How can CaCl2 be made? What is its use? See page 27. What else is used for a similar purpose? Symbolize and name the acid represented by Ca(ClO)2, and name this salt (page 107). It is one of the constituents of bleaching- powder, the symbol of which, though still under discussion, may be considered Ca(ClO)2 + CaCl2. This is made by passing Cl over Ca(OH)2 2 Ca(OH)2 + 4 Cl = Ca(ClO)2 + CaCl2 + 2 H2O. CHAPTER XLVII. MAGNESIUM, ALUMINIUM, AND ZINC. MAGNESIUM AND ITS COMPOUNDS. Examine magnesite, dolomite, talc, serpentine, hornblende, meerschaum, magnesium ribbon, magnesia alba, Epsom salt. 258. Occurrence and Preparation.--Mg is very widely distributed, but does not occur uncombined. Its salts are found in rocks and soils, in sea water and in the water of some springs, to which they impart a brackish taste. The most common minerals containing Mg are magnesite, MgCO3, dolomite, MgCO3 + CaCO3, and talc, serpentine, hornblende, and meerschaum. The last four are silicates, and often are unctious to the touch. What proportion of the earth's crust is composed of Mg? See page 173. 259. Metallic Mg is prepared by fusing MgCl2 with Na. Why is the process expensive? Write the reaction. Experiment 120.--With forceps hold a short strip of Mg ribbon in a flame. Note the brilliancy of the light, and give the reaction. Examine and name the product. Photographs of the interior of caverns, where sunlight does not penetrate, are taken by Mg light. Gun-cotton sprinkled with powdered Mg has recently been employed for that purpose. Mg tarnishes slightly in moist air. Compounds of Mg.--MgO, magnesia, like CaO, is very infusible, and is used for crucibles. Magnesia alba, a variable mixture of MgCO2 and Mg(OH)2, is employed in medicine, as is also Epsom salt, MgSO4 + 7 H2O. ALUMINIUM AND ITS COMPOUNDS. Examine aluminium, aluminium bronze, corundum, emery, feldspar, argillite, clay. Note especially the color, luster, specific gravity and flexibility of Al. What elements are more common in the earth than Al? What metals? Compare the abundance of Al with that of Fe. 260. Compounds of Al.--Al occurs only in combination with other elements. Feldspar, mica, slate, and clay are silicates of it. It occurs in all rocks except CaCO3 and SiO2, and in nearly 200 minerals. Though found in all soils, its compounds are not taken up by plants, except by a few cryptogams. Corundum, Al2O3, is the richest of its ores. Compute its percent of Al. Compounds of Al are very infusible and difficult of reduction. 261. Reduction.--Like most other metals not easily reducible by C or H, it was originally obtained by electrolysis, but more recently from its chloride, by the reducing action of strongly heated K or Na. Al2Cl6 + 6 Na = 6 NaCl + 2 Al.