alums crystallize with twenty-four molecules of crystal water. Potash-alum is K2SO4, Al2(SO4)3, 24 H2O; silver-alum is Ag2SO4, Al2(SO4)3, 24 H2O. Other trivalent elements may replace aluminum. Thus Na2SO4, Fe2(SO4)3, 24 H2O would be sodium ironalum. Chrome-alum is K2SO4, Cr2(SO4)3, 24 H2O. Aluminum Carbonate and Sulphide. The electro-positive properties of aluminum are so weak that even its salts with strong acids, e. g., the chloride and sulphate, are readily hydrolyzed. Its salts with weak acids cannot exist in the presence of water, being completely decomposed into the hydroxide and the free acid. When the acid is volatile, it of course escapes. This is the case with the carbonate. When aluminum salts are treated with the solution of a carbonate, the products are aluminum hydroxide and carbonic acid. Hence carbon dioxide escapes. (1) 2 AICI, +3 Na2CO3 → Al2(CO3)3 +6 NaCl. With the sulphide the result is similar. (1) 2 AlCl3+3 (NH4)2S Al2S3 +6 NH,CI. (2) Al2S3 +6 H2O 2 Al(OH)3+3 H2S. 479. Porcelain, Stoneware, Etc. Aluminum silicate, Al2(SiO3)3, is essentially the substance from which porcelain, etc., are made. In a pure form it is kaolin; in an impure form, clay (cf. § 393). Porcelain and china are made by mixing white kaolin and quartz with more fusible substances, such as feldspar, shaping the plastic mixture into form, and "firing" it in a pottery kiln. The more fusible portion (the feldspar) melts, and cements the whole together. The first heating is at a relatively low temperature. The article is then covered with the material of the glaze, a mixture of powdered feldspar and quartz, and fired for several days at a high temperature. Porcelain is hard and translucent, and withstands the action of heat and chemicals better than glass, hence it is used for many purposes in chemical laboratories. Stoneware is opaque, for it has not been heated enough to make the feldspar penetrate the kaolin as much as in porcelain. Earthenware is made from common clay, hardened by heat, but not fused. It is glazed by putting common salt into the furnace at the time of heating. This forms a covering of sodium aluminum silicate over the porous surface. Bricks, tiling, jugs, terra-cotta etc., are examples. Ultramarine is a blue coloring material, made by melting together kaolin, sodium carbonate, and sulphur. This substance was once very valuable, but thousands of tons of it are now made every year. 480. Exercises. 1. Write the equation for the following reactions: (a) Aluminum and magnetic iron oxide, heated. (b) Aluminum chloride and excess of barium hydroxide solution. (c) Decomposition of aluminum sulphate by heat. (d) Formation of potash-alum. (e) Hydrolysis of aluminum acetate. (f) Fusion of aluminum oxide with excess of sodium carbonate. 2." What are the formulas of aluminum phosphate, nitrate, carbide, and nitride? Of calcium metaluminate? Of ammoniaalum? 3. What element is next to aluminum in its abundance? Why is it so much more generally used? 4. How could you distinguish a solution containing an aluminum salt from one containing a calcium salt? A zinc salt? CHAPTER XXXV. IRON, NICKEL, AND COBALT. 481. Occurrence of Iron. - Iron is one of the most widely distributed metals, and, next to aluminum, the most abundant (cf. § 9). It is found in the soil, in natural waters, in the chlorophyll of plants, and in the hæmoglobin of the blood. Some reaches the earth in meteorites, and the spectroscope shows its presence in the sun and stars. The principal ores of iron are hæmatite (Fe2O3), magnetite (Fe3O4), brown iron ore [Fe2O3. 2 Fe(OH)3], and siderite, or spathic iron (FeCO3). Iron pyrites, FeS2, is used for its sulphur (cf. § 252). 482. Metallurgy of Iron.— The production of iron is carried out on an enormous scale, since iron is the most useful metal. In 1910 the world's production of pig-iron was 64,860,260 metric tons. The United States produced 27,303,567 long tons. The ores are oxides, or become oxides; hence they are reduced by heating them with carbon (coal or coke) and a flux (limestone; cf. §§ 316 and 434). The limestone combines with the silica and aluminum oxide of the ore and coal, and forms slag, a glassy mixture of calcium and aluminum silicates. METALLURGY OF IRON. 437 The reduction of iron ore is carried out in blast furnaces. Blast-Furnace. A blast-furnace (Figs. 88 and 89) is a structure about 100 feet high. The inner walls consist of fire-brick surrounded by brick. The outside is made of sheet-iron. The furnace is charged through the top with coke, ore, and limestone. A double "bell" (AB) permits new charges to be introduced without allowing gases to escape through the top. A blast of hot air is forced into the furnace through pipes, called tuyères (T,T), near the bottom. The walls of the furnace just above the tuyères are kept cool by a series of water pipes (W, W). The furnace gases escape through the downcomers (C, C) under the "bell." The iron collects (in liquid form) in the "crucible,' at the bottom of the furnace, and above it the slag. The iron is drawn off through the tap-hole (I) every four to six hours; the slag is run off through (S). The iron is run into molds, forming the bars called "pigs" (whence the name pig-iron), or it is transferred, while still molten, to "converters" (cf. § 487), or furnaces, and made into steel. In modern mills the melted iron is poured directly into steel molds. These are brought mechanically under the ladle, chilled in water, and emptied into a nearby car. Such a device is known as a "pig-machine." FIG. 88. The operations of the blast-furnace require careful attention. Ores must be analyzed, the composition of flux and reducing agent, and their amounts, must be determined, and the temperature regulated. When once started, blast-furnaces con |