During the last fifteen years the rapid development in the heat treatment of steel, owing to the increased demand for high-service machine parts, a more careful and scientific study of the heat treatment of steel has been possible, and this little work is nothing more or less than the collection of such data as has been obtained from time to time by our metallurgic force plus the cor rection of palpable errors, the reconciling of seeming inconsistencies and the deduction of a code of principles.