a bright red heat, is more difficult than silver to fuse, but fuses
more readily than gold. It absorbs oxygen while melting. There arises
from its surface a fine dust of metallic globules, which are covered
with the protoxide. The surface of the metal is likewise covered with
the protoxide. Copper exposed to moist air tarnishes, and is
converted into hydratic carbonate of copper. When ignited in the open
air, it is soon covered with the brownish-red protoxide.

([chi].) _Protoxide of Copper_ (Cu^{2}O).--This oxide occurs in
nature, crystallized in octahedrons of a ruby-red color, of a lamellar
structure, and transparent. Artificially prepared, it forms a powder
of the same color. It is decomposed by dilute acids into salts of
peroxide and metal. It is converted by ignition, with free access of
air, into peroxide.

([beta].) _Oxide of Copper_ (CuO).--This oxide is a dark-brown or
black powder. It is dissolved by acids, with a blue or green-colored
solution. It is soluble in aqua ammonia, and the solution is of a dark
blue color.

_Reactions before the Blowpipe._--Oxide of copper exposed upon
platinum wire to the inmost flame (the blue flame), communicates to
the external flame a green color. Heated upon charcoal in the
oxidation flame, it melts to a black ball, soon spreads over the
charcoal, and is partially reduced.

Exposed to the reduction flame, at a temperature which will not melt
copper, it is reduced with a bright metallic lustre, but as soon as
the blast ceases, the surface of the metal becomes oxidized, and
appears dark brown or black. If the temperature is continued still
higher, it melts to a metallic grain.

_Borax_ dissolves the oxide of copper in the flame of oxidation to a
clear green-colored bead, even if the quantity of oxide be quite
small, but by cooling, the bead becomes blue. In the flame of
reduction upon platinum wire, the bead soon becomes colorless, but
while cooling presents a red color (protoxide of copper). This bead is
opaque, but, if too much of the oxide is added, a part of it is
reduced to metal, which is visible by breaking the metallic grain.

Upon charcoal, the oxide is reduced to the metal, and the bead appears
colorless after cooling. With the addition of some tin, the bead
becomes brownish-red and opaque after cooling.

_Microcosmic Salt_ dissolves oxide of copper in the flame of oxidation
to a green bead, not so intensely colored as the borax bead. In the
reduction flame t