it tarnishes. It is easily dissolved by nitric acid, but very
slowly by dilute sulphuric or hydrochloric acid, producing hydrogen.

_Protoxide of Nickel _(NiO).--It is in the form of small greyish-black
octahedrons, or a dark, greenish-grey powder. Its hydrate is a green
powder. Both are unalterable in the air, and are soluble in nitric,
sulphuric, and hydrochloric acids, to a green liquid. The protoxide is
the base of the salts of nickel, which in the anhydrous state are
yellow, and when hydrated are green. The soluble neutral salts change
blue litmus paper to red. By ignition in the oxidation flame,
protoxide of nickel is unaltered. In the reduction flame and upon
charcoal, it becomes reduced, and forms a grey adherent powder, which
is infusible, and presents the metallic lustre by compression, and is
magnetic. Borax dissolves it in the oxidation flame very readily to a
clear bead, of a reddish-violet or dark yellow color, but yellow or
light red when cold. If there is but a small quantity of the oxide
present, it is colorless. If more of the oxide be present, the bead is
opaque and dark brown, and appears, while cooling, transparent and
dark red. By the addition of a salt of potassa (the nitrate or
carbonate) a blue or a dark purple colored bead is produced. The borax
bead, in the reduction flame, is grey, turbid, or completely opaque
from the reduced metallic particles. After a continued blast, the bead
becomes colorless, although the particles are not fused. If the nickel
contains cobalt, it will now be visible with its peculiar blue color.
Upon charcoal, and by the addition of some tin, the reduction of the
oxide of nickel is easily effected, while the reduced nickel fuses
with the tin.

The oxide of nickel is dissolved by microcosmic salt in the oxidation
flame to a clear bead, which appears reddish while hot, but yellow and
sometimes colorless when cooling. If a considerable quantity of nickel
be present the heated bead is of a brown color, but orange when
cooled. In the reduction flame, and upon platinum wire, the color of
the bead is orange when cold; but upon charcoal, and with the addition
of a little tin, the bead appears grey and opaque. After being
submitted to the blowpipe flame all the nickel is reduced, and the
bead becomes colorless.

Carbonate of soda does not affect it in the oxidation flame, but in
the reduction flame and upon charcoal, it is absorbed and reduced, and
remains, after washing off