ring, the other a three-ring combination, shows that two of the axes 

 of the cell rem n in constant, while the third has grown by an amount 



