128 PRECIOUS STONES.
formed from an equal weight of carbon, and hence established the identity.
According
to Moissan, Diamond unites with oxygen when a temperature of 620° to
840° C. is reached, and Lavoisier gives the temperature of ignition in
air as 910° C. In oxygen it burns with a pale blue flame, and continues
burning after the source of heat is withdrawn. It is insoluble in all
the ordinary solvents (see Sir W. Crookes' account of Moissan's
experiments under Artificial Production).
When
heated in the electric arc away from oxygen, a slow conversion to
Graphite takes place, and Despretz, by the use of an electric spark
from some five hundred Bunsen cells, detected some beads as though
fusion took place when the heating was very prolonged. Only specimens
containing impurities leave any ash on ignition. From the variety
Carbonado, Rivot found the ash varied from 0"24 to 2-03 per
cent.; the ash consists chiefly of the oxides of iron, silicon and
calcium. Inclusions of other minute Diamonds have been observed, and
also of Haematite, and possibly of Eutile or llmenite. As with many
other minerals, enclosures of liquid carbon dioxide are sometimes seen
; in some cases these are so numerous and so minute as to give the
crystal a clouded appearance.
Methods of Mining.
The
means taken to separate Diamond from its surroundings vary with the
nature of the deposit, but may be generally classified as to whether
the deposit is a river or alluvial one, or one in more solid material
as at Kimberley. In river workings the usual method adopted is to
divert part
