330 



gravity of 2.3, and conducts electricity. Natural graphite is 

 usually mixed with foreign matter, and even the purest specimen 

 leaves, when burned, from 2 to 5 per cent of ash. It is called also 

 plumbago, or black lead. 



Graphite (Greek, I write), moulded into blocks, is sawn into 

 rods for the cores_pf l jjeadj_[_pfincils (first used in the 16th cen- 

 tury). Clay is added in varying proportions to give different 

 degrees of hardness. Because of its infusibility, it is used to make 

 crucibles. Smeared on a plaster cast (non-conductor), it gives a 

 con-ducting surface on which metals (copper or silver) can be 

 deposited by electrolysis. A thin layer, used as stove-polish, 

 protects the iron from rusting. In electro-chemical industries it 

 is used for electrodes at which chlorine is to be liberated; all 

 other conductors interact chemically with this element and 

 are destroyed. It is employed also as a lubricant, when wooden 

 beams slide upon one another. 



Large amounts of pure graphite are now manufactured by 

 heating coke with some pitch and a little sand or ferric oxide 

 (Acheson's process). The mixture (3 to 3J tons) is piled (Fig. 

 87, p. 333) between the electrodes connected with a dynamo, and, 

 on account of its high resistance, becomes strongly heated. The 

 operation is complete in from 24 to 30 hours. 



Other Forms of Carbon. The apparently amorphous 

 varieties of carbon are numerous. They include wood-charcoal, 

 lampblack, animal charcoal, coal (e.g., bituminous coal and anthra- 

 cite) and coke. All of these substances will come up for dis- 

 cussion in later chapters. None of them, it may be noted here, is 

 composed of pure carbon, other elements being present, mostly 

 in combination with carbon, in very variable amounts. 



Examination of " amorphous " charcoal by X-ray methods 

 indicates that it possesses a crystalline structure identical with 

 that of graphite. Charcoal is not to be regarded, therefore, as a 

 supercooled liquid (see p. 94) like glass. It consists of tiny frag- 



