Page:Popular Science Monthly Volume 44.djvu/412

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the quality of resisting the forces of pulling, bending, and twisting, which tend to break the metal or separate its molecules. Equal volumes of aluminum and cast iron have about the same power of resistance to these actions. That of copper is not quite double, but that of wrought iron is more than three times, and that of steel about five times as great. For purposes, therefore, where this quality is demanded, aluminum offers no advantages; but there are numerous other uses in which the question of a greater or less resistance is of no interest; and the other qualities of aluminum—its ductility, conductibility, and lightness—may be dominant reasons for employing it. Its use has hitherto been limited by the consideration of cost.

This difficulty is fast passing away as improved processes are applied, and the use of aluminum has been greatly extended and diversified since Sainte-Claire Deville exhibited the first manufactured specimen. In 1856 it cost one hundred and eighty dollars a kilogramme; the next year Deville was able to prepare it at La Glacière under more favorable conditions, and the price fell to sixty dollars. A year afterward the factory was removed to Salindres, where fuel and bauxite were within convenient reach. The price gradually fell; cryolite, a new aluminum mineral, discovered in Greenland, was introduced, and the metal cost only eighteen dollars a kilogramme in 1883. The manufacture was undertaken at several places in England, with improved processes based on the method of Sainte-Claire Deville. Mr. Castner devised a method of producing sodium by which the cost of that metal was largely reduced, and the price of aluminum suffered another fall. Then Mr. C. Netto devised a direct process for producing sodium by exposing pulverized caustic soda to the action of incandescent charcoal, and the cost of aluminum fell to seven dollars a kilogramme.

The brightest promises for the future of aluminum are offered through the electrical processes. When the flame of the voltaic arc is turned upon a mixture of pulverized mineral and charcoal a fusion takes place, and the metal, relieved by dissociation, flows out fluid, limpid, and brilliant. So fine a result, however, can be obtained only under the most favorable conditions, to secure which, not always with certainty, great pains are required. An easier process is to turn the voltaic arc, not upon the pulverized mixture, but upon a bath of mineral substances which have been previously brought to a condition of igneous fusion, as is done in the Cowles electrical process. Complex phenomena are then produced, both calorific and chemical. Important factories have been established for obtaining by this process both pure aluminum and its alloys with other metals, particularly with iron and copper. By it the company at Pittsburg obtained almost