oofing, and in the manufacture of tin cans, cooking
vessels, and similar articles.

Many useful alloys contain tin, some of which have been mentioned in
connection with copper. When tin is alloyed with other metals of low
melting point, soft, easily melted alloys are formed which are used for
friction bearings in machinery; tin, antimony, lead, and bismuth are the
chief constituents of these alloys. Pewter and soft solder are alloys of
tin and lead.

~Compounds of tin.~ Tin forms two series of compounds: the stannous, in
which the tin is divalent, illustrated in the compounds SnO, SnS,
SnCl_{2}; the stannic, in which it is tetravalent as shown in the
compounds SnO_{2}, SnS_{2}. There is also an acid, H_{2}SnO_{3}, called
stannic acid, which forms a series of salts called stannates. While this
acid has the same composition as metastannic acid, the two are quite
different in their chemical properties. This difference is probably due
to the different arrangement of the atoms in the molecules of the two
substances. Only a few compounds of tin need be mentioned.

~Stannic oxide~ (SnO_{2}). Stannic oxide is of interest, since it is the
chief compound of tin found in nature. It is sometimes found in
good-sized crystals, but as prepared in the laboratory is a white
powder. When fused with potassium hydroxide it forms potassium stannate,
acting very much like silicon dioxide:

SnO_{2} + 2KOH = K_{2}SnO_{3} + H_{2}O.

~Chlorides of tin.~ Stannous chloride is prepared by dissolving tin in
concentrated hydrochloric acid and evaporating the solution to
crystallization. The crystals which are obtained have the composition
SnCl_{2}.2H_{2}O, and are known as tin crystals. By treating a solution
of stannous chloride with aqua regia, stannic chloride is formed:

SnCl_{2} + 2Cl = SnCl_{4}.

The salt which crystallizes from such a solution has the composition
SnCl_{4}.5H_{2}O, and is known commercially as oxymuriate of tin. If
metallic tin is heated in a current of dry chlorine, the anhydrous
chloride (SnCl_{4}) is obtained as a heavy colorless liquid which fumes
strongly on exposure to air.

The ease with which stannous chloride takes up chlorine to form stannic
chloride makes it a good reducing agent in many reactions, changing the
higher chlorides of metals to lower ones. Thus mercuric chloride is
changed into mercurous chloride:

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

If the stannous chloride is in excess, th