108 DESCRIPTIONS OF 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- 
larly 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 Ashford, 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 gioss 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. OC O,=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 choke-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 O, has been expelled by heat, 
a process carried on usually in a Limekiln. Tt 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 

