e reaction may go further,
producing metallic mercury:

SnCl_{2} + 2HgCl = SnCl_{4} + 2Hg.

Ferric chloride is in like manner reduced to ferrous chloride:

SnCl_{3} + 2FeCl_{3} = SnCl_{4} + 2FeCl_{2}.

The chlorides of tin, as well as the alkali stannates, are much used as
mordants in dyeing processes. The hydroxides of tin and free stannic
acid, which are easily liberated from these compounds, possess in very
marked degree the power of fixing dyes upon fibers, as explained under
aluminium.

LEAD

~Occurrence.~ Lead is found in nature chiefly as the sulphide (PbS),
called galena; to a much smaller extent it occurs as carbonate,
sulphate, chromate, and in a few other forms. Practically all the lead
of commerce is made from galena, two general methods of metallurgy being
in use.

~Metallurgy.~ 1. The sulphide is melted with scrap iron, when iron
sulphide and metallic lead are formed; the liquid lead, being the
heavier, sinks to the bottom of the vessel and can be drawn off:

PbS + Fe = Pb + FeS.

2. The sulphide is roasted in the air until a part of it has been
changed into oxide and sulphate. The air is then shut off and the
heating continued, the reactions indicated in the following equations
taking place:

2PbO + PbS = 3Pb + SO_{2},

PbSO_{4} + PbS = 2Pb + 2SO_{2}.

The lead so prepared usually contains small amounts of silver, arsenic,
antimony, copper, and other metals. The silver is removed by Parkes's
method, as described under silver, and the other metals in various ways.
The lead of commerce is one of the purest commercial metals, containing
as a rule only a few tenths per cent of impurities.

~Properties.~ Lead is a heavy metal (den. = 11.33) which has a brilliant
silvery luster on a freshly cut surface, but which soon tarnishes to a
dull blue-gray color. It is soft, easily fused (melting at 327 deg.), and
quite malleable, but has little toughness or strength.

It is not acted upon to any great extent by the oxygen of the air under
ordinary conditions, but is changed into oxide at a high temperature.
With the exception of hydrochloric and sulphuric acids, most acids, even
very weak ones, act upon it, forming soluble lead salts. Hot,
concentrated hydrochloric and sulphuric acids also attack it to a slight
extent.

~Uses.~ Lead is employed in the manufacture of lead pipes and in large
storage batteries. In the form of sheet lead it is used in lining the
chambers of sulphu