In the early days practically the only steels recognized—certainly the only ones desired—were of the high carbon or hardening variety. These were required for the manufacture of swords and other implements of war, for tools, etc., most of which had to have hard and sharp cutting edges. When softer and less brittle metal was desired, wrought iron was available, but in all probability high carbon steel was the material most largely used. Having but the two iron alloys and these of very different properties, it was not difficult to distinguish between them. A piece of metal could be heated to redness and plunged into cold water. If it became glass hard when cooled in this way it was thereby proved to be steel; if still soft, it was iron. But the problem is not so simple to-day. Medium, mild and yet softer steels, and other alloys which have steel characteristics have appeared and are used in immense quantities. Their advent introduced considerable complication. It will be well, therefore, before taking up our subject, “Cementation and Crucible Steel,” and the several steels which are to follow, to make sure that we all understand, as well as we may, what is “steel” as defined to-day, what are the best known varieties, and what are their characteristics? For a rough classification it is safe for us to divide the steel world into four general divisions as follows: 1. The harder, high carbon steels used for tools, dies, etc. 2. The mild and medium steels of which wire, rod, bar, plate, pipe and structural shapes for bridges, ships and “sky scrapers” are made. [Illustration: HARDENING A PIECE OF TOOL STEEL. READY TO QUENCH ] 3. Alloy steels, to which some metal such as nickel, manganese or chromium gives definite properties and the name. 4. Those other modern materials which are known as “self-hardening” and “high-speed steels.” [Illustration: BOWKNOT MADE FROM PIECE OF STEEL PIPE ] The two classes last named are not simple iron-carbon alloys and their properties are less directly derived from and do not so plainly depend upon carbon. Metallurgically, then, they are not steels in the exact former sense of the word; but as they do require carbon—though perhaps in lesser amount, are made by regular steel processes, have most of the characteristics of steel and are used for the same general purposes, they are undoubtedly entitled to the appellation “steel.” However, to distinguish, they are usually termed “alloy steels.” We are just now concerned only with the steels of classes one and two—the carbon steels. As explained in a previous chapter, these are alloys of iron with not more than 2 per cent of carbon. [Illustration: HIGH CARBON TOOL STEEL (1.25 PER CENT C) AS CAST (Magnification 70 diameters) ] [Illustration: LOW CARBON TOOL STEEL (.50 PER CENT C) ANNEALED (Magnification 70 diameters) ] [Illustration: LOW CARBON TOOL STEEL (.50 PER CENT C) AS CAST (Magnification 70 diameters) ] [Illustration: