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ALUMINIUM

the Allgemeine Elektricitats Gesellschaft united with the Swiss firm in organizing the Aluminium Industrie Actien Gesellschaft of Neuhausen to manufacture the metal on a larger scale. Heroult and Kiliani then reverted to the plan of winning the metal alone, and gradually the alloy process was abandoned. At present (1900) about 4000 h.p. obtained from the Rhine are employed at Neuhausen, and about 2000 h.p. at Rheinfelden, while some 3000 h.p. are similarly used by the Societe ElectroMetallurgique Erancaise of La Praz near Modane. In 1895 the British Aluminium Company was founded to mine bauxite and manufacture alumina in Ireland, to prepare the necessary electrodes at Greenock, to reduce the aluminium at Foyers, and to refine and work up the metal into marketable shapes at the old Milton factory of the Cowles Syndicate, remodelled to suit modern requirements. The river Foyers in Invemess-shire, which empties itself into Loch Ness, makes a total descent of 400 feet during the last 1^ mile of its course, and forms two cascades 40 feet and 165 feet high respectively. From a point some distance above the upper fall an 8-feet tunnel has been drilled through the solid rock, bringing the water at a high level to the brow of the hill that slopes down to the loch, which it descends in steel tubes to drive the turbines at the bottom. Lest the natural river should fail in times of drought, water-rights over some 100 square miles of Invemess-shire have been acquired, and a masonry dam has been built across the southern end of the mountain valley (700 feet high) that contained Lochs Faraline and Garth, so as to raise the water level and form one vast reservoir 5 miles long by f mile wide holding 4000 million gallons—sufficient without any rainfall to propel the machinery for 100 days. By the end of 1899 plant equal to 7000 h.p. had been installed, some of it, however, being employed in the manufacture of calcium carbide. Each dynamo is carried on the same vertical shaft with its turbine, and runs at a speed of 150 revolutions per minute, developing 700 e.h.p., and giving a current of some 8000 amperes at 60 volts. The Heroult cell consists of a square iron or steel box lined with carbon rammed and baked into a solid mass ; at the bottom is a cast-iron plate connected with the negative pole of the dynamo, but the actual working cathode is undoubtedly the layer of already reduced and molten metal that lies in the bath. The anode is formed of a bundle of carbon rods suspended from overhead so as to be capable of vertical adjustment. The cell is filled up with cryolite, and the current is turned on till this is melted ; then the pure powdered alumina is fed in continuously as long as the operation proceeds. The current is supplied at a tension of 3 to 5 volts per cell, passing through 10 or 12 in series ; and it performs two distinct functions—(1) it overcomes the chemical affinity of the aluminium oxide ; (2) it overcomes the resistance of the electrolyte, heating the liquid at the same time. As a part of the voltage is consumed in the latter duty, only the residue can be converted into chemical work, and as the theoretical voltage of the aluminium fluoride in the cryolite is 4 '0, provided the bath is kept properly supplied with alumina, the fluorides are not attacked. It follows, therefore, except for mechanical losses, that one charge of cryolite lasts indefinitely, that the sodium and other impurities in it are not liable to contaminate the product, and that only the alumina itself need be carefully purified. The operation is essentially a dissociation of alumina into aluminium, which collects at the cathode, and into oxygen, which combines with the anodes to form carbon monoxide, the latter escaping and being burnt to carbon dioxide outside. Theoretically 36°parts by weight of carbon are oxidized in the production of 54 parts of aluminium; practically the anodes waste at the same rate at which metal is deposited. The current density is about 700 amperes per square foot of cathode surface, and the number of rods in the anode is such that each delivers 6 or 7 amperes per square inch of cross-sectional area. The working temperature lies between 750° and 850° C., and the actual yield is 1 lb of metal per 12 e.h.p. hours. The bath is heated internally with the current rather than by means of external fuel, because this arrangement permits the vessel itself to be kept comparatively cool; if it were fired from without, it would be hotter than the electrolyte, and no material suitable for the construction of the cell is competent to withstand the attack of nascent aluminium at high temperatures. Aluminium is so light that it is a matter requiring some ingenuity to select a convenient solvent through which it shall sink quickly, for if it does not sink, it shortcircuits the electrolyte. The molten metal has a specific gravity 0f h f 1 lt e 1 ° . j Al cryolite saturated with alumina it 2-35, andUiV that of!A? the £ fluoride 2F6.2NaF saturated with alumina L97. The latter therefore appears the better material, and was originally preferred by Hall; cryolite, however, dissolves more alumina, and has been finally adopted by both inventors.

after prolonged, exposure to air, it has a slight violet shade. Its atomic weight is 27 (26'77, H = l, according to J. Thomsen). It is trivalent. The specific gravity of cast metal is 2‘583, and of rolled 2'688 at Propert'es4° C. It melts at 626° C. (freezing-point 654‘5°, Heycock and Neville). It is the third most malleable and sixth most ductile metal, yielding sheets 0000025 inch in thickness, and wires 0'004 inch in diameter. When quite pure it is somewhat harder than tin, and its hardness is considerably increased by rolling. It is not magnetic. It stands near the positive end of the list of elements arranged in electromotive series, being exceeded only by the alkalis and metals of the alkaline earths; it therefore combines eagerly, under suitable conditions, with oxygen and chlorine. Its coefficient of linear expansion by heat is 0,0000222 (Richards) or 0'0000231 (Roberts-Austen) per 1° C. Its mean specific heat between 0° and 100° is 0-227, and its latent heat of fusion 100 calories (Richards). Only silver, copper, and gold surpass it as conductors of heat, its value being 31-33 (Ag = 100, Roberts-Austen). Its electrical conductivity, determined on 99‘6 per cent, metal, is 60"5 per cent, that of copper for equal volumes, or double that of copper for equal weights, and when chemically pure it exhibits a somewhat higher relative efficiency. The average strength of 98 per cent, metal is approximately shown by the following table :—

Cast. Sheet Bars. Wire.

Ultimate Elastic Limit, Strength, of tons Reduction tons per Area per per square square inch. cent. inch. 3 7 15 11 35 H 12 40 7-13 13-29 60

Weight for weight, therefore, aluminium is only exceeded in tensile strength by the best cast steel, and its own alloy, aluminium bronze. An absolutely clean surface becomes tarnished in damp air, an almost invisible coating of oxide being produced, just as happens with zinc; but this film is very permanent and prevents further attack. Exposure to air and rain also causes slight corrosion, but to nothing like the same extent as occurs with iron, copper, or brass. Commercial electrolytic aluminium of the best quality contains as the average of a large number of tests, 0'48 per cent, of silicon and 0-46 per cent, of iron, the residue being essentially aluminium itself. The metal in mass is not affected by hot or cold water, the foil is very slowly oxidized, while the amalgam decomposes rapidly. Sulphuretted hydrogen has no action upon it, therefore articles made of it are not blackened in foggy weather or in rooms where crude coal gas is burnt. To inorganic acids, except hydrochloric, it is highly resistant, ranking well with tin in this respect; but alkalis dissolve it quickly. Organic acids such as vinegar, common salt, the natural ingredients of food, and the various extraneous substances used as food preservatives, alone, or mixed together, dissolve traces of it if boiled for any length of time in a chemically clean vessel; but when aluminium utensils are submitted to the ordinary routine of the kitchen, being used to heat or cook milk, coffee, vegetables, meat, and even fruit, and are also cleaned frequently in the usual fashion, no appreciable quantity of metal passes into the food. Moreover, did it do so, the action upon the human system would be infinitely less harmful than similar doses of copper or of lead. The highly electro-positive character of aluminium is most important. At elevated temperatures the metal decomposes nearly all other metallic oxides, wherefore it Aluminium is a white metal with a characteristic tint is most serviceable as a metallurgical reagent. In the which most nearly resembles that of tin; when impure, or casting of iron, steel, and brass, the addition of a trifling