340 PRINCIPLES OF CHEMISTRY 



probability, were submitted to the action of subterranean heat. 14 The 

 graphite in cast iron, and sometimes also natural graphite, occasionally 

 appears in a crystalline form in the shape of six-sided plates, but more 

 often it occurs as a compact amorphous mass having the characteristic 

 properties of the familiar black-lead pencil. 15 



The diamond is a crystalline and transparent form of carbon. It 

 crystallises in octahedra, dodecahedra, cubes, and other forms of the 

 regular system. 11 ' The efforts which have been made to produce 

 diamonds artificially, although they have not been absolutely fruitless, 

 have not as yet led to the production of large-sized crystals, because 

 those means by which crystals are generally formed are inapplicable 

 to carbon. Indeed, carbon in all its forms being insoluble and infu- 

 sible does not pass into a liquid condition by means of which crystalli- 



14 There are places where anthracite gradually changes into graphite, as the 

 strata sink. I myself had the opportunity of observing this gradual transformation in 

 the valley of Aosta, near Mont Blanc, not far from Courmayeur, near the warm mineral 

 waters. 



" l5 Pencils are made of graphite worked up into a homogeneous mass by disintegiu- 

 ting, powdering, and cleansing it from earthy impurities ; the best kinds are made of 

 completely homogeneous graphite sawn up into the requisite sticks. Graphite is found 

 in many places. In Russia the so-called Aliberoffsky graphite is particularly renowned ; 

 it is found in the Altai mountains near the Chinese frontier ; in many places in Finland 

 and likewise on the banks of the Little Tungouska, Sidoroff also found a considerable 

 quantity of graphite. 



Graphite, like most forms of charcoal, still contains a certain quantity of hydrogen, 

 oxygen, and ash, so that in its natural state it does not contain more than ( J p.o. of 

 carbon. 



In practice graphite is purified simply by washing it when in a finely-ground state, 

 by which means the bulk of the earthy matter may be separated. The following prr, 

 cess, proposed by Brodie, consists in mixing the powdered graphite with T T T part of its 

 weight of potassium chlorate. The mixture is then heated with twice its weight of strong 

 sulphuric acid until no more odoriferous gases are emitted ; on cooling, the mixture is 

 thrown into water and washed; the graphite is then dried and heated to a red heat; 

 after this it considerably shrinks in volume, and turns into a very fine powder, which is 

 then washed. When mixed with clay, graphite is used for making crucibles and pots 

 for melting metals. By acting on graphite several times with a mixture of potassium 

 chlorate and nitric acid heated up to 60, Brodie transformed it into a yellow insoluble 

 acid substance which he called graphitic acid, C U H 4 O 5 . The diamond remains unchanged 

 when subjected to this treatment, whilst amorphous charcoal completely oxidises. 

 Availing himself of this possibility of distinguishing graphite from the diamond or amor- 

 phous charcoal, Berthelot showed that when compounds of carbon and hydrogen are 

 decomposed by heat, amorphous charcoal is mainly formed, whilst when compounds of 

 carbon with chlorine, sulphur, and boron are decomposed, graphite is principally 

 deposited. 



16 Diamonds are sometimes found in the shape of small balls, and in that case it is 

 impossible to cut them, because directly the surface is ground or broken they fall into 

 minute pieces. Sometimes minute diamond crystals form a dense mass like sugar, ami 

 this is generally reduced to diamond powder and used for grinding. Some known 

 varieties of the diamond are almost opaque and of a black colour. Such diamonds a re 

 as hard as the ordinary ones, and are used for polishing diamonds and other precious 

 stones, and also for rock boring and tunnelling. 



