In general, any change in the composition of a steel results in some change in its properties. For example, the addition of certain metallic elements to a carbon steel causes, in the alloy steel thus formed, a change in position of the proper hardening temperature point. Tungsten or manganese tend to lower this point, boron and vanadium to raise it; the amount of the change is practically proportional to the amount of the element added. Just as a small proportion of carbon added to iron produces steel which has decidedly different-properties than those found in pure iron, so increasing the proportion of carbon in the steel thus formed, within certain limits, causes a variation in the degree in which these properties manifest themselves. For example, consider the property of tensile strength. In a ten-point carbon steel (one in which there is present but per cent of,carbon) the tensile strength is very nearly 25 per cent greater than that of pure iron. Adding more carbon causes the tensile strength to rise, approximately, at the rate of per cent for each per cent of carbon added. Carbon steels are divided into three classes according to the propor tion of carbon which they contain. The first of these embraces the unsaturated' steels, in which the carbon content is lower than per cent; the second, the saturated steels, in which the proportion of carbon is exactly per cent; and, the third, the supersaturated steels, in which the carbon content is higher than per cent.