INDUSTRIAL MINERALS DIAGRAM 20. 225 



Petroleum is now in universal use. Its name is derived from 

 two words meaning mineral oil. The use of petroleum has be- 

 come much more general since the discovery in America of true 

 subterranean springs of petroleum as abundant as springs of water. 

 When wells are dug, the petroleum immediately flows in 

 abundance, and sometimes rushes up to a considerable height, like 

 the water of an artesian well. 



Graphite. Graphite, also called plumbago or Hack lead, is found 

 in granitic formations, in small layers or masses. The last term 

 is improper, as it does not contain a trace of lead, but is a kind of 

 charcoal which burns with difficulty. When it is extracted in 

 blocks of sufficient size, it is sown into small square sticks, which 

 are then enclosed in two pieces of wood to make pencils. 



When the graphite is not of sufficiently good quality, it is 

 ground and made into a paste which is allowed to dry, and from 

 which the leads for the pencils are cut. Powdered plumbago is 

 used to blacken and polish grates and fire-irons. 



The most important and best known mine of graphite is at 

 BoiTowdale in Cumberland. 



Limestones. All soft or hard rocks are called by this name 

 which yield quick-lime when calcined in the fire. Limestones 

 have another peculiarity. If a drop of vinegar or any acid is 

 poured on a rock of this nature, it immediately produces an 

 effervescence of small bubbles of carbonic acid gas. 



Limestones may exhibit every degree of hardness, from that of 

 chalk to that of marble, and they may be of the most different 

 colours. There are black, yellow, red, white, and grey marbles, 

 and others which are veined with several colours. 



Marbles, on account of their hardness and beauty, are the 

 stones which are generally employed for monuments. They are 

 generally very expensive, especially white marble, which is used 

 for carving statues and busts. 



All limestones do not possess the hardness and beauty of 

 marble, but these stones are used almost everywhere for building 

 and stone-cutting. They contain many fossils. 



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