the
cause of the difficulty that exists in distinguishing it from the onyx
agate, which it much resembles. It is much harder than this stone, but
the difficulty of determining the degree of hardness without testing
the stone prevents the possibility of deciding whether a given
substance be really calcedonyx, except by the lapidary who tests it on
his wheel.
Barbot is said to have determined the, specific weight of this substance, which according to him is 2-6180.
It is often even more translucent than agate.
It
appears that it came from the East, and was not only known to, but
engraved by the ancients, and these engraved stones are the only
specimens in existence of the ealcedonyx, as, at present, none is
found in any part of the world that we are aware of.
In
the ancient collection of Genevosio, Caire saw an engraved ealcedonyx,
representing a warrior seated opposite a dead body. In the Viennese
cabinet of engraved gems there is another, which represents a young
man leaning on a column.
XX.
CARBON.
By
this name, of which every one understands the general signification, we
would specially indicate, technically, a mineral substance discovered
in Brazil in the year 1812, which was found mixed with deposits of
diamonds in alluvial lands and in the mud of rivers.
