The practice of using water, brine, oil and other liquids for quenching a steel tool after heating for hardening, varies with different blacksmiths. As was previously mentioned, two of the factors controlling the hardness of a piece of quenched steel are the rapidity of cooling and the range of temperature through which cooled. These may be combined as one condition, viz., the range of temperature through which a piece of steel is cooled in a given time. In any cooling liquid this condition depends (1) upon the difference in temperature of the steel and the liquid, and (2) upon the rapidity with which the liquid will conduct away the heat imparted to it by the metal. It is seen that the 2d item depends upon the ability of the liquid as a conductor of heat, upon its specific heat, and whether or not its vapor forms around the hot metal or whether a sticky film, as of oil, forms around the metal from charring. Experience is best to determine all these factors, and there is no mysterious virtue about any of the cooling media.