[98] Fe2O3: 2FeO:: 0.2877: 0.2589.
[99] 100 c.c. contain 1 gram of sulphuric acid.
[100] 3SiF4 + 4KCl + 2H2O = 2K2SiF6 + SiO2 + 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 (FeS2) 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 (SO2), which by the action of oxidising agents and water is converted into sulphuric acid. It forms two oxides, sulphurous (SO2) and sulphuric (SO3), which combine with bases to form sulphites and sulphates. Sulphites are of little importance to the assayer, and are converted into sulphates by the action of nitric acid and other oxidising agents.
The native sulphides, when acted on with hydrochloric acid, give off sulphuretted hydrogen; with nitric acid or aqua regia, sulphates are formed, and more or less sulphur separated.