The student will have perceived that the products of the iron manufacturer may be divided into three classes - cast iron, wrought iron, and steel, the differences in which are caused partly by the amount of carbon they respectively contain, and also by the processes they have undergone.

The following Table, from Bauermann's Metallurgy, gives the proportion of carbon in different varieties of iron and steel according to Karsten : -

1 P.I.C.E. 1884, p. 59. 2 Pole. 3 Matheson.

The great differences in the characteristics of cast iron, and wrought iron and steel, are briefly recapitulated below, and these determine the uses to which they are respectively applied.

Cast Iron has little tensile strength, but affords great resistance to compression.

It is hard, brittle, wanting in toughness and elasticity, and gives way without warning, especially under sudden shocks or changes of temperature. It is easily melted and run into various shapes.

The castings thus produced are liable to air-holes and other flaws, which reduce their strength. Small castings are stronger in proportion to their size than large ones.

Cast iron can be cut or turned with edge tools, but is not malleable either when cold or hot, nor is it weldable.

It is not so easily oxidised in moist air as wrought iron. In salt water, however, it is gradually softened and converted into plumbago.

Cast iron is peculiarly adapted for columns, bedding plates, struts, chairs, shoes, heads, and all parts of a structure which have to bear none but steady compressive strains; also for gutters, water pipes, railings, grate fronts, and ornamental work of nearly every description.

It has been much employed for girders, but is an untrustworthy material for those of large size, or in important positions. It is liable to crack and give way without warning under sudden shocks, and also under extreme changes of temperature, such as occur in the case of buildings on fire, where the girders may become highly heated, and then suddenly cooled by water being poured on them.

Malleable Cast Iron possesses originally the fusibility of cast iron, and eventually acquires some of the strength and toughness of wrought iron. It may be used for heads, shoes, and other joints in roofs, and for all articles in which intricacy of form has to be combined with a certain amount of toughness.

Wrought Iron has many most valuable qualities, though these differ considerably as to degree in different varieties of the material.

Its tensile strength is three or four times as great as that of cast iron, but it offers not half the resistance to compression.

It is, however, very tough and ductile, and therefore gives way gradually instead of suddenly snapping.

Its elastic limit is equal to about half its ultimate strength, and it will bear repeated loads below that limit without injury.

Wrought iron is practically infusible, is malleable hot and cold, is weldable at high temperatures, and can be forged into various shapes.

It is subject to "hot and cold shortness "produced by impurities, and to other defects. Large sections are more likely to contain flaws than small ones. Bars are, as a rule, stronger than plates, and plates are stronger with the grain than across it.

Malleable iron rusts quickly in moist air, but stands salt water better than cast iron.

The great tensile strength of wrought iron leads to its employment for tie-rods, bolts, straps, and all members of any structure which are exposed to tensile stress; it is also much used for members which undergo compression. It should be employed for all important iron beams and girders, especially those exposed to sudden shocks. In its various forms it comes into play in a variety of ways in roofs, braced girders, and iron structures of all kinds. Corrugated sheets are much used for roof coverings.

Steel differs even more than wrought iron in the characteristics of its several varieties.

It has a high tensile strength, much greater than that of wrought iron. Its resistance to compression is also much greater. Moreover, it has a harder surface, and is better able to resist wear and tear.

Hard steels, containing a large proportion of carbon, are fusible, easily tempered, have a high tenacity and elastic limit. Their resistance to compression is enormous, especially when they are tempered, but they cannot be easily welded or forged, are brittle, and very uncertain in quality.

Soft mild steels have a tenacity and resistance to compression, and an elastic limit somewhat higher than wrought iron. They can be hardened and tempered, but not easily. They are weldable and easily forged, and afford a very reliable and ductile material adapted for structures subject to sudden shocks.

Steel is more easily oxidised than wrought iron, and far more easily than cast iron.

Steel is at present hardly used at all by the builder. Sometimes bolts and cotters are made of steel for large roofs.

It is not adopted for engineering structures to anything like the fullest extent of which it is capable, but is required by the engineer for tools, rails, boilers, machinery, wheels, etc. etc., and is coming into use for some of the larger roofs and bridges.