It is
best to powder the oxide first produced, and recalcine.
[96] No magnetic oxide was formed.
[97] For example:--
CaO + 2HCl = CaCl_{2} + H_{2}O.
PbO + H_{2}SO_{4} = PbSO_{4} + H_{2}O.
MgO + 2HNO_{3} = Mg(NO_{3})_{2} + H_{2}O.
Al_{2}O_{3} + 6HCl = Al_{2}Cl_{6} + 3H_{2}O.
Fe_{2}O_{3} + 3H_{2}SO_{4} = Fe_{2}(SO_{4})_{3} + 3H_{2}O.
[98] Fe_{2}O_{3}: 2FeO:: 0.2877: 0.2589.
[99] 100 c.c. contain 1 gram of sulphuric acid.
[100] 3SiF_{4} + 4KCl + 2H_{2}O = 2K_{2}SiF_{6} + SiO_{2} + 4HCl.
CHAPTER XVI.
SULPHUR AND SULPHATES.
Sulphur occurs native in volcanic districts, and is mined in Sicily,
Italy, and California in considerable quantities. Combined with metals
(sulphides), it is common in all mineral districts. Iron pyrites
(FeS_{2}) is the most abundant source of this element. Sulphates, such
as gypsum, are fairly common, but have no value so far as the sulphur in
them is concerned. In coal it exists as an impurity, occurring partly as
a constituent of organic compounds.
Sulphur, whether free or combined with metals, forms, on burning,
sulphurous oxide (SO_{2}), which by the action of oxidising agents and
water is converted into sulphuric acid. It forms two oxides, sulphurous
(SO_{2}) and sulphuric (SO_{3}), which combine with bases to form
sulphites and sulphates.
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