364 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 60 



Figure 2. 



resulting in close-packed structures with not too rigidly fixed angles 

 and comparatively low melting points, because in the liquid state the 

 number of neighboring atoms has not greatly changed and tliey do not 

 have to be in particular directions with respect to each other. 



Graphite crystallizes in the hexagonal system; the atoms are ar- 

 ranged in layers; within each layer the pattern is not greatly differ- 

 ent from the arrangement in diamond, but the layers are separated by 

 comparatively large distances (fig. 3). It is this cliange in the 

 atomic arrangement that makes graphite a good lubricant, for the 

 layers can slip over one another under the action of weak binding 

 forces. It is also this change in spacing which accounts for the fact 

 that graphite is a conductor of electricity while diamond is an insu- 

 lator; it accomits also for the opaque black color, the lower density 

 and the different thermal conductivities of the two substances. In 

 the diamond the spacing between layers is 1.54 A. In the case of 

 graphite it is 3.35 A. This small difference is responsible for the 

 distinctive physical properties of the two substances. It is charac- 

 teristic of the diamond, as we shall see, that small effects produce 

 results out of all proportion. 



Wliile comparing graphite and diamond, there are two paradoxes 

 worthy of mention. First, one would imagine that the hard dia- 

 mond is the more stable structure. In fact, it is the other way about 



