ries of condensing vessels. The metal is purified by
distillation.

~Properties.~ Mercury is a heavy silvery liquid with a density of 13.596.
It boils at 357 deg. and solidifies at -39.5 deg.. Small quantities of many
metals dissolve in it, forming liquid alloys, while with larger
quantities it forms solid alloys. The alloys of mercury are called
amalgams.

Toward acids mercury conducts itself very much like copper; it is easily
attacked by nitric and hot, concentrated sulphuric acids, while cold
sulphuric and hydrochloric acids have no effect on it.

~Uses.~ Mercury is extensively used in the construction of scientific
instruments, such as the thermometer and barometer, and as a liquid over
which to collect gases which are soluble in water. The readiness with
which it alloys with silver and gold makes it very useful in the
extraction of these elements.

~Compounds of mercury.~ Like copper, mercury forms two series of
compounds: the mercurous, of which mercurous chloride (HgCl) is an
example; and the mercuric, represented by mercuric chloride (HgCl_{2}).

~Mercuric oxide~ (HgO). Mercuric oxide can be obtained either as a
brick-red or as a yellow substance. When mercuric nitrate is heated
carefully the red modification is formed in accordance with the equation

Hg(NO_{3})_{2} = HgO + 2NO_{2} + O.

The yellow modification is prepared by adding a solution of a mercuric
salt to a solution of sodium or potassium hydroxide:

Hg(NO_{3})_{2} + 2NaOH = 2NaNO_{3} + Hg(OH)_{2},

Hg(OH)_{2} = HgO + H_{2}O.

When heated the oxide darkens until it becomes almost black; at a higher
temperature it decomposes into mercury and oxygen. It was by this
reaction that oxygen was discovered.

~Mercurous chloride~ (_calomel_) (HgCl). Being insoluble, mercurous
chloride is precipitated as a white solid when a soluble chloride is
added to a solution of mercurous nitrate:

HgNO_{3} + NaCl = HgCl + NaNO_{3}.

Commercially it is manufactured by heating a mixture of mercuric
chloride and mercury. When exposed to the light it slowly changes into
mercuric chloride and mercury:

2HgCl = HgCl_{2} + Hg.

It is therefore protected from the light by the use of colored bottles.
It is used in medicine.

Most mercurous salts are insoluble in water, the principal soluble one
being the nitrate, which is made by the action of cold, dilute nitric
acid on mercury.

~Mercuric chloride~ (_corrosive sublimate_) (HgCl_{2}). Thi