[111] For the details of the titration, the student is referred to the
same place.
[112] N_{2}O_{5} + 6FeO = 3Fe_{2}O_{3} + 2NO.
CHAPTER XVIII.
SILICON, CARBON, BORON.
SILICON AND SILICATES.
In assaying, more especially products direct from the mine, there is
always found, when the rock is siliceous, a quantity of white
sandy-looking substance, insoluble in acids, which is sometimes
accompanied by a light gelatinous material very difficult to filter.
This is variously described as "insoluble," "sand," "insoluble
silicates," "gangue," or "rocky matter." It may be pure quartz; but
oftener it is mixed with silicates from the rock containing the mineral.
Some silicates, but not many, are completely decomposed by boiling with
hydrochloric acid or aqua regia; and others are partly so, they yield a
gelatinous precipitate of silica which greatly interferes with the
filtering. It is a common practice with assayers to carry the first
attack of the sample with acids to dryness, and to take up with a fresh
portion of acid. By this means the separated silica becomes granular and
insoluble, and capable of being filtered off and washed with comparative
ease.
This residue may be ignited and weighed; and be reported as so much per
cent. of "silica and silicates insoluble in acids." Unless specially
wanted, a determination of its constituents need not be made.
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