108 DESCRIPTIONS OP MINERALS. 



For the manufacture of the best pencils the granular 

 graphite was thought necessary, and hence the former great 

 value of the Borrowdale mine, where the texture was pecu- 

 liarly fine and firm. But now the graphite is ground up, 

 and then compressed under heavy pressure, and thus the 

 fine texture and firmness required may be obtained with any 

 pure graphite. At Sturbridge, Mass. , it is rather coarsely 

 granular and foliated, and has been extensively worked. 

 The mines of Ticonderoga and Fishkill Landing, N. Y. ; 

 of Brandon, Vt. ; and of Wake, North Carolina, are also 

 worked ; and that of Ash ford, Ct., formerly afforded a large 

 amount of graphite, though now the works are suspended. 



Graphite is extensively employed for diminishing the 

 friction of machinery ; also for the manufacture of crucibles 

 and furnaces ; and as awash for giving a gloss to iron stoves 

 and railings. For crucibles it is mixed with half its weight 

 of clay. 



Carbonic Acid. 



Carbonic acid — carbon dioxide of existing chemistry — is 

 the gas that gives briskness to the Saratoga and many other 

 mineral waters, and to artificial soda water. Its taste is 

 slightly pungent. It extinguishes combustion and destroys 

 life. 



Composition. C 2 = Oxygen 72-35, carbon 27-65 = 100. 



This gas is contained in the atmosphere, constituting 

 about 4 parts, by volume, in 10,000 parts ; and it is present 

 in minute quantities in the waters of the ocean and land. It 

 is given out by animals in respiration, and is one of the re- 

 sults of animal and vegetable decomposition ; and from this 

 source the waters derive much of their carbonic acid. This 

 gas is the choice-damp of mines, where it is often the occasion 

 of the destruction of life. It is often present also in wells. 



Carbon dioxide (or carbonic acid) is given out by lime- 

 stone (or calcium carbonate) when it is heated ; and quick- 

 lime is limestone from which C 2 has been expelled by heat, 

 a process carried on usually in a limekiln. It is also driven 

 from limestone by the action of sulphuric acid, with the for- 

 mation of gypsum (a hydrous calcium sulphate), or anhy- 

 drite (an anhydrous calcium sulphate). These processes are 

 often carried on in volcanoes, and hence carbonic acid gas is 

 common in some volcanic regions. The Grotto del Cane 

 (Dog Cave) at the Solfatara near Naples, is a small cavern 



