It is the comparative slowness of cooling in oil, the greater slowness of cooling in air, that make these by far the best tempering processes, because the molecular re-arrangement, in which tempering consists, requires time. Often the critical temperature, at which a desired re-arrangement takes place, is declared by the metal losing all power of response to a magnet: this fact affords the steel-maker welcome aid; he has only to shut off heat as soon as his steel ceases to attract a magnet and plunge the steel into water in order to obtain the hardness he wishes. The complex phenomena of heat treatment in steel manufacture are fully discussed by Professor H. M. Howe, in his “Iron, Steel and Other Alloys,” second edition, 1906. Steel for Railroad Rails. In another chapter of this book a word is said as to the form of rails at which Mr. P. H. Dudley has arrived as the outcome of years of experiment. He thus describes the properties which the steel should possess by virtue of due chemical composition and proper heat treatment:-- “Ductility to ensure power to resist the shock of the driving wheels, so that the steel may not break; resistance to abrasion, that it may not wear out; and high limit of elasticity, that it may not take a permanent set and be bent into a series of waves between its supporting ties, by the enormous pressures which the wheels of to-day throw upon it. The best composition is carbon 0.55 to 0.60 per cent., silicon 0.10 to 0.15, manganese 1.20, sulphur under 0.06, phosphorus under 0.06; with 50,000 to 60,000 granulations to the square inch. More granulations, or fewer, mean an increase of brittleness in the steel.”[17] [17] Henry Marion Howe, “Iron, Steel and Other Alloys.” Second edition. Published by Albert Sauveur, Cambridge, Mass., 1906. And a note from Mr. P. H. Dudley to the author, May 2, 1906. Invar: A Steel Invariable in Dimensions Whether Warmed or Cooled.