" The ordinary steel of commerce is carbon-steel; in other words, the distinctive features of two different grades are due for the most part to variations in carbon rather than to differences in other elements.

"There are often wide variations in manganese, phosphorus, silicon, etc., but it is rarely that the carbon content does not determine the class to which the material belongs.

"This selection of carbon as the one important variable arose primarily from the fact that primitive Tubal Cains could produce a hard-cutting instrument with no apparatus save a wrought-iron bar and a pile of charcoal; and the natural developments in manufacture have led to the conclusion that a given content of carbon will confer greater hardness and strength, with less accompanying brittleness than any other element.

"There are certain exceptions to be taken to this statement in the case of hard steels made by manganese, chromium, or tungsten, but it may be accepted as true in soft steel.

"It follows, therefore, that no limit should ever be placed to the carbon allowed in any structural material if a given tensile strength is specified. It is, of course, true that every increment of carbon increases the hardness, the brittleness under shock, and the susceptibility to crack under sudden cooling and heating, while it reduces the elongation and reduction of area; but the strength must be bought at a certain cost, and this cost is less in the case of carbon than with any other element."