It has already been stated that ordinary steel, as first cast, is porous, full of small cavities, which have to be removed by hammering before a sound metal is produced.

In order to remedy this evil, Sir Joseph Whitworth subjects the molten steel to a pressure of some six tons per square inch, by which all cavities are closed up, the gases contained in them driven out, the metal is compressed to about 7/8 of its bulk, its density and strength being greatly increased.

Sir J. Whitworth gives the steel a maximum ductility of about 30 per cent. He considers that more is unnecessary, "for cylinders of such metal do not fly into pieces when hurt, but simply open out or tear like paper, and a metal of greater ductility could not be required for structural purposes."1 The strength and ductility of the different varieties is given at p. 323.

Puddled Steel is produced by stopping the puddling process used in the manufacture of wrought iron before all the carbon has been removed.

The small amount of carbon that is left, i.e. from .3 to 1.0 per cent, is sufficient to form an inferior steel.

It is used chiefly for making inferior boiler plates and plates for shipbuilding.

A similar product resulting from imperfect refining is known as Natural Steel or German Steel.

Mild Steel contains from .2 to .5 per cent of carbon. When more carbon is present it becomes Hard Steel.

Mild steel is stronger and more uniform in texture than hard wrought iron, and superior to it in nearly every way.

It is used for welding, also for steel rails, spades, and hammers.

Mild steel made by the Bessemer and Siemens-Martin process is now coming greatly into use for boiler plates, shipbuilding, etc.

Tungsten, Manganese, and Chromium (or Chrome) Steels are made by adding a small percentage of the metals named to crucible steel; the result in each case being a steel of great hardness and tenacity, suitable for drills and other special tools.2

1 Proceedings Inst. Mech. Engineers, 1875.

2 Steel has lately been made containing 13.75 per cent of Manganese, and having a tensile strength of 60 tons per square inch, combined with 50 per cent elongation. It bids fair to become an important material.