deg., but it evaporates at
common temperatures. Pure mercury is unalterable. Upon being exposed
to the air, it tarnishes only by admixture with other metals, turns
grey on the surface, and loses its lustre. It is soluble in cold
nitric acid and in concentrated hot sulphuric acid, but not in
hydrochloric acid.

([chi].) _Protoxide of Mercury_ (Hg^{2}O).--It is a black powder,
which is decomposed by ignition into metallic mercury and oxygen. By
digestion with certain acids, and particularly with caustic alkalies,
it is converted into metallic mercury and peroxide. Some neutral salts
of the protoxide are only partly soluble in water, as they are
converted into basic insoluble and acid soluble salts.

Protoxide of mercury is completely insoluble in hydrochloric acid. Its
neutral salts change blue litmus paper to red.

([beta].) _Peroxide of Mercury_ (HgO).--This oxide exists in two
allotropic modifications. One is of a brick-red color, and the other
is orange. Being exposed to heat, they turn black, but regain their
respective colors upon cooling. They are decomposed at a high
temperature into metallic mercury and oxygen. They yield with acids
their own peculiar salts.

Mercury, in the metallic form, can never be mistaken for any other
metal in consequence of its fluid condition at ordinary temperatures.

Exposed to the blowpipe flame, it is instantly volatilized. This is
also the case with it when combined with other metals. The oxides of
mercury are, in the oxidation and reduction flames, instantly reduced
and volatilized. They do not produce any alteration with fluxes, as
they are volatilized before the bead melts. Heated with carbonate of
soda in a glass tube closed at one end, they are reduced to metallic
mercury, which is volatilized, and condenses upon a cool portion of
the tube as a grey powder. By cautious knocking against the tube, or
by rubbing with a glass rod, this sublimate can be brought together
into one globule of metallic mercury. Compounds of mercury can be most
completely reduced by a mixture of neutral oxalate of potassa and
cyanide of potassium. If the substance under examination contains such
a small quantity of mercury that it cannot be distinguished by
volatilization, a strip of gold leaf may be attached to an iron wire,
and introduced during the experiment in the glass tube. The smallest
trace of mercury will whiten the gold leaf in spots.

(_b._) _Arsenic_ (As).--This metal occurs in consid