Estimation of "Silica and Insoluble Silicates" in an Ore.—Take 2 grams of the powdered mineral, evaporate with nitric acid (if sulphides are present), treat the dried residue (or the original substance if sulphides are absent) with 10 or 20 c.c. of hydrochloric acid; again evaporate to dryness, take up with dilute hydrochloric acid, filter, wash, ignite, and weigh.
Estimation of Silicon in Iron.—Place 2 grams of the metal (borings or filings) in a four-inch evaporating dish, and dissolve (with aid of heat) in 25 c.c. of dilute nitric acid. Evaporate to complete dryness, take up with 20 c.c. of hydrochloric acid, and allow to digest for one hour. Boil down to a small bulk, dilute with a 5 per cent. solution of hydrochloric acid, boil, and filter. Wash with acid and water, dry, ignite in a platinum crucible, and weigh the SiO2. This, multiplied by 0.4673, gives the weight of the silicon. The percentage is calculated in the usual way.
PRACTICAL EXERCISES.
1. A certain rock is a mixture of 70 per cent. of quartz, 25 per cent. of potash-felspar, and 5 per cent. of potash-mica. What per cent. of silica will it contain?
2. Two grams of a mixture of silica and cassiterite left, after reduction in hydrogen, 1.78 grams. Assuming all the oxide of tin to have been reduced, what will be the percentage of silica?
3. The formula of a compound is 2FeO.SiO2. 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 (CaCO3) would you take to make 30 grams of a slag having the formula CaO.SiO2?
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).