In 1781 Professor Torbern Bergman, of the University of Upsala, in Sweden, showed that steel mainly differs from iron in containing about one fifth of one per cent. of plumbago, or carbon, as we would say now. Steels may contain all the way from one tenth to one and a half per cent. of carbon; the lower this percentage, the more nearly does the steel approach wrought iron in softness; as the proportion of carbon increases up to one per cent. the steel increases in tenacity, beyond one per cent. tenacity diminishes and brittleness is augmented. Hardness depends upon the percentage of carbon a steel contains. Physical conditions are almost as important as chemical composition; a mass of red-hot steel, carefully hammered or pressed is thereby strengthened, an effect due either to minimizing the process of crystallization, or to breaking up crystals as fast as they form. The microscope reveals many details of structure in steel, and has enabled the analysts greatly to economize the manufacture of desired varieties. Under the microscope steels much resemble crystalline rocks in structure, with constituents differing widely. Of these the most important is ferrite, a pure or nearly pure metallic iron, soft, weak, ductile, of high electric conductivity. Next in importance is cementite, an iron carbide (Fe₃C), harder than glass and nearly as brittle, but probably very strong under gradually and axially applied stress. A third constituent, austenite, is a solid solution of carbon, or perhaps of an iron carbide, in _gamma_ allotropic iron (there being also _alpha_ and _beta_ irons). Austenite is hard and brittle when cold, is stable at high temperatures, and is slowly transformed by reaction into compounds of ferrite or cementite. Several other ingredients of importance, as pearlite, illustrated on the opposite page, have also been studied.[14] [14] Henry Marion Howe, “Iron, Steel and Other Alloys.” Second edition. Published by Albert Sauveur, Cambridge, Mass., 1906. While carbon is the most decisive element in admixture, other ingredients have marked influence, silicon and manganese especially. The process invented by Bessemer, described by himself in another chapter of this book, as introduced in 1855, revolutionized the steel manufacture by its directness, cheapness and speed. It consists in burning out from pig-iron, by a hot air blast, all or nearly all its carbon. Then spiegeleisen, or other mixture, containing a definite quantity of carbon and manganese, is added to the molten mass, yielding steel of the quality desired. This method produces more rails for railroads than any competing method; in other fields it is being rivalled more and more severely by the open hearth process. [Illustration: Pearlite, magnified about 750 diameters.] [Illustration: Steel containing more than nine-tenths of one per cent of crystals of pearlite, surrounded by envelopes of cementite (Fe₃C). Magnified 200 diameters.]