26 MINERALOGY. 



various parts of India and of Brazil, in Africa, in the isl- 

 and of Borneo, and in the Urals of Russia. The original 

 rock of the diamond appears to be a quartz rock; but the 

 diamonds are almost always obtained from amid the 

 sands and pebbles which have come from the rocks, and 

 are scattered in the rivers and brooks. In Brazil, collec- 

 tions are made of these sands and pebbles, and by wash- 

 ing them in a series of boxes the diamonds are discovered 

 and gathered. If a negro be so fortunate as to find a 

 diamond weighing 17^ carats, he gains a boon which is 

 above the price of gems the boon of liberty. 



37. Graphite. Graphite, or Plumbago, commonly call- 

 ed black lead, although there is not a particle of lead in 

 it, is composed of carbon, with a small proportion of iron, 

 commonly to the amount of from 4 to 10 per cent. Some 

 have supposed it to be a carburet of iron ; but it is not a 

 chemical compound. It is only a mixture of the carbon 

 and iron, and that the iron is not essential is shown by 

 the fact that in some cases there is scarcely a trace of it. 

 Graphite is soft, and has a shining lustre. It is some- 

 times compact, and sometimes crystalline, usually in the 

 foliated form, but occasionally in six-sided prisms. It is 

 used in making lead-pencils, in lessening the friction of 

 machinery, in giving a gloss to stoves, etc. Commonly, 

 in preparing graphite for pencils, the solid mineral is cut 

 up into pieces of the requisite size ; but a method has of 

 late been adopted by which the mineral is finely pow- 

 dered, and then, by great pressure, made into solid sheets, 

 from which the pieces are obtained. 



38. Coal. The varieties of coal are divided into two 

 classes, the bituminous and the non-bituminous. The 

 former have, in addition to the carbon, hydrogen, which 

 in the combination produces carbureted hydrogen, or 

 common illuminating gas, and it is the burning of this, 

 that is, its union with the oxygen gas of the air, that 

 causes the flame. In the non-bituminous, the anthracite, 

 on the other hand, we have only the blue flame in the 



