The formula of a compound is 2FeO.SiO_{2}. What percentage of silica
will it contain?
4. Two grams of a sample of cast-iron gave 0.025 gram of silica. Find
the percentage of silicon in the metal.
5. What weights of quartz and marble (CaCO_{3}) would you take to make
30 grams of a slag having the formula CaO.SiO_{2}?
CARBON AND CARBONATES.
Carbon compounds enter so largely into the structure of organised bodies
that their chemistry is generally considered apart from that of the
other elements under the head of _Organic Chemistry_. Carbon occurs,
however, among minerals not only in the oxidised state (as carbonates),
but also in the elementary form (as in diamond and graphite), and
combined with hydrogen, oxygen, &c. (as in petroleums, bitumens,
lignites, shales, and coals). In small quantities "organic matter" is
widely diffused in minerals and rocks. In shales and clays it may amount
to as much as 10 or 20 per cent. (mainly as bituminous and coaly
matters).
The assayer has only to take account of the organic matter when it is of
commercial importance, so that in assays it is generally included under
"loss on ignition."
In coals, shales, lignites, &c., the carbon compounds are, on heating,
split up into oils and similar compounds. The products of distillation
may be classified as water, gas, tars, coke, and ash.
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