Trimming the results of an analysis to make them fit in more closely with the calculations from the formula would be foolish as well as dishonest. There can be no doubt that the actual analytical results represent the composition of the specimen much more closely than the formula does; although perhaps other specimens of the same mineral would yield results which would group themselves better around the calculated results than around those of the first specimen analysed. It must be remembered that substances are rarely found pure either in nature or in the arts; so that in most cases the formula only gives an approximation to the truth. In the case of hydrated salts there is generally a difficulty in getting the salt with exactly the right proportion of water. PRACTICAL EXERCISES. The following calculations may be made:-- 1. Calculate standards in the following cases-- (a) Silver taken, 1.003 gram. Standard salt used, 100.15 c.c. (b) Iron taken, 0.7 gram. Bichromate used, 69.6 c.c. 2. Calculate percentages:-- (a) Ore taken, 1 gram. Solution used, 65.2 c.c. Standard, 0.987 gram. (b) Ore taken, 1 gram. Barium sulphate got, 1.432 gram. Barium sulphate contains 13.73 per cent. of sulphur, and the percentage of sulphur in the ore is wanted. (c) Barium sulphate is BaSO_{4}. Calculate the percentage of sulphur it contains, for use in the preceding question. 3. A method of estimating the quantity of peroxide in a manganese ore is based on the following reactions:-- (1) MnO_{2} + 4HCl = MnCl_{2} + Cl_{2} + 2H_{2}O. (2) Cl + KI = KCl + I. To how much MnO_{2} is 1 gram of Iodine (I) equivalent? 4. A mineral has the following composition:-- Carbonic acid (CO_{2}) 19.09 Copper oxide (CuO) 71.46 Water (H_{2}O) 9.02 What is its formula? 5. How much copper is contained in 1.5 gram of crystallized copper sulphate (CuSO_{4}.5H_{2}O)? How much of these crystals must be taken to give 0.4 gram of copper? 6. How much ferrous sulphate crystals (FeSO_{4}.7H_{2}O) must be taken to yield 2 litres of a solution, 100 c.c. of which shall contain 0.56 gram of iron? 7. Galena is PbS, and hæmatite Fe_{2}O_{3}. What percentages of metal do these minerals contain? CHAPTER VIII. SPECIFIC GRAVITY.