SnCl2 + 2HgCl2 = SnCl4 + 2HgCl.
If the stannous chloride is in excess, the reaction may go further, producing metallic mercury:
SnCl2 + 2HgCl = SnCl4 + 2Hg.
Ferric chloride is in like manner reduced to ferrous chloride:
SnCl3 + 2FeCl3 = SnCl4 + 2FeCl2.
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.