CARBON. 



figures bounded by 48 curved triangular faces, but can always 

 be cleaved in the direction of the faces of the octohedron, which 

 possess that particular brilliancy characteristic of the diamond. 

 The diamond is the hardest of the gems. An edge of its crystal 

 formed by flat planes only scratches glass, but if the edge is 

 formed of curved faces, like the edge of a convex lens, it then, 

 besides abrading the surface, produces a fissure to a small depth, 

 and in the form of the glazier's diamond is used to cut glass. 

 The diamond is remarkably indestructible, and may be heated 

 to whiteness in a covered crucible without injury, but it begins 

 to burn in the open air, at about the melting point of silver, 

 charcoal sometimes appearing on its surface, and is entirely 

 converted into carbonic acid gas. It is more quickly consumed 

 in fused nitre, when the carbonic acid is retained by the potash; 

 this is a simple mode of analyzing the diamond, by which 

 it has been proved to be perfectly pure carbon. The diamond 

 is a non-conductor of electricity. Its density varies from 3.5 to 

 3.55. 



Graphite. This mineral, which is also known as black lead 

 and plumbago, occurs in rounded masses deposited in beds in 

 the primitive formations, particularly in granite, micaschist and 

 primitive limestone. Borrowdale in Cumberland is a celebrated 

 locality of graphite, and affords the only specimens which are 

 sufficiently hard for making pencils. It is occasionally found 

 crystallized in plates which are six-sided tables. Graphite may 

 also be produced artificially, by putting an excess of charcoal in 

 contact with fused cast iron, when a portion of the carbon dis- 

 solves, and separates again on cooling, in the form of large and 

 beautiful crystalline leaflets. In the condition of graphite, car- 

 bon is perfectly opaque, soft to the touch, possessed of the me- 

 tallic lustre, and of a specific gravity about 2.5. It always con- 

 tains a small quantity of iron, often amounting to 5 per cent, 

 but in some specimens, as in those from Barreros in Brazil, not 

 more than a trace, which is to be considered an accidental con- 

 stituent, and not essential to the mineral. Neither in the form 

 of diamond nor graphite does carbon exhibit any indication of 

 fusion or volatility under the most intense heat. Anthracite is 

 nearly pure carbon, but always contains a portion of hydrogen, 

 and is more analogous to coal than to graphite. 



Charcoal. Owing to its infusibility carbon presents itself 

 under a variety of aspects, according to the structure of the 



